A Perspective on the PotentialDevelopment of EnvironmentallyAcceptable Light-duty Diesel VehiclesRobert Hammerle, Dennis Schuetzle, and Willi AdamsFord Motor Company, Research Laboratory, Dearborn, Michigan

Between 1979 and 1985, an international technical focus was placed upon potential human health effects associated with exposure to diesel emis-sions. A substantial data base was developed on the composition of diesel emissions; the fate of these emissions in the atmosphere; and theeffects of whole particles and their chemical constituents on microorganisms, cells, and animals. Since that time, a number of significant develop-ments have been made in diesel engine technology that require a new look at the future acceptability of introducing significant numbers of light-dutydiesel automobiles into the European and American markets. Significant engineering improvements have been made in engine design, catalysts,and traps. As a result, particle emissions and particle associated organic emissions have been reduced by about 10 and 30 times, respectively, dur-ing the past 10 years. Research studies to help assess the environmental acceptability of these fuel-efficient engines include the development of anemissions data base for current and advanced diesel engines, the effect of diesel emissions on urban ozone formation and atmospheric particle con-centrations, the effect of fuel composition, e.g., lower sulfur and additives on emissions, animal inhalation toxicology studies, and fundamental mole-cular biology studies. - Environ Health Perspect 1 02(Suppl 4):25-30 (1994).

Key words: catalysts, diesel, environment, health effects, PAH, particulate traps

IntroductionThe presence of polycyclic aromatic hydro-carbons (PAH), compounds that are sus-

pected to cause cancer, was reported first invehicle exhaust in the 1950s and early1960s (1-4). In the late 1970s and 1980s,there was a substantial international effortto chemically characterize the PAH in gaso-

line and diesel vehicle exhaust and to test

the effect of these emissions on microorgan-isms, cells, and animals. This effort becameconcentrated on diesel vehicles because ofmuch higher PAH and particulate emis-sions (4), which were found to cause cancer

in some laboratory animals (5). Thisprocess culminated in the classification ofdiesel particulate as a probable carcinogen(4) and in regulations to reduce diesel par-ticulate emissions in the United States (6).Over the last 10 to 15 years, there has

been an equally substantial effort to lowerdiesel emissions. It began with detailedmapping of diesel engine operation to defineoperating modes with low emissions andprogressed to improved engine control to

maintain the engine in those low emissionmodes (7). This trend continues today(8,9). More recently, the effort has resulted

This paper was presented at the Symposium on RiskAssessment of Urban Air: Emissions, Exposure, RiskIdentification and Risk Quantitation held 31 May-3June 1992 in Stockholm, Sweden.

Address correspondence to R. Hammerle, FordMotor Company, P. 0. Box 2053, Research Laboratory,Mail Drop 3061, Dearborn, MI 48121.

in modification of the engines, which hasled to still lower emissions. The fuel injec-tion process, for example, was modified fordirect injection engines to distribute thefuel better so that it could be burned morecompletely. Together, such engine andengine control modifications have alreadylowered particulate emissions by about afactor of 3 (10). Additional modifications,which are planned for the near future, suchas higher pressure fuel injection and indi-vidual control of each injector are expectedto lower emissions further.The effort to lower diesel emissions has

also led to the introduction of exhausttreatment devices, namely catalysts andparticle traps. Today, the catalysts that areused on most new gasoline vehicles arecapable of removing roughly 95% of thehydrocarbon (HC) and nitrogen oxide(NOX) emissions and about 90% of thecarbon monoxide (CO) emissions (11).Catalysts are capable of removing about90% of the organic and CO emissions fordiesel vehicles, but they cannot lower theNO emissions because diesel exhaust has

x

excess oxygen. However, the catalysts thathave been used for the last few years ondiesel vehicles are less active than thoseused on gasoline vehicles; they typicallyremove less than 50% of the organic emis-sions (12). More active catalysts cannot beused because the sulfur present at high lev-els in commercial diesel fuel would be con-verted to particulate sulfate. Sulfur levels

in diesel fuel are being reduced, both in theUnited States and Europe.While particle traps are effective and have

been tested on diesel engines extensively formore than 10 years, their commercial intro-duction has been very slow because they aredifficult to control. The problem is regular-ly burning the collected soot withoutrestricting the engine exhaust flow or melt-ing the trap. Solutions based on using cat-alytic metallic compounds in the fuel havebeen tested successfully (13,14), but notimplemented fully, apparently because ofcontrol difficulties and concern over theemissions of metallic aerosol.

