what is the ozone layer and how does it affect you?arn.org/currpage/curr31.pdfvery dry air around 15...

Volume 3, Number 1 1994Access Research Network

even lethal, ultraviolet radiation emittedby the sun. This filtering ability isparticularly remarkable when youconsider the relative scarcity of ozonemolecules. For every billion moleculesin the atmosphere, only around 300 areozone.

Imagine you could poke a tubethrough the entire atmosphere over yourhead and bring all the ozone moleculesin the tube down to the surface. If theywere then subjected to the sametemperature and atmospheric pressure(standard temperature and pressure orSTP) as you are, they would form a layeronly about 3-millimeters thick.

Although they may be formed inmany different ways, all the ozonemolecules in the stratosphere areidentical. An oxygen molecule (O2) iscomposed of two oxygen atoms (O).Ultraviolet radiation can split an O2

molecule and leave behind two free Oatoms. Various chemical reactions leaveO atoms as a byproduct. In either case,the free O atom can merge with an O2

molecule to form triatomic oxygen (O3),more commonly known as ozone.

The Two Ozone Layers

The term “ozone layer” generallyrefers to a relatively high concentrationof ozone in the stratosphere, a layer ofvery dry air around 15 to 35 kilometers(9 to 22 miles) above the Earth’s

By Forrest M. Mims, III

In this issue:

• The Ozone Hole:

Sorting Out the Facts .......10

• Further Reading .............. 10

• Ultraviolet and

Your Health ..................... 11

Forrest Mims is a science writer with over 500

articles and more than 70 books to his credit. Among

his accomplishments is the invention of a hand-held

instrument that measures atmospheric ozone. For

this accomplishment, Mims won a 1993 Rolex Award,

which included a cash prize of 50,000 Swiss Francs

(about $30,000). Mims used his prize money to

produce about 30 instruments, which have become

part of the Sun Photometer Atmospheric Network

(SPAN) – a worldwide network that has been set up

to monitor ozone. For more information about SPAN,

write to Mims at 433 Twin Oak Road, Seguin, TX,

78155. For more information about his instrument,

called MicroTOPS, contact Advanced Concept Elec-

tronics, 116 W. 19th Street, POB472, Higginsville,

MO 64037-0472. The phone number is (816) 584-

7121.

Mims has measured ultraviolet radiation and the

thickness of the ozone layer almost daily since 1989.

What Is the OzoneLayer and How Does

It Affect You?

Copyright 1993 by Forrest M. Mims III

Sprinkled throughout theatmosphere are pale bluemolecules of a toxic gas that are

essential to most life on Earth. This gasis ozone.

Ozone is toxic because it is highlyreactive. This is why it can sterilizedrinking water, eliminate odors, bleachcolors, and decompose rubber.Fortunately, the amount of ozone atground level is usually too low for theseeffects to be observed. However, highconcentrations of various air pollutantsand sunlight can increase ozone levelsnear the ground from a few tens ofmolecules per billion molecules of air(ppb) to a few hundred ppb. These levelsof ozone can damage plants, cause eyeirritation, inflame mucous membranesand impair the performance of athletes.

Ozone is essential to life because itshields the Earth from the damaging,

in Science, Technology, & Society

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Ozone is essential tolife because itshields the Earthfrom the damaging,even lethal, ultra-violet radiationemitted by the sun.

Figure 1 . A simplified view of how ozone molecules can beformed and destroyed.

over from this reaction combines with an O2

molecule to form an ozone molecule. Variousother gases in the atmosphere can combine withNO to form more NO2, which then can cause abuildup of ozone. Gaseous organic chemicals, inthe presence of nitrogen oxides and sunlight, canalso contribute to ozone production.

The same photo-chemistry that formsozone can also destroyit. Indeed, somephotochemicalprocesses in theatmosphere are called“do-nothing” re-actions since theydestroy as muchozone as they create.Ultraviolet radiationleaking through theozone high in thestratosphere can alsocreate and destroyozone in the tropos-phere.

Measuring Tropo-spheric Ozone

There are severalways to measure

tropospheric ozone. Since ozone is a strongoxidizer, it changes the color of some chemicalcompounds and solutions. For example, papersoaked in a mixture of starch and potassium iodidewill change color when exposed to ozone.

The reaction of ozone with various chemicals,gases, and even some lubricating oils causes a faintluminescence that can be detected by a sensitivephotomultiplier tube. Detection systems such asthis are known as chemiluminescence detectors.

Since ozone absorbs ultraviolet radiation soeffectively, many kinds of ozone detectorsincorporate a UV lamp and a detector. Air ispassed through a chamber, and any attenuation isassumed to have been caused by ozone. A problemwith this method is that attenuation can also becaused by dust. Therefore, it’s common practice touse two chambers, one of which receives air fromwhich any ozone has been scrubbed by a catalyticconverter. Alternatively, scrubbed and unscrubbedair can be passed in sequence through the samechamber. Either way, the error caused by dust andother contaminants in the ozone-free sample canthen be determined by subtraction.

Sulfur dioxide and other chemicals caninterfere with the chemical and UV detection of

surface. However, about 10 percent of the totalozone is found in the troposphere, the lowestportion of the atmosphere. The tropospherecontains 90 percent of the atmosphere andnearly all of the atmosphere’s water vapor.Storms form in the troposphere and usually staythere. But the tops of especially powerfulthunderstormsoccasionally pokeinto the lowerstratosphere.

The tropopause,the border betweenthe troposphere andthe stratosphere,ranges from around10 to 15 kilometersabove the surface.The heat of summerincreases the heightof the tropopause; theheight is reduced inwinter.Other thingsbeing equal, when thetropopause is low, theamount of stratos-pheric ozone is highand vice versa.

TroposphericOzone

The ozone between the surface and thetropopause forms only a fraction of the ozoneover most locations. Nevertheless, it absorbssolar UV more efficiently than an equal amountof stratospheric ozone. This is because scatteringcaused by dust and aerosols increases thedistance that rays of sunlight travel on their wayto the surface. In spite of this benefit, tropos-pheric ozone is often referred to as “bad” ozonebecause of its adverse effects in high concen-trations. If the same ozone were somehow todrift into the stratosphere, it would be called“good” ozone.

Forming and DestroyingTropospheric Ozone

Tropospheric ozone is produced in manyways. Some is formed by lightning or by UVradiation from the sun. Most is formed bychemical reactions which take place in thepresence of sunlight.

