RF and Microwave Safety

A Self-Paced Safety ClassPresented ByEnvironmental Health and SafetyCornell University

EH&S Rev 1 12/2010

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magnet safety

EHS-RF Safety

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This Program is Presented By

•Department of Environmental Health and Safety, Research and Radiation Safety Section

•Contact person:▫Jeff Leavey, Radiation Safety Officer▫Phone: 255-8200 or 255-7397▫Email: jal247@cornell.edu

Purpose of this Program

•To increase awareness in RF / microwave safety▫Applies to RF and microwave transmitters

such as radio, cell phone, data communications, paging radios, etc.

▫Review the hazards, exposure limits, and safety requirements.

•Download the Cornell RF & Microwave Safety Guide▫RF & Microwave Safety Guide

Program Outline

•Some definitions•RF fundamentals•RF & microwave hazards•Exposure limits•Safety guidelines and requirements•RF measurements•Final comments

Definitions•Specific absorption rate (SAR)

▫ Power absorbed by the body when exposed to electromagnetic radiation. The current SAR is 4 W/kg. To be conservative, standards are usually set to 1/10 of this value.

•Power density▫ Power per unit area, typically milliwatts/cm2 or W/m2 .

•Near field▫ The region close to an RF emitter (i.e. antenna) where the

electric and magnetic fields are not perpendicular to each other and their orientation to each other varies from point to point. Extends a distance about 2 to 5 wavelengths from the emitter. Power density measurements are not useful in the near field.

Definitions

•Far field▫ The region where the electric and magnetic fields are

perpendicular to each other and do not vary from point to point. Power density can be measured in the far field. Generally beyond about 5 wavelengths.

•ISM bands▫ Frequencies designated for industrial, scientific, and

medical (ISM) use where radiated power is not regulated. Some typical ISM frequencies are 3.56 MHz, 27.12 MHz, 9.5 MHz, 2450 MHz, and 5800 MHz. Because emitted power is not regulated, safety hazards may be created due to high radiated power.

Definitions

•E field▫Electric field strength component of RF energy

measured in volt/meter (V/m).•H field

▫Magnetic field strength component of RF energy measured in amps/meter (A/m).

RF & Microwave Fundamentals

•Some useful conversions▫1 W/m2 = 0.1 mW/cm2

▫E = electric field strength in V/m▫H = magnetic field strength in A/m▫Power density (PD) = E x H V-A/m2 =

W/m2

▫Approximate PD can be found from PD = E2 / 3770 mW/cm2

PD = 37.7 x H2

Impedance of free space = 377 ohms (Ω)

RF & Microwave Fundamentals

•Electromagnetic radiation▫Energy waves propagated through space.▫Characterized by wavelength or frequency.

RF & MicrowaveFundamentals• RF and

microwave have long wavelengths.

f = c / l E = h * f

f = freq sec-1 (Hz)l= wavelength cmc = speed light 3x1010 cm/sE = wave energy eVh = Planck’s const 4.13x10-15 eV-s

E = 1240 / l nm

RF & Microwave Fundamentals

•Extremely low freq (ELF) ~30 Hz to ~300 Hz.

•Radiofrequency ~300 Hz to ~300 MHz.•Microwave ~300 MHz to ~300 GHZ.

•3 regions▫Reactive field – very close, 1 to 2 λ.▫Near field – 2 to ~5 λ.▫Far field – > ~5 λ.▫Highly dependent on antenna design.

RF & Microwave Biological Hazards•Energy absorption

▫Atoms and molecules have “favorite” or resonate frequencies.

▫Macro bodies more strongly absorb when wavelength matches size of body.

▫Whole body resonate freq 40 to 90 MHz.▫Arm resonate freq 175 MHz, head 375 MHz.

•Energy absorption causes atoms/molecules to vibrate creating heat.

•Can affect internal organs, lens, whole body.

RF & Microwave Biological Hazards• Whole body

▫Greater than 10 GHz mostly outer skin absorption.

▫10 GHz to 2.5 GHz penetrate 3 mm to 2 cm.▫2.5 GHz to 1 GHz penetrate even deeper.▫2.45 GHz is water resonance frequency (ovens).

• Wireless computer equipment operates at 2.45 GHz in ISM band (Industrial, Scientific, Medical band at low power unregulated by FCC), cordless phones at 915 MHz, 2450 MHz, and 5800 MHz.