Using a full range of the emission con-trol technology developed for diesel vehi-cles over the past 10 years, the particleemissions and the particle-associated organ-ic emissions can be reduced by about 10and 30 times, respectively. The success ofparticulate emission control efforts raisesthe issue of whether the environmentalacceptability of diesel vehicles should bejudged only on their potentially carcino-genic particulate emissions or whether itshould be judged by its overall affect,including other problems such as urbanozone and global warming.

Public Health Risk fromDiesel Particulate MatterDiesel particulate matter (PM) containsmany PAH and their chemical derivativeswhich are suspected carcinogens (4). Most

Environmental Health Perspectives 25

HAMMERLE ETAL.

of these compounds have relatively lowvapor pressure and condense along withother organic material on carbonaceous coreparticles before they leave the exhaust pipe.Together, the condensed organic materialand the dry soot make up the PM at ambi-ent temperatures. After collecting the PM,the PAH and derivatives can be extractedwith solvents, so they are considered part ofthe soluble organic fraction (SOF).

In 1989, inhaled diesel exhaust was clas-sified as probably carcinogenic (4). Thisclassification means that, while there is apositive association between diesel exhaustand cancer in humans, chance or bias couldbe responsible. At the same time, itrequires a positive association between can-cer in animals and diesel exhaust thatchance or bias cannot negate. A very briefsummary of the health risk of inhalingdiesel PM is included here for complete-ness. The interested reader is referred to anexcellent review elsewhere (4).

"Definitive evidence of carcinogenicityin humans can only be provided by epi-demiological studies" (4). There havebeen several epidemiological studies ofworkers exposed to high levels of dieselexhaust; they show associations betweenexposure and lung cancer, but the associa-tions are too weak to be free of possiblebias (4). One major problem stems fromcigarette smoking, which causes the over-whelming majority of the lung cancersobserved in such studies (4).The largest of these studies was done for

U.S. railroad workers who started railroadwork between 1939 and 1949 (4,15-17).Using only exposure data from the 1980s,this study reconstructed the workers' dieselexhaust exposure from their careers and clas-sified them according to that exposure.Classes with the greatest exposure hadroughly five times greater exposure thanthose with the lowest exposure, but they had1.45 times higher risk of developing lungcancer. This higher risk was barely signifi-cant with the 95% confidence interval,which ranged from a relative risk of 1.11 to1.89. Classes with intermediate exposureshad 1.2 times higher risk with a 95% confi-dence interval of 1.1 to 1.3. The limitedsignificance of this result may be confound-ed by different smoking habits of the variousclasses, which was not considered (4).The risks for the railroad workers devel-

oping other cancers were also studied, butthey were nearly independent of the dieselexposure classes. For example, classes withthe largest exposure had 1.03 times greaterrisk of developing bladder cancer thanthose with the lowest exposure. Here the

95% confidence interval ranged from a rel-ative risk of 0.88 to 1.19. Bladder prob-lems could result from inhalation of dieselexhaust because particulate matter deposit-ed in the lung is normally cleared from thelung and swallowed. In other studies, peo-ple with bladder cancer appeared to havebeen associated with diesel exhaust moreregularly than those without (4).

After these human population studies,the case for a health risk from dieselexhaust inhalation rests on animal studies(4). While several animal species havebeen exposed to diesel exhaust, only the ratdevelops lung cancer. There is no evidencethat the exposed animals develop bladdercancer (4). Many studies have shownunequivocally that the rat develops lungtumors after many hours of daily exposurefor at least 24 months to very high levels ofdiesel PM, above about 4 mg/im3. Thishigh level of PM overloads the naturalself-clearing mechanism so that diesel parti-cles build up in the lung. No tumors werefound in animals exposed to filtered dieselexhaust from which the PM was removed.