One such reaction is the conversion ofnitrogen dioxide (NO2) into nitrogen oxide (NO)in the presence of solar UV. The O atom left

CURRENTS in Science, Technology & Society 3

ozone. When scientists at the Montsourisobservatory near Paris became aware of thisproblem in 1905, they built a second chemicalozone detector. The air inlet for the new detectorwas fitted with a 4-meter (13-feet) hose of naturalrubber, which completely destroyed any ozonepassing through it. In this way any errors in theoriginal detector caused by gases other thanozone could be eliminated.

Several times my son Eric and I havemeasured the amount of tropospheric ozonebetween the bottom and top of mountains in NewMexico. We do this by measuring the totalamount of ozone in the atmosphere with a UV-sensitive instrument that is pointed at the sun.One of us goes to the top of a mountain while theother stays at the base. We then make a series ofobservations at prearranged times. Later, wesubtract the ozone measured at the mountaintopfrom the measurements made at the base todetermine the amount of ozone in between. So farour results (a few Dobson Units per verticalkilometer; see Figure 5) have agreed withmeasurements made from balloons by theNational Oceanic and AtmosphericAdministration (NOAA).

Tropospheric Ozone Cycles

The amount of ozone above any given spotof Earth is rarely constant. Consider the diurnalor daily ozone cycle over Albuquerque, NewMexico.

Early on a July morning at the base of theSandia Mountain aerial tramway just northeast ofthe city, ground-level ozone concentration mightbe, say, 20-30 ppb. As the sun rises high in thesky, photochemical ozone production increases,especially when the wind is from the southwestand the clean mountain air is spiked withnitrogen oxides and hydrocarbons fromautomobile exhaust. Although little or nophotochemical smog may be visible, the ozoneconcentrations might reach 40-60 ppb by lateafternoon. As the sun sinks behind the volcanocinder cones west of Albuquerque, the ozonelevel also falls. Late that evening, the ozonereturns to its normal “background” level.

Tropospheric Ozone Trends and Effects

The ozone measured at the ground nearParis, France, from 1876-86 was only around athird to a half of what is usual in unpolluted areastoday. The increase since then is generallybelieved to be caused by human activities. Atleast two-thirds of the nitrogen oxides arebelieved to come from the burning of fossil fuels,wood, forests and agricultural wastes. Nitrogen

Figure 2 . How ozone is distributed in the atmosphere.


vegetation down-wind from thecentral city. Theyconcluded thatozone is reducedonly when bothhydro-carbonsand nitrogenoxides arereduced.

Therelationship oftrees and ozone isparticularlyinteresting. Toomuch ozone candamage or evenkill trees. Ironic-ally, trees emithydrocarbons thatparticipate inchemical re-actions thatproduce ozone.Several years agoWilliam

Chameides of the Georgia Institute of Tech-nology studied satellite images of Atlanta andfound that 57 percent of the city was wooded. Heand his co-workers concluded that Atlanta’s treesemitted at least as many hydrocarbons as thecity’s cars, trucks, buses and factories.

Stratospheric OzoneMost references to the ozone layer mean the

ozone found in the stratosphere. There it forms avaporous shield that protects life on Earth fromthe lethal effects of the sun’s UV radiation. Ifyou’ve flown in the Concorde, then you haveprobably travelled through the bottom of thestratospheric ozone layer.

Forming and DestroyingStratospheric Ozone

Ozone in the stratosphere is formed by anatural photochemical process when ultravioletradiation from the sun splits molecules of oxygeninto the individual oxygen atoms from whichthey are formed. The free O atoms soon reactwith O2 molecules to form O3.

This process works both ways: O3 moleculesthat are unlucky enough to be struck by UVradiation are split back to an O2 molecule and afree O atom. The free O atom can merge with anO2 molecule to once again form an O3 molecule.

Ozone molecules that drift lower down in the

Figure 3 . Ozone levels near street level and at the 110th floor of the World Trade Center in New YorkCity on July 21, 1989.

A governmentofficial who makesozone measurementsat fixed sites told methat ozone levels canbe much higher inthe air over NewYork and in thesurrounding regionsthan at ground levelin the city. Appar-ently the highnumber of airpollutants, people,rubber tires, and thelike, suppressesozone concentra-tions at street level.

oxides are also produced naturally by lightning,forest fires, and soil. Organic chemicals, such asmethane and hydrocarbons, can be byproducts ofplants, animals, and human activity.

It's interesting to compare the amount ofozone in a vertical column of air adjacent to amountain with that over a city. In the summer of1989, Eric and I measured 5.8 DU (Dobson Units)of ozone between the base and crest of SandiaMountain, an altitude difference of 1,164 meters(3,819 feet). A few weeks earlier I had measuredabout 5 DU of ozone between street level and anobservation deck atop the 110th floor of theWorld Trade Center in New York City (420meters or 1,377 feet). There was much moreozone in the air above New York City than in theair near Sandia Mountain.

Surprisingly, however, much of the air atstreet level has less ozone than you might expect.A government official who makes ozonemeasurements at fixed sites told me that ozonelevels can be much higher in the air over NewYork and in the surrounding regions than atground level in the city. Apparently the highnumber of air pollutants, people, rubber tires, andthe like, suppresses ozone concentrations at streetlevel.

When California forced a significantreduction of hydrocarbon emissions fromautomobiles, the ozone in downtown Los Angelesfell. Ozone levels downwind, however, continuedto rise. The scientists who puzzled over thisdilemma noticed that there is considerably more


stratosphere are protected from the destructiveeffects of UV radiation by the overlying blanketof ozone molecules. But even molecules of ozonedeep in the ozone layer are not entirely safe, forthey can be destroyed by reactions involvingsunlight and oxides of nitrogen, hydrogen,chlorine, and bromine. Although all thesechemicals can arise from natural sources, they canalso arise from human activity. For example,manufactured chlorofluorocarbons (CFCs) havebeen a concern since 1970 when James E.Lovelock detected their presence in air. In 1974,Sherwood Rowland and Mario Molina proposedthat CFC molecules could eventually drift into thestratosphere, where UV radiation would breakthem down into chlorine monoxide (ClO) andother ozone-destroying compounds.

In recent years, evidence has beenaccumulating that CFCs may indeed be causing agradual reduction of ozone, particularly in regionsnear the poles during early spring. The questionnow being researched is how much ozone theymight ultimately destroy. Fortunately solar UV isconstantly creating new ozone. Therefore, CFCscannot destroy all the ozone.

Measuring Stratospheric Ozone

Ozone in the stratosphere can be measureddirectly using instruments on aircraft, rockets,and–especially–balloons. Many of the same kindsof sensing systems used for measuring ozone atthe surface have been modified for these roles.

Thanks to ozone’s well-known ability toabsorb ultraviolet radiation, the total amount ofozone (troposphere plus stratosphere) can bemeasured indirectly from the surface or fromspace. Several kinds of optical instruments havebeen developed for measuring ozone from thesurface, including the Dobson spectrophotometerand various instruments that use filters ordiffraction gratings to measure narrow bands ofultraviolet.