RF & Microwave Biological Hazards

RF & Microwave Biological Hazards• Body must deal with extra heat load.• Organs with poor blood flow at risk e.g. eye

lens.• Specific Absorption Rate (SAR)

▫4 W/kg hazardous▫Limit 0.4 W/kg▫Body resting is ~1 W/kg, activity is ~5 W/kg,

heart’s metabolic rate is ~33 W/kg▫Temp ~102oF can impair body function, fatal

~107 to 108oF• Non-uniform exposure can cause hot spots.

RF & Microwave Biological Hazards•Eye lens

damage causes cataracts.

•Animal studies show critical freq is 800 to 10,000 MHz.

RF & Microwave Biological Hazards• Electric shock and burns a larger issue at lower

frequencies.▫ 3 kHz to 100 MHz range.▫ Induced currents in body causes shock when contacting or

near a conductor.▫ AM radio tower workers – shielded mesh suits.

• Non-thermal effects (animal studies).▫ Behavioral changes.▫ Immunological changes.▫ Possible synergistic cancer effects (cell phones) being

studied. No conclusive results to date.▫ Power freq (50/60 Hz) HV line childhood leukemia being

studied.

RF & Microwave Exposure Limits•Lots of different but similar limits

▫OSHA, FCC, IEEE/ANSI, ICNIRP, countries, etc.

•OSHA▫10 mW/cm2 to protect against thermal, 6

min avg▫Freq independent

•FCC, IEEE/ANSI▫Cornell follows FCC, see next slide

•Limits set to meet 0.4 W/kg SAR

RF & Microwave Exposure Limits

FCC MPEMaximumPermissibleExposure

Note: for 100 GHz to 300 GHZthe 100 GHZ limit applies

Safety Guidelines and Requirements

•Perform regular visual inspections▫General safety / housekeeping conditions in the

surrounding area.▫Presence and functionality of safety interlocks.▫High voltage safety features (e.g. grounding

rods, etc.)▫Condition of cables and coax lines that

transmit RF.▫Presence and condition of RF shielding.

Safety Guidelines and Requirements

•Perform regular operational inspections▫Monitoring or survey of RF levels, especially in

occupied spaces.▫Test of interlocks to inhibit RF generation

when tripped.▫Verify proper use of “lockout / tagout” for RF

and/or high voltage equipment.

Safety Guidelines and Requirements

•RF shielding▫Shields can be solid panels or metallic mesh

screening. Materials for shielding E fields include tin, copper, aluminum, silver, and gold. These have high reflective losses for E fields. H field shielding materials include ferrous materials like iron, steel and special alloys (e.g. Hypernom and Permalloy).

▫See the RF & Microwave Safety Guide for assistance in designing shielding mesh dimensions.

▫Equipment enclosures, access panels, doors, etc. should be sealed with metal braid or other conductive gaskets.

Safety Guidelines and Requirements

•RF shielding▫Grounding straps and wires should not be bent

at sharp angles - sharp angles act as antenna to radiate RF.

▫Conductors and straps carrying RF should bend gently and blend smoothly at connection points. Screws used to connect conductors should have smooth, rounded heads, not sharp edged angular heads.

▫Solder connections should be as large in area as possible.

Safety Guidelines and Requirements

•Signs and posting requirements▫A warning sign is required

at the entrance to areas where RF fields can exceed limits (e.g. usually at the entrance to the lab).

▫Signs are availablefrom EH&S.

Use This SignAt the entrance to an area where RF may exceed the PUBLIC limits.

Use This SignAt the entrance to an area where RF may exceed the OCCUPATIONAL limits.

Use This SignAt the entrance to an area where RF exceeds the OCCUPATIONAL limits.

RF Measurements• Reactive field

▫Avoid measurements in this region, highly variable.

• Near field▫Measure E and H separately, use formula

to calc equivalent plane PD, use highest value.

• Far field▫Measure PD directly, or can measure E

and H and calc PD, should be similar.• Measure where person exposed

▫Head to toe, determine 6 min Time Weighted Average (TWA).

RF Measurements

•Estimate PD from source parameters▫Distance to far field = A /(2 * l) or

(( * p r2) / (2 * )).l▫Max PD in near field = (4 * P) / A.▫Max PD in far field = (P * A) / (l * r)2.

▫Where A = antenna area l = wavelength P = output power r = distance from antenna.

RF Measurements

•For pulsed sources e.g. radar.▫Duty cycle = pulse width (sec) * PRF

(pulses/sec).▫Avg power P = peak power * duty cycle.

•EH&S can assist with measurements, contact 255-8200.

Final Comments

•Please contact EH&S if you have any questions or wish assistance with RF/microwave safety.

Thank you for taking this EH&S safety class!!!