Similar studies show that rats developlung cancer after exposure to inert PM,which consists of insoluble particles such astitanium oxide or carbon black that is freeof organic material. In these tests, the inci-dence of lung tumors in rats exposed toinert PM was equivalent to that in ratsexposed to unfiltered diesel exhaust(18-20). Apparently, inhalation of thesuspected carcinogenic PAH on the dieselPM has no additional effect. In otherwords, the risk of inhaling diesel exhaustwas the same as inhaling similar levels ofother particulate matter. These results sug-gest that overloading the lung clearancemechanism and not the PAH may beresponsible for the lung tumors in rats(21). Atmospheric particulate levels aretypically below 100 pg/m3 in the UnitedStates, or more than fourty times lower the4 mg/ m3 used in the above animal tests.

Recent Developments inDiesel EnginesOver the last 15 years, there has been a sub-stantial effort to lower diesel emissions. Earlydiesel vehicles were designed for optimumfuel economy and performance. They pro-vided approximately 15 to 30% better volu-metric fuel efficiency, miles per gallon of fuel,than similar gasoline engines of comparablepower level. No one could be certain thatthey could maintain this advantage with adiesel designed for low emissions. The effortsto lower emissions began with detailed map-ping of the diesel engine operating modes

that provide lower emissions (6). Carefulcontrol of the fuel injection and the additionof exhaust gas recirculation (EGR) were nec-essary to meet the emission standards. Forthe lowest emissions, injection parametersand EGR rate must change as precise func-tions ofvehide and engine speeds, acceleratorpedal position, inlet air temperature and pres-sure, and engine temperature (22).

In the early 1980s, the fuel injection andEGR on most diesel passenger vehicleswere controlled by mechanical systems thatcould drift out of tune. More precise con-trol of fuel injection and EGR wasachieved with the introduction of electron-ic control units in the late 1980s (23,24).These electronic control units wereequipped with sensors that monitored theengine operation and helped maintain lowemissions throughout the vehicle's life.The trend toward greater electronic controlof diesel engines will probably continueover the next 5 to 10 years (8).The precision of fuel injection has been

improved substantially by the developmentof new fuel detergents and new fuel injec-tors that reduce injector fouling (25,26).In the early 1980s, the flow area of theinjectors typically was oversized by nearly40% to allow better engine operation overthe lifetime of the vehicle as the inevitablebuildup of the carbonaceous deposits grad-ually reduced the injection rate (25). Newashless detergents (25,26) and new designsfor injectors are available to reduce thefouling and lower emissions greatly. Thedetergent composition is often proprietary;however, one detergent is a high molecularweight polyisobutylene with amine func-tionality (27). Using properly sized injec-tors that remain clean, the HC and partic-ulate emissions can be reduced by roughly25% compared to fouled injectors.

In addition, modifications to the sealbetween piston and cylinder wall havereduced the emission of engine lubricatingoil substantially (28). In the 1980s, lubri-cation oil usually contributed to the domi-nant portion of the SOF of the PM (29).Sufficient oil must flow between the pistonand cylinder wall to prevent wear, but littleoil should be drawn into the combustionprocess. A number of improvements suchas chromium faced piston rings (28), opti-mized ring design (30), control of ring andcylinder bore corrosion and deposits, lowdistortion cylinder bores with controlledsurface finishes (24), and optimized honingpatterns, have reduced the oil contributionto the PM greatly.

Together these engine and engine-controlmodifications have lowered PM emissions

Environmental Health Perspectives26

PERSPECTIVE ON ENVIRONMENTALLYACCEPTABLE LIGHT-DUTY VEHICLES

Table I. Particulate matter and soluble organic fraction for diesel engines without traps and catalysts.

Vehicle PM, mg/km SOF, mg/km SOF/PM, % References

Circa 1980, IDI 246 124 50 (4)1983 1.6-LalDI 200-250 NA NA (41)1983 1.6-L a IDI turbocharged 150-200 NA NA (41)1986 3.0-LalDl turbocharged, 1900 kg b 231 18.5 8 (34)1987 1.6-LalDI 1200kgb 122 27.5 22.5 (34)1989 1.8-L a IDI turbocharged, 1360 kg b 82 16 20 (24)

Abbreviations: PM, particulate matter; SOF, soluble organic fraction; IDI, indirect injection; NA, not analyzed.a Engine displacement. bInertia weight used in the emission test.

by a factor of 3, from 246 to 82 mg/ km,and the SOF by a factor of 7, from 123 to16 mg/km (Table 1). The base emissionlevels, 246 mg/km with 50% SOF, appearto be representative of diesel engines in thelate 1970s and early 1980s and were citedby International Agency for Research onCancer in 1989 when it classified diesel PMas a probable carcinogen. Table 1 shows thePM and SOF emissions for several dieselvehicles including many, but not all, of theimprovements indicated above. Thus, thisdownward trend is expected to continue.

