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Carcinogenic air pollutants in the exhaust from a European car operating on various fuels
Atmospheric
Environment
Vol.
8, pp.
693-705.
Pergamon Press 1974.
Printed in Great Britain
CARCINOGENIC AIR POLLUTANTS FROM A EUROPEAN CAR OPERATING
IN THE EXHAUST ON VARIOUS FUELS
A. CANDELI, V. MASTRANDREA,G. MOROZZI and S. TOCCACELI Istituto di Igiene della Facolta di Scienze MM.FF.NN. della Universita di Perugia. Via Dante Alighieri. 06 100 Perugia, Italy (First received 11 October
1973 and injnal
form
25 October 1973)
A&met-The influence of some fuel variables on polycyclic aromatic hydrocarbons (PAH) emission was studied The fuel variables taken into consideration were: tetra-ethyl-lead (TEL) content (0 and 0.63 g 1-l); fuel aromaticity (0, 6 and 48 per cent) and type of aromatic at constant total level of aromatics (benzene, xylene and hydrocarbons with 9 and 10 carbon atoms). Each test consisted of European test cycles on a dynamometer bench, using a European engine of average displacement for European cars (1608 cm’) and without deposits in the combustion chamber. The results obtained show that it is not possible to generalize about the effect of TEL on PAH emission: in two cases the addition of TEL reduced and in one case it increased the PAH emission. Increasing fuel aromaticity increases PAH emission, but these results apply only to the three leaded tested fuels, and not to the same fuels unleaded In three synthetic unleaded fuels at constant total aromatics the type of aromatic present strongly affected the PAH emissions: it seems that simple aromatic hydrocarbons, such as benzene oy xylene, produce less PAH than C, and C,, hydrocarbons INTRODUCTION
The difference between urban and rural mortality from lung cancer represents a real finding, but it has not yet been clearly established which factors should be held responsible (Wynder and Hammond, 1962; Stocks, 1966; Candeli and Mastrandrea, 1971); some workers have suggested that the high urban rates are due to air pollution, since various carcinogenic substances have been shown to be present both in factories and in the general atmosphere in towns (Hueper, 1966; Waller and Commins, 1967; Sawicki, 1967). According to Waller (1967) and Lawther (1970), it is extremely difficult to determine the extent to which general air pollution can be blamed for the production of lung cancer and it is doubtful whether pollution by vehicle exhaust can be shown to be harmful. Nevertheless it is important to minimize the emission of polycyclic aromatic hydrocarbons (PAH) from any sources including motor vehicles as “the available data can neither be said to support, nor to refute the hypothesis of an influence of air pollution on recorded lung cancer rates” (Pedersen et al., 1969). The present investigation relates in particular to the PAH produced by the incomplete combustion of fuels used in internal combustion engines (Kotin et al., 1954; Lyons, 1959; Hoffmann and Wynder, 1962 a,b; Hoffmann et al., 1965; Del Vecchio et al., 1970; Candeli et al., 1970; Morozzi, 1970). In the present work we describe some fuel variables that were studied in order to know better which can affect the quantitative production of the polycyclic aromatic hydrocarbons in European internal combustion engines, and so to contribute to the reduction of the carcinogenicity of the atmosphere, at least as far as a reduction of automative air pollutants is concerned. 693
694
A. CANUELI rtul
Many studies dealing with this problem have been published, and the results obtained, both from laboratory burner tests and from engine tests, indicate that the type of hydrocarbons used as fuels can influence the formation of PAN. The study carried out by Boubel and Ripperton (1963) on the production of benzo-(a)-pyrene (BaP; also known as 3: 4 benz-pyrene) during the combustion of four fuels of different molecular structure, but with the same carbon number (benzene, cyclohexane, hexene-1 and hexane), in a laboratory burner, showed that the amount of BaP during the combustion of benzene ranged from 10 to 30 times as much as that produced with any of the other fuels. Likewise Begeman (1962), Hoffmann and Wynder (1962a) and Hoffmann rt al. (1965) using an American V-8 engine operating on a city driving schedule on a dynamometer and burning various fuels(50 per cent benzene + 50 per cent xylene; 50 per cent xylene + 50 per cent aliphatic hydrocarbons; standard petrol with 36 per cent aromatics; isoctane; di-iso-butylene) showed that benzene and its derivatives produced more PAH than did aliphatic hydrocarbons. According to Begeman (1962) the BaP emission rate with the