The Dobson spectrophotometer plays a keyrole in ground-based ozone monitoring efforts.Invented in the late 1920’s by G. M. B. Dobson,this instrument divides sunlight into a spectrumwith a prism and measures the ratio of two UVwavelengths about 20 nanometers (nm) apart.Dust and aerosols can cause errors in ozoneobservations by scattering one wavelength morethan another. Dobson observations are usuallymade at two pairs of wavelengths to cancel outthis error.

The Dobson instrument is expensive, nearly 2meters (6 ft) long, and heavy–about 40 kilograms(85 pounds). Since it measures the ratio of two ormore ultraviolet signals, it provides no informa-

In recent years,evidence has beenaccumulating thatCFCs may indeed becausing a gradualreduction of ozone,particularly inregions near thepoles during earlyspring. The questionnow being re-searched is howmuch ozone theymight ultimatelydestroy. Fortunatelysolar UV is con-stantly creating newozone. Therefore,CFCs cannot destroyall the ozone.

tion about the amount of solar ultraviolet.

The Brewer ozonometer uses a diffractiongrating to separate the sun’s ultravioletwavelengths. (A diffraction grating consists ofseveral parallel grooves that split light up intoseveral wavelengths. A compact disk produces aneffect much like a diffraction grating–except thatthe rainbow colors are produced by parallel rowsof pits instead of grooves.) This expensiveinstrument is smaller than the Dobson and well-suited for automated data taking. It also measuressulfur dioxide, a gas that can interfere with ozonemeasurements. Some scientists believe that theBrewer measures ozone with higher precisionthan the Dobson. Indeed, Canada has switchedfrom Dobsons to Brewers.

A third kind of instrument uses optical filtersto measure two or more UV wavelengths. Suchinstruments are cheaper, smaller, and easier touse than the Dobson and the Brewer. They alsoprovide information about the sun’s directultraviolet. For these reasons, I selected the filterapproach when designing an instrument tomeasure total ozone several years ago.

Some sensors on satellites can measure theamount of ozone at various altitudes by observingthe sun as it rises and sets through the atmosphereabove the Earth’s limb (an astronomical termreferring to the edge of a planetary body’s disk).Other satellite sensors measure the amount of thesun’s ultraviolet that is scattered back into spacefrom the atmosphere below. Since thesewavelengths are absorbed by ozone, processingthe backscatter from one or more pairs ofwavelengths permits one to estimate the amountof ozone between the satellite and the ground.

It’s important to note the latter kind ofinstrument cannot measure all the ozone betweenthe top of the atmosphere and the surface. Cloudscan get in the way, and little or none of the UVbackscattered from the lowest few kilometersabove the surface can penetrate back through theozone layer. An estimate of lower troposphericozone is added to the satellite ozone equation tocorrect this problem. The estimate is based onmeasurements made from the ground duringannual calibration checks and measurementsmade by balloon sensors.

Stratospheric Ozone Cycles

Superimposed on the daily ozone cycle nearthe ground are seasonal changes in the amount ofstratospheric ozone. In the northern hemisphere,the total amount of ozone is lowest during winter.The amount of ozone begins to rise rapidlyduring spring and gradually diminishes in thesummer and fall.


In the northernhemisphere, the totalamount of ozone islowest during winter.The amount of ozonebegins to riserapidly duringspring and graduallydiminishes in thesummer and fall.

This gradual seasonal variation in ozone ismarked by sharp spikes and dips associated withweather systems. Passage of a cold front, forexample, may cause the amount of ozone toincrease 20 percent or more for a day or two. Awarm front may cause an comparable decrease.Meterologists refer to regions of diminishedozone as ozone minimums, and regions of highozone as ozone maximums.

I’ll never forget the giant ozone maximumthat passed over South Texas on March 16, 1990(See Figure 6, page 8). That morning I made afew ozone observations around 11:00 a.m. andwas surprised to find the highest amount of ozoneI had ever measured, around 360 DU. Since theozone amount kept climbing as noon approached,I assumed something was wrong with theinstrument. But both filters were clean, no wireswere dangling in front of them, and the batterywas fresh.

After 1:00 p.m. the ozone amount climbedeven faster than it had before noon. By 1:30 p.m.it was more than 440 DU and shortly before 2:00p.m. it reached 460 DU. The ozone amount thenbegan a sharp slide to pre-noon levels.

Several months later, data from the TOMS(Total Ozone Mapping Spectrometer) instrumentaboard the Nimbus-7 satellite confirmed theextraordinarily high ozone levels of March 16.My son Eric wrote a Pascal program that

transformed the TOMS data into a color-codedozone map of the United States. The mapdisclosed an enormous tongue of high ozonereaching down from Canada and ending in SouthTexas. A check of weather records revealed thatthis ozone maximum was associated with a giantweather system that moved in from the Pacificand crossed the United States over a 3-dayperiod.

Stratospheric OzoneTrends and Effects

Recall that most ground observations of theozone layer measure the total amount of ozone ina column between the instrument and the top ofthe atmosphere. Therefore, these measurementsinclude the total amount of both tropospheric andstratospheric ozone.

Daily measurements of the amount of totalcolumn ozone have been made at Arosa,Switzerland since 1926. The total amount ofozone back to 1912 has been determined by acareful analysis of solar measurements made bythe Smithsonian Institution. Since 1957, morethan 70 Dobson spectrophotometers have maderegular measurements of ozone.

These measurements show that the totalamount of ozone varies in cycles that may last a

decade or more. For example,during the 1960’s, scientists atNOAA found that ozone overNorth America increased byabout 5 percent. Since 1970,however, ozone over thenorthern hemisphere hasdeclined around 5 percent.Since this decline is seen inboth measurements from theground and from satellites,there is little disagreement thatit is real. But there is consider-able disagreement concerningthe reason for the decline, andits significance.

Some scientists believe thedecline is primarily abyproduct of natural metero-logical cycles, a changingclimate and possibly the solarcycle. They support their caseby pointing to ozone cyclesover the past 50 or more years.Others believe the decline is inlarge part caused by contami-nation of the atmosphere bypollutants that contribute to thedestruction of ozone–

Figure 4 . How ozone is measured from space and the ground.


Figure 5 . Annual cycles are obvious in this graph of the total ozone in South-Central Texas fromNovember 1, 1978 to May 25, 1992. Light gray represents Nimbus-7 satellite observations. Darkgray represents observations by Forrest Mims, III using TOPS-1. (Satellite observations shownhere for 1989 are incomplete and may be 8% too low.)

particularly CFCs. Some believe the eruption ofmajor volcanoes such as El Chichon in 1982 andPinatubo in 1991 exacerbate the problem. Stillothers believe ozone is impacted both by naturalmeteorological trends and ozone-destroyingchemicals.