Exhaust Treatment SystemsIn addition to modifying the engine, theeffort to lower emissions has led to the devel-opment of catalysts and particle traps. Earlyattempts to use catalysts on diesel engineswere limited by the buildup ofPM in the cat-alyst (31). As the soot built up, it blockedthe diffusion of the organic gases and oxygento the catalytic surfaces, greatly reducing therate of oxidation. In extreme cases, the sootcould restrict the flow of exhaust gases fromthe engine. Engine modifications that low-ered the particulate emissions can eliminatethis catalyst plugging problem.

Catalysts are capable of removing about90% of the organic emissions of a diesel vehi-cle (12). They can oxidize gaseous organiccompounds, both the HC and the SOF thatis gaseous in the catalyst, but not the dry, car-bonaceous soot. Catalysts must be above 250to 300°C to oxidize organics effectively.Typically, cold engines must operate for a fewminutes for the catalysts to reach this temper-ature. During this cold start period, the lightmolecular weight HC with high vapor pres-sures are emitted from the vehicle uncat-alyzed. However, the cold catalyst cools thegases emitted from the engine rapidly, caus-ing the heavier HC present in diesel exhaustto condense on its walls. As the catalystwarms up and becomes active, the condensedorganic material is oxidized. The result isthat very little of the gaseous organic materialescapes unoxidized (12).To effectively remove the organic com-

pounds, the catalyst must become very

active at the lowest possible temperature.Unfortunately, the sulfur in the diesel fuelcauses two problems. The sulfur emittedfrom the engine as sulfur dioxide is con-verted to sulfur trioxide or, at lower tem-peratures and in the presence of water, sul-furic acid, or sulfate. The acid emissionscan cause environmental problems and addto the particulate mass emitted. Some cat-alyst formulations can oxidize the organicsselectively while hardly oxidizing the sulfurdioxide. These catalysts are used today.They typically can remove about 50% ofthe SOF (12). However, if sulfur were notpresent, substantially more active catalystscould be used. They can remove about90% of the SOF (12). This has been rec-ognized, and diesel fuel sulfur in the UnitedStates will be reduced from about 2800ppm in 1991 to less than 500 ppm startingin October 1993. Even at this level, thesulfate can contribute about 25 mg/km (40mg/mi), assuming 25% conversion of thefuel sulfur to sulfate by the catalyst and12.7 km/1 (30 mi/gal) fuel economy(Figure 1). This is about 50% of the U.S.50 mg/km (80 mg/mi) PM standard.The second, more fundamental problem

is that sulfur raises the temperature at whicha catalyst becomes active. It does this

because sulfur dioxide is absorbed stronglyby the catalyst at low temperatures, prevent-ing the absorption of oxygen, which is need-ed to catalyze the organics. The sulfur diox-ide begins to desorb at about 2300C,depending on the catalytic material. TheU.S. Auto/Oil program found that loweringthe gasoline sulfur level from 466 to 49 ppmlowered the HC emissions of 1989 modelcars by more than 15% (32). Because theexhaust temperatures of diesel vehicles arelower than that of gasoline vehicles, oneexpects a greater reduction in HC emissionsif the sulfur level in diesel fuel were reducedfrom 500 to 50 ppm. Catalysts couldreduce the SOF by as much as 95%. Inaddition, the sulfate emissions would bereduced to a few mg/km. Today, fuel sulfurlimits the application of catalysts to dieselvehicles like lead limited their application togasoline vehides in the 1970s.