aromatic fuel was 37 times that with the di-iso-butylene fuel and 2.7 times that with standard petrol. McKee and McMahon (1967) have surveyed the relevant literature up to 1967. More recently Begeman and Colucci (1970), Griffing et al. (1971) and Gross (1972 a,b), have confirmed these findings and Griffing et al. found that the BaP levels in exhaust gas increased linearly with the percentage of aromatic constituents (from 12 to 48 per cent). According to Begeman and Colucci (1970) and Rinehart et al. (1971) the average emission of PAW from “Indolene” petrol, which is rich in aromatics, was higher than from commercial premium petrol. The second relevant factor is lead anti-knock. For a given octane number however, anti-knock and aromatic contents of the fuel are not independent of one another, since the aromatic hydrocarbons exert a markedly anti-knock effect, and these two factors have therefore often been studied together. In the work already cited, Griffing et al. (1971), who studied several leaded and unleaded fuels at different levels of aromaticity, pointed out that the addition of tetra-ethyl-lead (TEL) does not affect BaP emission. Similar results were reported by Begeman and Colucci (1970) although slightly higher emissions of BaP were found with leaded “lndolene” petrol than with the same petrol unleaded. Rinehart et al. (1970) experimented both with commercial premium fuel and with “Indolene”, with and without lead, and their results showed that lead did not appreciably affect BaP emission when the comparison was made between fuels at constant aromaticity. Gross (1972a) obtained similar results since, according to the author, “the addition of anti-knock lead to gasoline had variable effects on PNA emission; both increases and decreases were observed.” Other factors that have been studied and connected with the PAH emission are: the presence of the PAH themselves in unburnt fuel (Commins, 1962; Begeman and Colucci, 1968, 1970; Gross, 1972 a,b); the presence of deposits rich in PAH in the combustion chamber (Gross, 1972 a,b); the carry-over of the PAH from one engine test to another (Griffing et al., 1971; Gross, 1972b);and the oil consumption (Hoffmann et al., 1965 ; Begeman and Colucci, 1970; Gross, f972b). EXPERIMENTAL
This study was designed to determine the concentration of PAH in the exhaust from a European car operating with each of 9 fuels. These were chosen in order to study the
Carcinogenic air pollutants in car exhaust
influence of tetra-ethyl-lead anti-knock, aromaticity level of aromaticity, on PAH emission.
695
and type of aromatics at a constant
Fuels
In Tables 1,2 and 3 respectively, the distillation curves, the physical constants and the composition of the fuels used in our experiments are reported. The fuels A, B, C and the same fuels, plus 0.63 g 1-l of TEL, which we have called A-TEL, B-TEL and C-TEL, are special blends prepared to provide increasing concentrations of aromatics: 0, 6, 48 per cent, and different levels of aliphatics: 100,47,52 per cent respectively; only fuel B contains olefins (47 per cent). The distillation curves of these fuels are typical of commercially available fuels. Fuels D, E and F are synthetic unleaded fuels all having the same aromatic content as fuel C (48 per cent), but containing different types of aromatics: benzene in fuel D, mostly xylenes in fuel E and mostly hydrocarbons with 9 and 10 carbon atoms in fuel F. The aromatic component of fuel C is a blend characteristic of catalytic reforming. Engine and complete tar collection system
The tests were done on a typical European car engine (1608 cm3) without emission control, following the standard European city driving schedule on a dynamometer bench. The combustion chamber was cleaned after each test in order to remove deposits which could contain PAH (Gross, 1972 a,b), as the same engine was used for all our tests. In each test the European cycle of 195 s duration was repeated until 50 1. of petrol had been used; the average simulated speed was 19 km h-l. Prior to experimental use, the vehicle was operated on rollers by a driver, following the European cycle. During the tests the engine throttle position and the engine speed in rev min- 1 were recorded continuously. These parameters were then imposed on the same engine at the dynamometer bench. To Table 1. Distillation curves of fuels A, B and C ASTM D 36
A
B
C
IBP”C* (% vol. evap.) 5 10 20 30 40 50 60 70 80 90 95
37
39
39
57 65 77 86 94 100 103 106 111 124 169
50 53 57 61 67 73 82 91 102 116 125
53 58 67 74 87 101 118 133 144 158 169
FBP”Ct
183
142
188
98 1 1
98 1 1
(%Ivol.) Recovery Residue Loss * Initial boiling point. t Final boiling point.