These issues are being studied and debated bythe scientists who study ozone. Meanwhile, as hasbeen widely reported in the press, there has beenconsiderable political fallout over the issue ofdeclining ozone.

That’s one reason why more than 70 nationshave signed the Montreal Protocol, an agreementto eventually ban production of most CFCs.Because it is believed that CFSs can remain in theatmosphere for decades, even a total ban will notrestore the ozone to its pre-1970 condition–ifindeed CFC’s are the principal culprit. Instead,ozone will continue to decline at, perhaps, a fewpercent or so per decade until CFCs are no longerpresent.

Ultraviolet RadiationIf the spectral sensitivity of a honey bee’s

eyes could somehow be added to yours, rainbowswould have an additional streak of color adjacentto the violet band. This invisible band of “blacklight” is known as ultraviolet radiation.

Ultraviolet radiation is divided into three

These measure-ments show that thetotal amount ofozone varies incycles that may lasta decade or more.

bands. The wave-lengths below 290nm are referred toas UV-C. Thewavelengthsbetween 290 and320 nm are referredto as UV-B. Andthe wavelengthsbetween 320 and340 nm are knownas UV-A. Theozone layer blocksall UV-C. The UV-B that leaksthrough is whatcauses sunburn.

The UltravioletSky

The sky is bluebecause most of itsmolecules are justthe right size toscatter the bluewavelengths ofsunlight. Thesemolecules also

scatter UV wavelengths. This means that theentire daytime sky is a gigantic source of UV-Aand UV-B (remember that all the UV-C isabsorbed by ozone).

The UV radiation from the sky can bedescribed as direct, diffuse, or global. Directradiation is that which comes directly from the


In short, exposedparts of your bodycan receive a signifi-cant does of UVeven when shadedfrom the direct sun.

sun. Diffuseradiation is thatscattered fromclouds andmolecules of air.Global is the sumof direct anddiffuse UVradiation. Mostnatural materialsreflect UV ratherpoorly. But snow isan excellent UVreflector; and so iswater. In short,exposed parts ofyour body canreceive a signifi-cant does of UVeven when shadedfrom the direct sun.

UltravioletHazards

The energy of electromagnetic radiation isinversely related to its wavelength. In otherwords, radiation with short wavelengths–like x-rays–has much higher energy than radiation withlonger wavelengths, such as visible light. For thisreason, ultraviolet radiation is more energetic thanvisible light.

That’s why the sun’s UV-B radiation readilycauses sunburn while UV-A and visible sunlightdo not, even though much more UV-A and visiblelight reach the earth. (Of course, even visiblesunlight will cause sunburn if it is concentratedwith a lens or reflector.) The sun’s UV-B can alsodamage the chromosomes in human skin cells–which can eventually lead to skin cancer.Excessive UV-B can also cause cataracts and alterthe immune system.

Plants can be damaged by excessive UV-B.So can organisms that live at least part of their lifecycle near the surfaces of lakes, rivers and oceans.More research is needed to better understand thenature of such damage and the amount and wave-lengths of UV that are responsible.

Ultraviolet Benefits

The public has been frequently remindedabout the dangers of UV-B exposure. It’simportant to realize, however, that solar UV alsoplays an essential role in human health.

Perhaps the most important contribution ofsolar UV is its ability to stimulate the productionof vitamin D in the outer layers of the skin. This

gives UV the ability to prevent and to curerickets and to maintain a healthy skeleton. Bothnatural and artificial UV radiation are also usedto treat psoriasis.

Measuring Ultraviolet Radiation

If you’ve ever retrieved a rolled upnewspaper which has lain in the summer sun fora few hours, you know that newsprint darkenswhen exposed to solar UV. So does freshly cut orsanded pine and other woods. Colored construc-tion paper and fabrics are bleached by solar UV.

A few years ago during a field trip to NewMexico, my son and I tacked a strip of freshlysanded pine atop the crate bolted in the back ofour pickup that carried our instruments andsupplies. We covered the wood with a strip oftape, several centimeters of which we removedeach day. After 10 days on the UV-drenchedhighways of Texas and New Mexico, the strip ofwood was divided into 10 segments, eachslightly darker than the next.

Some chemicals will also change color whenexposed to solar UV. But an electronic instru-ment is necessary to quantitatively measure theintensity of UV. The two principle kinds of UVsensors used with such instruments are photo-tubes and solid-state detectors. Specializedphototubes known as photomultipliers can bemade exceptionally sensitive to UV. But they areexpensive, fragile, and require a high operatingvoltage. Solid-state detectors are not as sensitive.

Figure 6 . Unusually high ozone measured by the author in Seguin, TX on March 16, 1990.


In addition, they are are cheaper, sturdier, andmuch easier to use.

An optical filter can be placed between aUV detector and the sun to enable the detector torespond only to specific regions of the UVspectrum. Alternatively, a prism or grating canbe used to select specific wavelengths.

Various methods are used to measure direct,diffuse and global UV. Direct UV is easilymeasured by placing the detector inside acollimator tube that limits its field of view.Ideally, the field of view of the collimator shouldnot exceed a few degrees.

Global UV is measured by placing a diffuserplate (such as ground silica or diffuse UV-trans-mitting plastic) over the detector and its filter.

Diffuse UV can be measured with a globaldetector by placing a small disk so that itsshadow completely covers the detector. Thisblocks direct sunlight so that the detector signalis entirely from the diffuse sky. Subtracting thediffuse amount from the global value gives thedirect UV.

Ozone vs. Sunlight

Ozone absorbs some of the sun’s infraredradiation. It even weakly absorbs thewavelengths around 600 nm, which appear

As noted, it’s impor-tant to understandthat the scatteringcaused by dust andaerosols can causeozone near theground to absorbmore UV than anequal amount ofozone in the strato-sphere.

orange to the human eye. Its most importantabsorption, of course, occurs at UV wavelengthsbelow around 340 nm. The absorption increases sorapidly below 320 nm that little or no measurableradiation below 295 nm reaches the surface at sealevel.

As noted, it’s important to understand that thescattering caused by dust and aerosols can causeozone near the ground to absorb more UV than anequal amount of ozone in the stratosphere. Thishelps explain why some scientific studies havefound a slight decrease in UV-B over the pastdecade, a time during which the amount of stratos-pheric ozone has decreased and the amount oftropospheric ozone has increased.