Diesel particulate traps are capable ofremoving more than 80% of the particulatemass emissions (14). Traps remove theSOF and the dry, carbonaceous soot, whilecatalysts remove only the SOF. However,traps are substantially more difficult tooperate on the vehicle than catalysts, andthis has slowed their introduction onlight-duty vehicles greatly. The problemoccurs as the particulate material builds upin the trap; this causes the exhaust backpressure to increase, and higher back pres-sure reduces fuel economy and perfor-mance (13). Therefore, the collected par-ticulate must be burned periodically toregenerate the vehicle's performance. Theregeneration can occur naturally when theengine is used at high speed and loads.However, for many applications, naturalregeneration is too infrequent to maintaingood vehicle performance.

Sulfate Emissions (mg/ km)I50

100

50

00 0.05 0.1 0.15 0.2

Fuel Sulfur Level (wt"/6)Figure 1. Sulfate particulate emissions.

Volume 102, Supplement 4, October 1994 27

HAMMERLE ETAL.

Table 2. Particulate matter and soluble organic fraction emissions for diesel engines with traps and catalysts.

Vehicle PM, mg/km SOF, mg/km SOF/ PM, % References

1986 3.0-L a IDI turbocharged 1900 kg b trap and fuel cat 31 3.4 12.5 (34)1987 1.6-La IDI 1200 kg b trap and fuel cat 26 12 46.5 (34)1987 3.0-L a IDI turbocharged trap and fuel cat 8.75 NA NA (42)1992 2.5-L a IDI catalyst 120 12 10 (35)

Abbreviations: PM, particulate matter; SOF, soluble organic fraction; IDI, indirect injection; NA, not analyzed.a Engine displacement. bInertia weight in the emission test.

If regeneration is delayed, some of theSOF collected by a trap at low temperaturecan be volatilized later when it becomeshotter. The diesel PM in a trap will notignite until its temperature is about 500 to550°C (13), thus all of the trapped SOFthat becomes gaseous below 500°C can beemitted. In addition, if regeneration isdelayed, the direct-acting TA98 muta-genicity of the trapped PM can be greaterthan that of the directly emitted PM (33).This appears to be due to the reaction ofthe PM with the exhaust gases flowingthrough the trap to form nitrated PAH.Together these problems could eliminateany benefit of reduced biological activityprovided by a trap.A promising regeneration method is the

use of fuel catalysts such as organic com-pounds containing iron or copper (14).These compounds are burned in the engineand then are emitted as finely dispersedmetal particles that are collected in the trapand intermixed with the diesel particulate.They act as catalysts by reducing the tem-perature at which the PM will burn so thatregeneration occurs more frequently.Their presence reduces the PM ignitiontemperature to below 300 C (14), greatlyreducing problems with vaporization ornitration of the trapped SOF (34). Verylow concentrations of fuel catalyst, about0.05 gm/ L of fuel, are sufficient for regen-eration. Over 95% of the metal used as

fuel catalyst is collected in the trap. Testsshow that some metals, such as calciumand copper can be purged from the trapsduring the high temperatures of regenera-tion (34). Great care must be taken toavoid release of the metal particles becausethey could cause environmental problems.

Together, exhaust treatment and enginemodifications have lowered PM emissionsby a factor of 10, from 246 to 26 mg/km,and the SOF by a factor of 40, from 123 to3 mg/ km (Table 2). Again, the base emis-sion levels for this comparison is 246mg/km with 50% SOF. Table 2 showsthe PM and SOF emissions for severaldiesel vehicles, including some, but not all,of the improvements indicated above. Thisdownward trend is expected to continue.

Biological Activity ofEmissions from CurrentDiesel Vehicles

To our knowledge, the biological activity ofthe PM emitted from current light-dutydiesel vehicles, especially that from catalystsand traps, has not been determined com-pletely. The Ames assays that have beenconducted show that the TA98 mutagenicactivity is decreased by between 50 and 90%(34,35). The PM from current diesel vehi-cles equipped with traps has fewerrevertants/ km than that from gasoline vehi-cles made in the early 1980s (Table 3).

Limited supporting data from heavy-dutydiesel vehicles shows that traps decreased theTA98-S9 activity of the SOF by 54 to 94%(36) and that oxidation catalysts decreasedthe TA98-S9 activity by about 65% (37).