97 1.5 1.5
696
Yia
i
-
-
--
697
Carcinogenic air pollutants in car exhaust Table 3. Composition of the aromatic component of test fuels (C, D, E and F) at constant total aromatic@
Compounds Benzene Toluene Ethyl-benzene para-Xylene meta-Xylene ortho-Xylene iso-Propyl-benzene n-Propyl-benzene 1: 3- plus 1: Cmethyi-benzene 1:3.5Trimethyl-benzene 1 :ZMethyl-ethyl-benzene 1:2:4-T~methyl-benzene 1: 2: 3-Trimethyl-benzene Aromatics C,,,
C*
Et
Ft
(% vol.)
(% vol.)
(% vol.)
0.39 0.28 9.98 7.47 19.58 9.14 0.11 0.06 0.27 0.08 0.06 0.24 0.06 0.28
0.42 0.14 0.08 0.12 0.36 0.55 0.12 1.36 9.14 3.27 2.47 12.35 5.31 12.31
3.04 12.46 2.59 3.76 6.66 3.57 0.12 0.65 4.09 1.53 0.84 4.44 1.47 2.47
47.91 0.09
* Fuel with commercially typical distillation curve. t Synthetic fuels. $ Aromatics 48 per cent, paraffin 52 per cent
avoid variation in the emissions due to different equivalence ratios* (Commins, 1969; Begeman and Colucci, 1970) and thus to obtain comparable data for fuels differing in physical properties, engine carburation was adjusted to the same equivalence ratio (1.10) for all tests. The engine was operated strictly according to the European cycle by experienced motor engineers at the Petroleum Products Laboratory of Snam Progetti. All particulate matter and condensable vapours were collected by a total condensation system similar to that described by Begeman and Colucci (1962) and described in detail in our previous paper (Candeli et al., 1970). The benzene-soluble material and the methanol-soluble material were recovered from the various parts of the collecting system as follows: (a) from the condensed water by extraction with benzene in a Scheibel countercurrent liquid-liquid extraction column: (b) from the filter medium by extraction with a mixture of benzene and method (4: 1, v/v); and (c) from the condenser, accumulator, transfer drum, and other parts of the exhaust system by rinsing with benzene. Most of the solvent from tar solutions was stripped from the tar by distillation through an Oldershaw distillation column, operated at 2: 1 reflux ratio. The remaining solvent was removed under vacuum with a “‘rotavapor” rotary evaporator. These three fractions of tar were combined, and a dried sample of about 5 g, to which a known quantity of tritiated BaP had previously been added as an internal standard, was used for the analysis. Analytical procedure
Column chromatography on silica gel deactivated with 5 per cent of water and eluted with hexane, was adopted for a preliminary purification of the tar sample, mostly from paraffins. The residue of the eluate of this first column was fractionated by chro~tography on alumina, activated at 120” for 8 h and deactivated with 2 per cent of water. Analysis by * Equivafence ratio = [air to fuef (stoichiome~ic)]/[air A.1’. X/7--a
to fuel (actual)].