Conclusion

Evaluating claims about the ozone layerrequires basic knowledge not only about ozoneand its effects on ultraviolet, but about theresearch methods that have been used to under-stand the ozone layer’s dynamics. Hopefully, thisarticle has provided some of that knowledge–andperhaps even sparked some interest in doing someindependent investigating. If so, be sure to checkout some of the resources mentioned on page 10(“Readings about Ozone and Ultraviolet.”) Also,be sure to read the other articles in this issue, onthe ozone hole and on the health effects of UV.

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The Ozone Hole:Sorting Out the Facts

by Forrest M. Mims, III

In recent years, much attention hasbeen given to the “ozone hole” over Ant-arctica. This phenomenon is observed eachyear in October during the Antarctic spring.After several weeks, the Antarctic vortex,a whirling weather system that encirclesand isolates the South Pole during winter,breaks up and ozone levels rapidly rise.

In meterological terms, the Antarcticozone hole is a significant ozone minimumand not a literal “hole” through the entireozone layer. Nevertheless, for a brief timeozone levels within the hole can plummetto 100 DU. (Normal levels are about 300DU.) At the same time, the ozone levels ina broad belt encircling the hole are thehighest on earth.

Because various scientific studies haveconcluded that the ozone hole is caused inpart by chlorine believed to come frommanufactured chemicals–especially CFCs–some scientists, politicians, and govern-ment agencies have sounded an alarm aboutthe prospect of severe ozone depletionleading to ozone holes elsewhere. In a

widely publicized statement two years ago,then-Sen. Albert Gore raised the possiblitythat an ozone hole might appear overKennebunkport, Maine. Although a promi-nent NASA scientist discounted this possi-bility, other scientists held a press confer-ence to express alarm about possible seri-ous ozone depletion over the Arctic. De-velopments like these led to many scaryreports in the media.

Fortunately, the Antarctic hole is aphenomenon preceded by the very coldtemperatures and darkness found insidethe winter Antarctic vortex, which is muchstronger than the Arctic vortex.

Long before the ozone hole was iden-tified in 1985, G.M.B. Dobson, inventor ofthe Dobson spectrophotometer, discoveredsomething very different about Antarcticozone. In a paper titled “Forty Years’Research on Atmospheric Ozone at Ox-ford: A History” (Applied Optics, March1968), Dobson described an ozone moni-toring program that began at Halley Bay,Antarctica, in 1956.

When the data began to arrive, “thevalues in September and October 1956were about 150 [Dobson] units lower thanexpected. . . . In November the ozonevalues suddenly jumped up to those ex-pected. . . . It was not until a year later,when the same type of annual variationwas repeated, that we realized that theearly results were indeed correct and thatHalley Bay showed a most interesting

Over the past half century, thousandsof scientific papers, articles and bookshave been published about ozone and solarUV.

If you want to study the history of thesubject, the best paper by far is one byG.M.B. Dobson, inventor of the Dobsonspectrophotometer, a ground-based instru-ment for measuring atmospheric ozone.The paper is titled “Forty Years’ Research

all seasons.”

For a summary of current knowledgeof ultraviolet solar radiation reaching theearth’s surface, see “Ultraviolet SunlightReaching the Earth’s Surface: A Reviewof Recent Research,” by John Frederick(Photochemistry and Photobiology, vol.57, no. 1, pp. 175-178, 1993).

You can find many other articles aboutozone and ultraviolet at any library. Uni-versity libraries are best since they havemany of the scientific journals that publishpapers about ozone. Several books aboutozone have also been published.

If you really want to dig into thescientific literature on utlraviolet and

difference from other parts of the world.”

The ozone decline reported by Dob-son was not nearly as severe as the one thatcharacterizes the Antarctic ozone hole to-day. However, two scientists who reviewedold ozone records recently reported thatthe ozone amount in the spring of 1958 fellto only 110 DU at the French AntarcticObservatory at Dumont d’Urville.

In Annales Geophysicae (November,1990), P. Rigaud and B. Leroy observedthat in 1958, the Antarctic vortex, wherethe most significant ozone depletion oc-curs, was centered over Dumont d’Urville,on the opposite side of the South Pole fromHalley Bay. They reported that the con-centration of CFCs in the atmosphere in1958 was much lower than it is today andconcluded that natural phenomena, suchas volcanic aerosols in the stratosphere,may also lead to ozone destruction.

Writing in a recent issue of Science,however NASA scientist Paul Newmanhas convincingly refuted Rigaud’s andLeroy’s ozone measurement methods. Hepointed out that Rigaud and Leroy reliedon photographic plates, an unreliablemethod for measuring stratospheric ozone.

Meanwhile, the chemistry and dy-namics of the atmosphere inside the Ant-arctic and Arctic vortices remain the sub-jects of extensive research using variouskinds of ground-based instruments,instrumented balloons, high-flyingaircraft, and satellites.

Readings AboutReadings AboutOzone andOzone andUltravioletUltraviolet

continued on page 16

on Atmospheric Ozone at Oxford: A His-tory” (Applied Optics, vol. 7, no. 3, March1968, pages 387-405). Another outstand-ing paper, by one of Dobson’s contem-poraries, F.W. Paul Gotz, is “Ozone in theAtmosphere” (Compendium of Meteorol-ogy, American Meteorological Society,1951, pages 275-291).

The most recent comprehensive sci-entific paper on ozone trends is “MeasuredTrends in Stratospheric Ozone” by Rich-ard Stolarski, Rumen Bojkov and severalothers (Science, vol. 256, April 17, 1992,pages 342-349). This paper presents a de-tailed comparison of satellite and ground-based measurements and concludes thereis “an apparent downward trend in the totalcolumn amount of ozone over mid-lati-tude areas of the Northern Hemisphere in


parency.

Cataracts have been associated withmany risk factors, including smoking,diabetes, steroids, episodes of severedehydration, and–of course–UVRexposure.

As with skin damage, cataractformation is associated more withchronic exposure than acute exposure–which is not to say that acute exposure isrecommended. Indeed, acuteoverexposure can lead to permanent ortemporary blindness.

Snow Blindness

One particularly excruciating resultof acute overexposure of the eyes iskeratoconjunctivitis, or snow blindness.Snow blindness is essentially a sunburnon the surface of the eye (i.e. the corneaand conjunctiva).

Symptoms include redness of theeyes and a gritty feeling, whichprogresses to pain and an inability totolerate any kind of light. The pain hasbeen compared to rubbing sandpaperacross one’s eyes. Fortunately, snowblindness is usually only temporary.

Sun and snow is an idealcombination for getting snow blindness.Snow is an outstanding UVR reflector,and the combination of direct andreflected sunlight is a double whammyfor unprotected eyes. Skiers should thusbe careful to protect their eyes whenthey hit the slopes.