Future WorkSince the late 1970s, the primary healthconcern for diesel vehicles has been theemissions of PM and the PAH containedon the PM. This concern led to particulateemissions standards and efforts to reducethese emissions. As shown above, the PMand SOF emissions have been reduced byfactors of 10 and 30, respectively. There isthe potential for additional reductions,especially for vehicles with catalysts usingvery low sulfur fuel or with traps using fuelcatalysts or other regeneration aids. Theemissions from such systems have not beencharacterized fully to help establish thepotential health effects, so more work mustbe done in this area.At the same time, the health effects of

inhaling diesel PM emissions remain uncer-tain, especially for low-emission vehicles.In the mid 1980s, chronic inhalation expo-sure of rats to their maximum tolerateddose of diesel PM was shown to cause lungtumors (4). However, by the early 1990s,it became clear that similar chronic expo-sure to inert PM such as titanium oxide alsocaused tumors in rats (18-20). In general,it has been found that chronic, maximumtolerated exposure of "more than half of allchemicals tested (both natural and synthet-ic) are carcinogens in rodents, and a highpercentage of these carcinogens are notmutagens" (38). Maximum tolerated dosesappear to simulate cell division (38), andthere is growing evidence that induced celldivision has a dominant role in carcinogen-esis (39). This suggests that tests at maxi-mum tolerated exposure provide little

Table 3. Mutagenicity of the soluble organic fraction from various diesel vehicles.

Mutagenicity TA98 Mutagenicity TA98Vehicle PM, mg/km SOF, mg/km -S9 rev/im +S9 rev/im References

Circa 1980 IDI diesel 246 123 595 240 (4)Circa 1980 gasoline with catalyst 62 10 61 180 (4)1986 3.0-LaIDI diesel 232 b 019 b 49 b 486b (34)1986 3.0-L a IDI diesel with trap and fuel cat 31 c 3 c 67 c 39 c

% Reduction 87% 84% 86% 92%

1987 1.6-L IDI diesel 123 b 28 b 521 b 422 b (34)1987 1 .6-L IDI diesel with trap and fuel cat 26 c 12 c 108 c 71 c

% Reduction 79% 57% 79% 83%

1992 IDI diesel 81 b NA 14 b NA (35)1992 IDI diesel with catalyst 75 c 6 c

% Reduction 7% 60%

Abbreviations: PM, particulate matter; SOF, soluble organic fraction; IDI, indirect injection; NA, not analyzed. a Engine displacement. Engine out. c Tail-pipe.

Environmental Health Perspectives28

PERSPECTIVE ONENVIRONMENTALLYACCEPTABLE LIGHT-DUTY VEHICLES

information about low dose risk. More needsto be done to define the health risk of inhal-ing atmospheric levels of diesel exhaust.

If, as suspected, the health risk of inhalingdiesel exhaust can be reduced substantiallyby lowering their PM and SOF emissions,then a wider view of the environmentalacceptability of light-duty diesel vehiclesbecomes appropriate. Such a view should

also consider local and regional levels ofozone, carbon monoxide, toxic compoundsand PM, and global climate change.The impact of diesel and reformulated

gasoline vehicles appear similar, and changesto current uncertainties could easily affecttheir relative environmental standing (40).Currently available data shows that dieselvehicles emit less nonmethane organic com-

pounds, carbon monoxide, and carbondioxide than gasoline vehicles but morenitrogen oxides and PM than gasoline vehi-cles with state-of-the-art control systems(40). More data on current and futurediesel vehicles is needed to assess the effectof diesel emissions on urban ozone forma-tion, atmospheric particle concentrations,and attendant health risk.

REFERENCES

1. Kotin P, Falk HL, Thomas M. Aromatic hydrocarbons. II.Presence in the particulate phase of gasoline-engine exhaust and thecarcinogenicity of exhaust extracts. Arch Ind Hyg Occup Med9:164-177 (1954).

2. Kotin P, Falk HL, Thomas M. Aromatic hydrocarbons. III.Presence in the particulate phase of diesel-engine exhaust and thecarcinogenicity of exhaust extracts. Arch Ind Health 11:113-120(1955).

3. Begeman CR, Colucci JM. Apparatus for determining the contri-bution of the automobile to the benzene-soluble organic matter inair. Natl Cancer Inst Monogr 9:17-57 (1962).