A. CANDELI
69X
e?nl.
U.V.spectrophotometry of each fraction eluted with cyclohexane was used for identification and quantification of PAHs; when it was necessary, in order to isolate polycyclic hydrocarbons with absorption spectra which closely resembled those of pure reference compounds, further separation was achieved by ~hro~tography on acetylated paper (~offmann and Wynder, 1962 ; Candeli et al., 1967). To determine the absolute amount of BaP in the examined tar samples, we employed the tracer technique (Hoffmann and Wynder, 1962b) using tritiated BaP(400 mCi mM- ‘) supplied by the Radiochemical Centre at Amersham (Candeli and Morozzi, 1969). RESULTS
The amounts of tar residue collected from 50 1, of each of the 9 fuels together with the weights per litre of fuel are summarized in Table 4. Table 4. Tar recovered from the exhaust gas of test fuels Fuel A*
A-TEL* B*
B-TEL* c* C-TEL* Dt Et Ft
Total tar (g)
Tar (g I-’ fuel)
35.26 27.10 28.64 28.05 44.60 34.00 47.17 68.59 I195.73
0.71 0.54 0.57 0.56 0.89 0.68 0.95 1.37 3.91
* Fuels with commercially typical distillation curves. t Synthetic fuels.
Table 5 lists the 13 PAH’s identified and shows the quantities of each that were collected per litre of fuel. The data concerning the three leaded fuels, A-TEL, B-TEL and C-TEL, and the same unleaded fuels A, B and C are also displayed in Figs. I,2 and 3, which show the ratios of the quantities of PAH emitted by leaded fuels to those emitted by unleaded, expressed as percentages in order better to demonstrate the differences between leaded and unleaded fuels. The data showing the effects of increasing aromaticity and of different types of aromatics are shown in Figs. 4-6. These data indicate that unleaded fuels give more tar than leaded fuels, the percent differences between fuels with the same composition being: 23.1 per cent (between A and A-TEL); 2.8 per cent (between B and B-TEL) and 23.8 per cent (between C and C-TEL). Likewise, and with only two exceptions, the amounts of PAHs are higher for fuels C and A without lead antiknock than for the leaded fuels, whereas exactly the reverse happens when fuel B is considered (Table 5 ; Figs. i-3). The percentage differences between the amounts of PAH found in fuel C-TEL and fuel A-TEL emissions (Fig. 4) are of the same order of magnitude as the percent differences found in the case of the two unleaded fuels, C and A (Fig. 5). With leaded fuels, when aro~ti~ity increases from 0 to 6 per cent and then to 48 per cent, the amounts of tar and of PAH emission (with the exception of chrysene) increase too, but for unleaded fuels the findings are no longer consistent (Table 5, Figs. 4 and 5). In fact the PAH emissions are always higher in fuel C, with the highest concentration of
1.6 (3.2)t 2.2 0.5 3.6 2.0 1.4
3:CBenzpyrene 1.1 (1.9)t * 0.2 2.7 0.7 1.6
18.2 28.2 17.8 37.1 7.3 31.6 *
Fuel A-TEL
0.5 5.1 1.9 4.7
(&
41.7 38.6 24.0 45.7 11.1 29.8 3.6
B
Fuel
4.1 (6.5)t 0.3 6.1 31.2 10.4 7.0
50.6 85.1 52.0 52.9 13.6 50.9 4.6
Fuel E-TEL
4.9 3.6 111.8 25.5 52.0
(26.0)t
16.6
392.5 570.5 235.5 251.5 32.4 64.9 38.9
Fuel C
* The quantity could not be established owing to impurities and to low concentration of the compounds. t Values corrected by the trace technique using tritiated 3:4-benzpyrene.