Surfers should also be careful.Reflected light from the water can havethe same effect as reflected light fromsnow.

Skin Cancer

Three kinds of skin cancer havebeen associated with UVR exposure:basal cell carcinoma (BCC), squamouscell carcinoma (SCC), and melanoma.

By far the most common form ofskin cancer is BCC, which makes up 75to 90 percent of all skin cancers. It isstrongly linked with sun exposure, and israrely found on skin surfaces not

UltravioletandYour

Health

continued

by Mark Hartwig

in this range are known as UV-B.1 How-ever, UVR between 320 and 400 nm–called UV-A–can also give you a burn.UV-A is less energetic than UV-B, but itcan penetrate the top layers of skin,damaging the lowest level. It is also ab-sorbed less efficiently by the atmo-sphere. Consequently, the ratio of UV-Ato UV-B will increase as the sun getslower in the sky, and its contribution tosunburn will be relatively high in theearly morning and late afternoon.

Sun-Damaged Skin

Of course, sunburn is not the onlyeffect of UVR. One effect of long-termexposure is sun-damaged skin–even inthe absence of sunburn.

Much of what was once attributed toaging is now known to be caused by sundamage. Old age can bring aboutroughness, fine wrinkling, andlooseness of the skin. Sun-exposed skin,however, is also marked by coarsewrinkling and elastosis, which gives theskin a pebbly, yellowed quality. Bothwrinkling and elastosis are caused bydamage to elastic fibers in the lowestlevel of skin, the dermis.

In addition to these effects, sun-exposed skin is also prone to irregularhyperpigmentation and depigmentation,and actinic keratoses–which are rough,red patches of precancerous skin cells.

Cataracts

Another long-term effect of UVRexposure is the formation of cataracts. Acataract is any change in the structure ofthe lens that leads to a loss of trans-

Regardless of what you thinkabout ozone depletion and CFCs,ultraviolet radiation (UVR) is

very real. So are its effects. Even if weshould see no long term increase inUVR, most places on earth receive morethan enough to cause real problems ifyou often go outside without protectingyourself.

Sunburn

Perhaps the best-known conse-quences of excessive UVR exposure iserythema, or sunburn. Sunburns can bemild or severe. If you’ve had a severesunburn, you’ll not likely forget it. Suchcases are marked by bright pink or evenscarlet-colored skin, swelling, blistering,and exquisite pain. An extremely severecase may also be accompanied by nau-sea, fever or chills, and tachycardia (aracing heart beat). Because of water lostthrough the skin, sunburns can also leadto dehydration.

Actually, the painful symptoms ofsunburn are caused more by the body’sresponse to UVR skin damage than bythe damage itself. Although no one re-ally understands the whole process,UVR damage apparently triggers an in-crease of several chemical substances,including prostaglandins and histamines.Both substances contribute to inflamma-tion. Whole body exposure can also leadto increased levels of serum interleukin-1 and interleukin-6, which could partlyaccount for some of the symptoms asso-ciated with an extremely severe sunburn.

Sunburn is primarily caused by theUVR wavelengths between about 295and 320 nanometers (nm). Wavelengths


Indeed, the development of skin cancersmay well be–at least in part–a result ofimmune system damage.

For example, in one study, skincancers were induced in mice byexposing them to UVR. When these skincancers were transplanted into normal,genetically identical mice, most wererejected by the new host’s immunesystem. However, when transplanted tomice that had been subjected to a shortcourse of UVR exposure, the tumorsgrew and eventually killed them.

Similarly, another study showed thatafter human subjects had undergonetwelve 30-minute exposures to artificialUVR in a commercial tanning bed, thefunctions of T cells and Natural Killercells (which play a role in fighting viralinfections and are cytotoxic to sometumor cells) were negatively affected.

How does UVR exposure affectimmunity? One way is by the damage itdoes to Langerhans cells. Langerhanscells, which make up about 4-7 percentof the cells in the epidermis, areresponsible for communicating with Tcells and initiating a response to foreigninvaders. UVR can damage these cellsso that they can no longer perform thatfunction. Instead, a suppressive responsemay be initiated, which actuallyprevents an immune response against theinvader.

Health Benefits of UVR

UVR is not all bad. For one thing, itassists in the production of vitamin D inskin cells. This vitamin D is absorbed bythe body and then used in the up-take ofcalcium from the intestinal tract. Thisvitamin is essential for the growth anddevelopment of healthy bones.Fortunately, brief exposure to sunlighton a regular basis is enough to produceall the vitamin D most people need. Thevitamin can also be obtained fromdietary sources.

UVR is also useful for treatingpsoriasis and alopecia areata. But suchtreatment may also increase the risk ofskin cancer, and should be undertakenonly under the supervision of aknowledgeable physician.

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Balancing Technology

continued from page 11

exposed to the sun. The only exceptionsusually involve arsenic or radiationexposure, or complications from tattoos,scars, burns, or vaccinations.

Fortunately, BCC does notmetastasize (except in some AIDSpatients) and is slow growing.Nonetheless, if it is left untreated, it candamage or destroy underlying tissue andcause disfigurement.

The next most common kind of skincancer is SCC, accounting for about 20percent of all cases. Like BCC, SCCmost often occurs on sun-exposed skin.It can also occur in scar tissue, infectionsand ulcerations, areas of previousradiation exposure, areas of chronicirritation, and non-healing wounds.

Although less common than BCC,SCC is a more serious matter because itcan metastasize. About 95 percent of allBCC can be cured if treated early.Nonetheless, SCC claims the lives of asmany as 2,000 people a year.

Both BCC and SCC are thought toresult from chronic UVR exposure ratherthan one or more acute episodes.Malignant melanoma, however, the mostdeadly form of skin cancer, seems to bemost common in white people who havehad intermittent sunburns–especially inchildhood or adolescence. Indeed, somescientists believe that one painfulsunburn in children 15 or under cantriple their odds of getting melanomalater on.

Also, indoor workers who vacationin the sun and get occasional burns aremore likely to end up with melanomathan those who work in the sun.

Other possible factors includechemical carcinogens, viruses, andimmune deficiencies.

Melanoma is easily cured if treatedearly. Once it spreads to the lymphnodes, the survival rate dropsdramatically.

Immune System Deficiencies

Finally, excessive UVR can alsoproduce immune system deficiencies.

Protection from the UVR

An important part of protectingyourself from excessive UVR exposureis to recognize that UVR intensity canvary as a result of many differentfactors, including:

Time of day. UVR is most intensewhen the sun is highest in the sky.That’s because it has to pass through lessof the atmosphere to reach you.

Time of year. Because the sun ishighest in summer, UVR is also at itsmost intense–other things being equal.