4. IARC. Monograph on the Evaluation of Carcinogenic Risks toHumans, Diesel, and Gasoline Engine Exhausts and SomeNitroarenes, Vol 46. Lyon:International Agency for Research onCancer, 1989;13-185.

5. Heinrich U, Muhle H, Takenaka S, Ernst E, Fuhst R, Mohr U,Pott F, St6ber W. Chronic effects on the respiratory tract of ham-sters, mice and rats after long inhalation of high concentrations offiltered and unfiltered diesel engine exhaust. J Appl Toxicol6:383-395 (1986).

6. Clean Air Act Amendments. Public Law 101-549, 101st U.S.Congress 104 Stat. 2399-2712, November 15, 1990.

7. Wade WR. Light-duty diesel NO,-HC-particulate trade-off studies.Society ofAutomotive Engineers Transactions 89:65-86 (1980).

8. Flaig U, Seiber A. Electronic control for diesel engines. AutomotiveEngineering 96:49-53 (1988).

9. Herzog PL, Buergler L, Winklhofer E, Zelenka P, Cartellieri W.NOX reduction strategies for DI diesel engines. Society ofAutomotive Engineers Transactions 101 (3):820-836 (1992).

10. U.S. EPA. Control of air pollution from new motor vehicles andnew motor vehicle engines, federal certification test results for the1990 model year. Ann Arbor, MI:United States EnvironmentalProtection Agency, 1990;141-142.

11. Hurley RG, Hansen LA, Guttridge RL, Hammerle RH, MatzoAD, Gandhi HS. The effect of mileage on emissions and emissioncomponents durability by the fuel additive methylcyclopentadienylmanganese tricarbonyl (MMT). Paper No. 920732. Society ofAutomotive Engineers, 1992.

12. Horiuchi M, Koichi S, Ichihara S. The effects of flow-through typeoxidation catalysts on the particulate reduction of 1990s dieselengines. Society of Automotive Engineers Transactions99(3):1268-1278 (1990).

13. Wade UR, White JE, Florek JJ. Diesel particulate trap regenerationtechniques. Society of Automotive Engineers Transactions90:502-523 (1981).

14. Rao VD, White JE, Wade WR, Aimone MG, Cikanek HA.Advanced techniques for thermal and catalytic diesel particulatetrap regeneration. Society of Automotive Engineers Transactions94:111-127 (1985).

15. Garshick E, Schenker MB, Munoz A, Segal M, Smith TJ, WoskiSR, Hammond SK, Speizer FE. A retrospective cohort study oflung cancer and diesel exhaust exposure in railroad workers. AmRev Respir Dis 137:820-825 (1988).

16. Woski SR, Smith TJ, Hammond SK, Schenker MB, Garshick E,Speizer FE. Estimation of diesel exhaust exposure of railroad work-ers. I. Current exposures. Am J Ind Med 13:381-394 (1988).

17. Woski SR, Smith TJ, Hammond SK, Schenker MB, Garshick E,Speizer FE. Estimation of diesel exhaust exposure of railroad work-ers. I. National and historical exposures. Am J Ind Med13:395-404 (1988).

18. Lee KP, Trochimowicz HJ, Reinhardt CF. Pulmonary response ofrats exposed to titanium dioxide by inhalation for two years.Toxicol Appl Pharmacol 79:179-192 (1985).

19. Lee KP, Henry NW, Trochimowicz HJ, Reinhardt CF. Pulmonaryresponse to impaired lung clearance in rats following excessiveTiO2 dust deposition. Environ Res 41:44-167 (1986).

20. Heinrich U, Dungworth DL, Pott RF, Peters L, Dasenbrook C,Levsen K, Koch U, Creutzenberg 0, Schulte A. The carcinogeniceffects of carbon black particles and tar/pitch condensation aerosolafter inhalation exposure of rats. Ann Occup Hyg (Suppl) (inpress).

21. Vostal JJ. Factors limiting the evidence for chemical carcinogenicityof diesel emissions in long-term inhalation experiments. In:Carcinogenic and Mutagenic Effects of Diesel Engine Exhaust(Ishinishi N, Koizumi A, McClellan R, Stober W, eds). NewYork:Elsevier Science Publishers, 1986;381-396.