10: 1 1-Benzfluoranthene 11: 12-Benztluoranthene 1: 12-Benzperylene Indeno(l,2,3-cd)pyrene Coronene
25.9 61.4 57.1 70.2 8.6 64.2 *
A
Fuel
Anthracene Pyrene Ffuoranthene 2: 3-Benztluorene 1: ZBenzanthracene Chrysene 1: 2-Benzpyrene
PAH
10.2 (18S)t 9.2 18.7 54.5 21.7 41.8
268.1 192.1 183.8 153.5 29.7 27.9 16.9
Fuel C-TEL
Table 5. Emission of polycyclic aromatic hydrocarbons (pg 1- ’ fuel burnt)
(i$ 0.3 14.3 4.0 9.3
91.1 58.4 37.4 ’ * 10.6 38.7 4.7
Fuel D
0.74 27.3 11.7 15.9
3.5 (7;6)+
161.5 123.6 109.5 116.5 10.4 102.5 15.9
Fuel E
48.1 (85.4)+ 40.5 1.9 266.7 102.1 46.4
1577.6 1564.3 1557.7 2708.6 233.0 202.9 90.8
F
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of some PAHs from engine burning various unleaded fuels, relative from the fuel containing the most aromatics (fuel C).
to those
aromatics, than in fuels B and A; but comparing these two latter fuels without TEL (Fig. 5) it is clear that smaller quantities of some polycyclic hydrocarbons, and greater quantities of others, are emitted from fuel A (0 per cent aromatics), than from fuel B (6 per cent aromatics). The data conerning the influence of the type of aromatics in unleaded fuels at a constant level of aromaticity (48 per cent), on the amount of tar and on the amount of PAH emitted, are shown in Tables 4 and 5 and in Fig. 6. Fuel F (C, and C,, aromatics) gives higher emission values than fuel E (xylenes) and the latter gives higher emission values than those found in fuel D (benzene), concerning both residue and PAH; BaP corrected values, for instance, are respectively: 85.4; 7.6; 6.7 ,ugl- ‘. The data found for fuel C, containing a mixture of aromatics including C9 and C,,, (Table 3), are situated between those found for fuels E and F, with the exception of chrysene and coronone. DISCUSSION
Engine emissions are sensitive to slight changes in operating conditions and we would have liked to duplicate our experiments in order to ascertain how reproducible the results were. Unfortunately this was not possible. Nevertheless the figures obtained show a consistent pattern which may be summed up as follows: (1) Of the three fuels tested with and without TEL the two which did not contain olefines
Carcinogenic air pollutants in car exhaust
703
•ll E xylenes 00 benzene
Fig. 6. Emission of PAH from engine burning fuels of the same aromatic content (48 per cent) but with different aromatic components.
produced less PAH when TEL was added It is known that PAH formation involves free radical reactions and this is therefore in accordance with the theory that TEL acts as a free radical inhibitor (Griffing et al., 1971; Zaghini et al., 1972): for the fuel B containing olefinic hydrocarbons (47 per cent olefins, 6 per cent aromatics and 47 per cent of paraffinic hydrocarbons), the presence of TEL had the opposite effect. This lack of a consistent effect is in line with the findings of Rinehart et al. (1970), of Begeman and Colucci (1970), of Griffing et al. (1971) and of Gross (1972a). Our data, relating to the weights of the tar in the exhaust from leaded and unleaded fuels, are not in agreement with those reported by Ter Haar et al. (1972), but it must be pointed out that whereas our results relate to both condensable and particulate material the data of Ter Haar et al. deal only with particulate matter. (2) The increasing emission of PAH with the increasing level of fuel aromaticity (0, 6 and 48 per cent) verified for our leaded fuels, is in agreement with the data reported by Hoffmann and Wynder (1962b), Hoffmann et al. (1965), Begeman (1962), Begeman and Colucci (1970), Griffing et al. (1971) and Gross (1972a), all of whom worked with American engines operating on California cycle; they are also in agreement with the data referred by Boubel and Ripperton (1963), who experimented with a laboratory burner. In his latest paper (1972b) Gross plays down the effect of aromaticity on PAH emissions, emphasizing the role of the PAH content of the unburnt fuels. Research on this problem