Latitude. You’ll burn much fasterin Hawaii than in Maine.

Elevation. Because ozone blocksout UV-B whether it’s at ground level orin the stratosphere, you’ll generally burnfaster on top of a mountain than at itsbase.

Reflective surfaces. Reflectivesurfaces like sand, water, and especiallysnow, can greatly increase your UVRexposure, leading to burns. Indeed,reflected UVR can give you a burn evenwhen you’re sitting under an umbrella.

Stratospheric ozone. Observationshave shown that stratospheric ozone canfluctuate dramatically in a relativelybrief time. In addition to the sharpincrease Mims noted in his article, hehas also observed periods of extremelylow ozone (as low as 230 Dobson units),along with correspondingly high UVRlevels. This occurred in June, 1993 andis occurring again this summer.

continued on page 16


News & Notes

Dimpled Baseball Bats

A technical instructor in the MITDepartment of Aeronautics andAstronautics hopes to strike paydirt withhis patented idea for a dimpled baseballbat. Jeffrey Di Tullio was working withthe students in one of his lab courses onthe problem of how to reduce drag oncylinders when he realized theapplicability of the work to baseballbats. A conventional bat pushes air outof its way. A dimpled bat allows someair to flow around the contour of the bat.Less air to push means less drag, andthat would mean higher bat speed, moremomentum and better results for a hitter,he reasoned.

The aerodynamic principles thatmake his bat move through the air fasterthan a conventional bat are not new.However, almost all the publishedresearch on this subject involves the useof various types of surface roughness orbumps. Bumps on the surface of abaseball bat would not be acceptable forobvious reasons. So Di Tullio came upwith another solution: dimples similar tothose found on a golf ball.

Di Tullio made a die and presseddimples into some wooden "brands"(bats without markings burned intothem). He pressed rather than drilled thedimples so he wouldn't remove anymaterial that would consequently makethe bat lighter. With prototypes in hand,he moved into a wind tunnel at MIT totest his hypothesis that dimpled bats can

Environment

be swung faster. It worked, and thedevelopment process was underway.

Eco-Friendly Pesticides

Two MIT chemists are developingnon-traditional environmentally benignstrategies for pest control. Most studiesin the area of natural “communication”chemicals produced by plants and theirpests have focused on luring pests intotraps where they can be destroyed. Thenew research by Professor RickDanheiser and Graduate StudentAlexandre Huboux involves naturalchemicals produced by plants startingtheir spring growth processes. In manycases, the same chemicals also “wakeup” insect pests.

Danheiser and Huboux are design-ing a practical synthesis of glycinoclepinA, a natural substance that stimulates thesoybean cyst nematode, a seriousagricultural pest, to hatch. The substancewould then be spread over fields in verysmall amounts during the winter,causing the nematodes to hatch into thecold and die.

The work is one of nine projectsorganized through the MIT Initiative inEnvironmental Leadership and fundedby the V. Kann Rasmussen Foundation.The projects focus on the environmentalimpacts of chlorine. The Danheiser/Huboux work addresses a way to replacesome of the chlorine-dependent pesti-cides currently on the market withnatural alternatives.

Research

High-Latitude IceClouds May Be Visible inUnited States in Future

Noctilucent ice clouds, striking,silvery-blue apparitions that appear inthe far northern latitudes each summer,are projected to creep south into theUnited States during the next century asa result of greenhouse gas increases inEarth’s middle atmosphere.

Professor Gary Thomas of theUniversity of a Colorado at Boulder saidnew calculations indicate the high-altitude clouds will be five to 10 timesbrighter in the 21st century and will bevisible for the first time in the continen-tal U.S. Basking in the sunlight 50 milesabove the Earth’s surface and into themiddle atmosphere, said Thomas, who isaffiliated with CU-Boulder’s Laboratoryfor Atmospheric and Space Physics. Themethane then reacts with sunlight toform large quantities of water vapor thateventually freeze and circulate to the topof the mesosphere, facilitating noctilu-cent cloud formation.

The process is hastened by increas-ing amounts of rising carbon dioxidefrom Earth, he said. While CO2 isthought to contribute to greenhousewarming in the lower atmosphere, thegas ironically cools the middle andupper atmosphere and creates conditionseven more conducive to noctilucentcloud formation.

“In a sense it’s a double whammy,”he said. “This increase in both moistureand colder air is the most favorablecondition for noctilucent cloud forma-tion.”

A paper on the subject was pre-sented by Thomas at the spring meetingof American Geophysical Union inBaltimore May 23 to May 27. Otherauthors of the paper included Eric

Atmospheric Sciences


problem, decided to solve it. In anelectrical engineering course in whichstudents choose their own assignments,he developed a far cheaper color-matchmachine. The machine merely analyzesa point’s red, green and blue componentsinstead of checking every frequency inthe color spectrum, a far more complexprocedure.

“Red, green and blue are all oureyes are equipped to see,” Paschke said.“That’s the basis on which colortelevision works: It mixes the primarycolors to make pictures of every color.”

Paschke’s analyzer, which cost $120in parts, shared a $1000 prize forcreativity given during the U. of I.’sannual Engineering Open House inFebruary.

The sample is placed below a smallturntable that has three diodes attachedto it. The analyzer shines light fromthree flashlight bulbs onto the sample.Each diode, which converts light intoelectricity, is covered by an inexpensivefilter. One permits the passage only ofgreen light, one permits blue to passthrough, and the third permits thepassage of red light. When a motorrotates the turntable, each diode absorbsone of the three colors as reflected fromthe sample. The greater the amount oflight passing through each filter, thestronger the current put out by eachdiode. The current is converted to avoltage, digitized and representedelectronically on a graph on a videomonitor.

The zero point on the graph is setfor black—the absence of color—andwhite—the combination of all color—represents the highest reading. Todetermine intermediate readings, a paintcompany’s carded samples are fed intothe analyzer. Color values are stored inthe memory of a small circuit board,which costs $18.

In Paschke’s latest version of theanalyzer, he incorporated a new diodecapable of transmitting all three primarycolors. Thus, the analyzer doesn’t needfilters, turntable, motor and certain now-redundant circuitry. “The new model ischeaper and simpler,” Paschke said. Andit may be in place this summer in hisparent’s store.

1960s that is not explainable by methaneand carbon dioxide increases, he said.He speculated CFC emissions could playsome part in the increase, since ozonelevels are one of the key components ofthe mesosphere’s atmospheric chemistry.

Thomas and his colleagues also areinvestigating what, if any, effects theclouds may have on Earth’s climate. It ispossible the clouds could reflect sunlightback into space and cool Earth, or theycould heat the planet by trappinggreenhouse gases in the lower andmiddle atmospheres.