22. Stumpp G, Polach W, Muller N, Warga J. Fuel injection equip-ment or heavy duty diesel engines for U.S. 1991/1994 emissionlimits. Society of Automotive Engineers Transactions98(3):1559-1571 (1989).

23. Schwarzbauer G, Weiss H. Electronic diesel control of the BMUturbodiesel model 324td. Motortechnische Zeitschrift 49:37-41(1988).

24. Lawrence PJ, Evans RW. The Ford 1.8-L four cylinder tur-bocharged diesel engine for passenger car applications. Society ofAutomotive Engineers Transactions 99:2032-2044 (1990).

25. Reading K, Roberts DD, Evans TM. The effects of fuel detergentson nozzle fouling and emissions in IDI diesel engines. Paper No.912328. Society ofAutomotive Engineers, 1991.

26. Smith AK, Dowling M, Fowler WJ, Taylor MG. Additiveapproaches to reduce diesel emissions. Paper No. 912327. SocietyofAutomotive Engineers, 1991.

27. Herbstman S, Virk K. Use of dispersants! detergents in diesel injec-tor keep clean and clean up studies. Society of AutomotiveEngineers Transactions 100(4):820-830 (1991).

28. McGeehan JA, Kulkarni AV. Wear control in diesels: influence oflubricants. Auto Engin 96:56-61 (1988).

29. Richards RR, Sibley JE. Diesel emission control for the 1990s.Auto Engin 96:63-69 (1988).

30. Burnet PJ. Relationship between oil consumption, deposit formation,and piston ring motion for single-cylinder diesel engines. Society ofAutomotive Engineers Transactions 101 (3): 149-161 (1992).

31. Widdershoven J, Pischinger F, Lepperhoff G, Schick KP, Strutz J,Stahlhut S. Possibilities of particle reduction for diesel engines.Society ofAutomotive Engineers Transactions 95:89-101 (1986).

32. Benson JD, Burns VR, Gorse RA, Hochhauser AM, Koehl WJ,Painter LJ, Reuter RM. Effects of gasoline sulfur level on massexhaust emissions: Auto/ Oil air quality improvement research pro-gram. Society of Automotive Engineers Transactions100(4):715-747 (1991).

33. Bagley ST, Dorie LD, Leddy DG, Johnson JH. An investigationinto the effect of a ceramic particle trap on the chemical mutagens

Volume 102, Supplement 4, October 1994 29

HAMMERLE ETAL.

in diesel exhaust. Research Report No. 5. Cambridge, MA:HealthEffects Institute, 1987.

34. Smith LR. Characterization of exhaust emissions from trap-equippedlight-duty diesels. Report No. SWRI-1280. San Antonio,TX:Southwest Research Institute, 1989.

35. Berg W, Holmes JG. Oxidation catalysts for automotive diesels-emissions performance and European experience. Book No. VIP-23. Air and Waste Management Association, 1992.

36. Kantola TC, Bagley ST, Gratz LD, Leddy DG, Johnson JH. Theinfluence of a low sulfur fuel and a ceramic particle trap on thephysical, chemical and biological character of heavy-duty emissions.Society of Automotive Engineers Transactions 101(4):675-690(1992).

37. McClure BT, Bagley ST, Gratz LD. The influence of an oxidationcatalytic converter and fuel composition on the chemical and bio-

logical characterizations of diesel exhaust emissions. Paper No.920854. Society ofAutomotive Engineers, 1992.

38. Ames BN, Gold LS. Too many rodent carcinogens: mitogenesisincreases mutagenesis. Science 249:970-971 (1990).

39. Butterworth BE, Slaga T. Chemically Induced Cell Proliferation:Implications for Risk Assessment. New York:Wiley-Liss, 1991.

40. Chang TY, Hammerle RH, Japar SM, Salmeen IT. Alternativetransportation fuels and air quality. Environ Sci Technol25:1190-1193 (1991).

41. Weidmann K, Menrad H, Reders K, Hutcheson RC. Diesel fuelquality effects on exhaust emissions. Society of AutomotiveEngineers Transactions 97:863-873 (1988).

42. Holmes JG, Berg W. Advanced diesel systems as clean fuel vehicles.Book No. 90.149. Air and Waste Management Association, 1990.

30 Environmental Health Perspectives