704
A. CANDELI etat.
is in progress in our Institute on the same fuels as were tested in the experiments described above. The ratio between BaP values in the 48 and 0 per cent aromatic leaded fuel emissions is lQ’1.9 = 9.7; the corresponding ratio calculated from Griffing’s data, for the Federal Test procedure* is 548,/142 = 3.9; the figure 142, for BaP emission from fuel containing 0 per cent aromatics, was obtained from those for six observations with fuels containing from 10 up to 48 per cent aromatics. The same ratio for unleaded fuels tested in our experiments is 26.0/3.2 = 8.1. For the unleaded fuels PAH emission no longer increases in a consistent manner with fuel aromati~ity as the olefinic fuel B, with 6 per cent of aromatics, gives higher values for some PAHs, and lower values for other PAHs, than those found in the emission of the paraffinic fuel A with 0 per cent aromatics. Nevertheless the quantities of PAH emitted by the unleaded fuel with the highest percentage of aromatics (48 per cent) are always higher than those found for the other two fuels with 6 and 0 per cent of aromatics. (3) As a third and final point we have found, al~ough in only a few tests, that at a constant level of aromatics the type of aromatic compound in the fuel strongly affects the PAH emission. With unleaded fuels containing aromatics the quantity of PAH’s emitted increased with increasing number of carbon atoms (6 in benzene; 8 in xylenes; 9 and 10 in straight line substituted benzene). The amount of BaP, for instance, was 11 times higher in the emission of fuel at 48 per cent aromatics of the type C, and C, 0, than in the emission when the engine was burning fuel confining xylene, and 13 times higher than when the fuel contained benzene. These findings are consistent with Badger’s theory (1962) according to which C6-C4 fragments are intermediate compounds which are closely involved in the formation of 3 : 4benzpyrene which occurs in pyrolysis and in incomplete combustion (Zaghini et al., 1973). ~C~now~e~ge~en~s-The authors wish to thank the Petroleum Products Laboratory of Snam Progetti for providing the samples of tar produced from a European engine, and the Ente Nazionale Idrocarburi (ENI) for supporting the whole research. REFERENCES Badger G. M. (1962) Mode of formation of carcinogens in human environment. Natl. Cancer Instit. ~onogrup~ 9, i-16. Begeman C. R. (1962) Carcinogenic Aromatic Hydrocarbons in Automobile ESJIuents. SAE Technical Progress Series, Vol. 6. McMillan, New York 1964. (Presented to Automotive Engineering Congress Detroit, Michigan, January 1962). Begeman C. R. and Colucci .l. M. (1962) Apparatus for determining the contribution of the automobile to the benzene-soluble organic matter in air. Natl. Cancer Instit. Monograph 9, 17-57. Begeman C. R. and Colucci J. M. (1968) Benzo(a)pyrene in gasoline partially persists in automobile exhaust. Science 161,271.
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Candeli A., Mastrandrea V. and Morozzi G. (1970) idrocarburi aromatici policiclici nei gas di scarico di un motore a combustione interna alimentato con diverse benzine de1 commercio. Riu. Ital. Ig. 29/30, 190-204. * Federal Register, Department of Health, Education and Welfare. 33, No. 108, Part II (4 June 1968).