Data for the cloud observationswere obtained from CU’s Solar Mesos-phere Explorer satellite in the 1980s andother orbiting spacecraft and rocketexperiments, said Thomas. Two pro-posed unmanned NASA missions toexplore the mesosphere involving CU-Boulder scientists—TIMED andTECHSAT—could provide additionaldata on the spectacular and troublingclouds.

Student Develops Less Expen-sive Way to Analyze and MatchPaints

Most parents look at their child’seducation as a long-term investment. ButFred and Debbie Paschke are getting animmediate payback from their son, John,a senior electrical engineering student atthe University of Illinois.

The Paschke’s, who own a smallhardware and lumber store in Mt.Carroll, Ill., couldn’t justify spendingmany thousands of dollars to purchasethe color-matching machines used bytheir competitors—larger hardwarestores and paint retailers. The machines,which use elaborate programming andtop-of-the-line computer hardware,allow stores to blend paint to match theexact color of paint chips brought in bycustomers.

John Paschke, aware of his parents’

Jensen of NASA’s Ames ResearchCenter in Moffet Field, Calif., CU-Boulder doctoral student RobertPortmann and Rolando Garcia ofBoulder’s National Center for Atmo-spheric Research.

The noctilucent ice clouds, alsoknown as polar mesopheric clouds, arethe highest on Earth and occur in thecoldest part of Earth’s system, Thomassaid. They have been visible for the pastcentrum during June and July fromCanada, Alaska, Scotland, Finland,Sweden, Norway, Greenland andnorthern Russia. They also are visible atcomparable latitudes in the SouthernHemisphere in the correspondingsummer months of November andDecember.

Ice-core records indicate methaneemissions have doubled in the pastcentury and are likely to double againsometime in the 21st century, Thomassaid. The model created by Portmannand his colleagues predicts a roughly 50percent increase in water vapor in themesosphere next century as a result ofthe increases.

Carbon dioxide records fromNOAA’s Mauna Loa Observatory inHawaii since 1958 have triggeredspeculation that carbon dioxide emis-sions also will double sometime in the21st century, said Thomas. The increasewill probably cause the temperature inthe upper mesosphere to decrease byabout 18 degrees Fahrenheit.

“While they are a beautiful phenom-enon, these clouds may be a messagefrom Mother Nature that we are upset-ting the natural equilibrium of theatmosphere,” he said. “And it has takenus 100 years to decipher this message.”

The clouds were first observed in1885 after the eruption of Krakatoa nearJava injected an estimated 100 milliontons of water vapor into the normally dryupper atmosphere, he said. But theypersisted each summer long after theeffects of Krakatoa should have dissi-pated, causing Thomas and his col-leagues to suggest in the late 1980s thatthe clouds may be a byproduct of theIndustrial Revolution.

A puzzling increase has occurred innoctilucent cloud activity since the mid- &

Technology


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Evolution: A Theory in CrisisMichael Denton1985,368 pages, hardback Item# B003 19.95

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changes in ozone distributions; factors thataffect ozone distributions; absorption spec-tra of atmospheric molecular species; pho-tolysis rates of atmospheric species ab-sorbing in the UV-B region; aerosols;clouds; measurements of UV-B radiation;and the effects of UV-B radiation on man,plants, animals, and materials.

Grant has drawn on several sourcesfor his bibliographic material: atmosphericscience and general science journals; Cur-rent Contents and the Science CitationIndex, published by the Institute for Scien-tific Information of Chicago, Illinois; en-vironmental journals; The Global ClimateChange Digest, Center for EnvironmentalInformation, 46 Prince Street, Rochester,NY 14607-1016, 716-271-0606; informa-tion supplied by researchers on UV-B; andgeneral news sources. He also used bibli-ographies in published and unpublishedcompilations, and thanks Sasha Madronich,of the National Center for AtmosphericResearch (NCAR) for the use of two of hisbibliographies on UV-B radiation.

A copy of the bibliography (on a 3.5inch disk) is available for the asking bywriting to William B. Grant, 803 MarlbankDrive, Yorktown, VA, 23692-4353.

It’s important to understand that somebooks and articles about ozone are not asobjective as most scientific papers on thesubject. The clash over ozone is especiallywell demonstrated by the cover story in theFebruary 17, 1992, issue of Time and aresponse in the form of a cover story in theApril 6, 1992, issue of Insight. If you readthe first (“Vanishing Ozone”) be sureto read the second (“Vanishing Facts”).

greatly in recent years.

Excessive exposure to UVR canhave many undesirable consequences.But by taking a few sensible precautions,you can protect yourself and still haveplenty of fun in the sun.

Notes

1Actually the UV-B spectral region

includes wavelengths from 280 to 320nm. But very little UVR reaches theground at wavelengths shorter than 295

n m .2SPF is a ratio that tells you how much

energy it takes to produce a minimalsunburn through a sunscreen productcompared to how much energy it takes

to produce the same sunburn withoutthe sunscreen. Thus, if you normally

get a minimal burn in 20 minutes, itwould take you 15 times as long, orfive hours, to get one using a sunscreen

with an SPF of 15. Of course, thesenumbers are generalizations, and will

vary with several conditions, includingskin type. Also, SPF refers only to UV-B. UV-A protection is rated by the

percentage of UV-A that thesunscreen blocks.

So, what can you actually do toprotect yourself from excessive UVRexposure?

1. If possible, avoid going outsideduring the midday hour, when solarUVR reaches its peak intensity.(Remember, midday is not necessarily at12 p.m. It’s when the sun is highest inthe sky.)

2. If you do go outside, protect youreyes by wearing a hat and sunglasses.Studies have shown that using both ofthese is very effective in reducing UVRreaching the eyes–and in reducing riskof cataracts.

3. Protect your skin by wearing asunscreen that provides both UV-A andUV-B protection, and has a sunprotective factor (SPF) of 15 or more.2

Sunscreen products are available withvery high SPF ratings–45 and up. Butthe chemicals used in these products aremore concentrated than they are at thelower ratings, and may cause skinirritation for some people.

You should also know that somechemicals can cause an allergicreaction–either by themselves or incombination with UVR. One suchchemical is para-aminobenzoic acid(PABA), which was used in some of theearliest sunscreens developed. Becauseso many people are sensitive to PABA,its popularity and use has declined

Ultraviolet andYour Health


Readings AboutReadings AboutOzone andOzone andUltravioletUltraviolet


ozone, William B. Grant of NASA hascompiled a comprehensive bibliography.Specific topics include: solar radiation;

what is the ozone layer and how does it affect you?arn.org/currpage/curr31.pdfvery dry air around 15...

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