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Candeli A. and Mastrandrea V. (1971) Fattore urban0 e cancro polmonare. Giorn. Ig. Med. Preu. 12, 89-120. Commins B. T. (1962) The spectrophotometric determination of polycyclic aromatic hydrocarbons in polluted air. Ph.D. Thesis. University of London. ComminsB. T. (1969) Formation of polycyclic aromatic hydrocarbons during pyrolysis and combustion of hydrocarbons. Atmospheric Environment 3,347-354. Del Vecchio V., Valori P., Grella A. and Melchiorri C. (1970) Inquinamento atmosferico da autoveicoli e i problemi relativi al sue controllo. Sperimentazione di un post-combustore eterogeneo. Nuoui Ann. Ig. Microbial. 21, l-136. Griffing M. E.. Maler A. R., Borland J. E. and Decker R. R. (1971) Applying a new method for measuring benzo(a)pyrene in vehicle exhaust to the study of fuel factors. Presented at the Symposium on current approaches to automotive emission control, American Chemical Society, Los Angeles, California. Ethyl Corporation Research Laboratories, Detroit, Michigan p. 13. Gross G. P. (1972a) The effect of fuel and vehicle variables on polynuclear aromatic hydrocarbon and phenol emissions. Paper No. 720210 presented at the SAE Automotive Engineering Congress and Exposition. Detroit, Michigan. Gross G. P. (1972b) Gasoline composition and vehicle exhaust gas polynuclear aromatic content. Third annual report. CRC-APRAC Project No. CAPE 6-68 EPA Contract 68%04-0025. Esso Research and Engineering Company Products Research Division, Linden, New Jersey. Hoffmann D. and Wynder E. L. (1962a) A study of air pollution carcinogenesis-II. The isolation and identification of polynuclear aromatic hydrocarbons from gasoline engine exhaust condensate. Cancer 72,93-102. Hoffmann D. and Wynder E. L. (1962b) Analytical and biological studies on gasoline engine exhaust. N&l. Cancer Instit. Monograph 9,91-l
17.
Hoffmann D., Theisz E. and Wynder E. L. (1965) Studies on the carcinogenicity of gasoline exhaust. J. Air. Polk. Control Ass. 15, 162-165. Hueper W. C. (1966) Occupational and Enuironmental Cancers qfthe Respiratory System. Springer, Berlin. Kotin P., Falk, H. L. and Thomas M. (1954) Aromatic hydrocarbons-II. Presence in particulate phase of gasoline-engine exhaust and carcinogenicity of exhaust extracts. A.M.A. Arch. Indust. Hyg. 9, 164-177. Lawther P. J. (1970) Air pollution and its effect on man. Proc. Roy. Instn. Gt. Er. 44, 74-94. Lyons M. J. (1959) Vehicular exhaust; identification of further carcinogens of polycyclic aromatic hydrocarbons class. &it. J. Cancer 13, 126-131. McKee H. C. and McMahon W. A. (1967) Polynuclear aromatic content of vehicle emissions. Southwest Research Institute. Technical Report-No. 1. Presented to Committeefor Air and Water Conservation, American Petroleum Institute, pp. 37. Morozzi G. (1970) Significato di alcuni indici ottenibili dallo studio quantitativo degli Idrocarburi Aromatici Policiclici nella atmosfera e nei gas di scarico. Giorn. Ig. Med. Preu. 11, 135-153. Pedersen E., Magnus K., Mork T., Hougen A., Bjelke E., Hakama M. and Saxen E. (1969) Lung cancer in Finland and Norway. An epidemiological study. Acta path. microbial. stand. Suppl. 199,74. Rinehart W. E., Gendermalik S. A. and Gilbert L. F. (1970) Fuel factors in automotive tailpipe emissions. Presented at American Industrial Hygiene Conference. Detroit, Michigan, Paper No. 127 pp. 15. Sawicki E. (1967) Airborne carcinogens and allied compounds. Arch. environ. Hlth 14,46-53. Stocks P. (1966) Recent epidemiological studies of lung cancer, cigarette smoking and air pollution with discussion of a new hypothesis of causation. &it. J. Cancer 20,595-623. Ter Haar G. L., Lenane D. L.. Hu J. N. and Brandt M. (1972) Composition, size and control of automotive exhaust particulates. J. Air. Polk. Control. Ass. 22, 39-46. Waller R. E. (19671 Bronchi and lungs-air oollution. In The Prevention of Cancer.(Edited bv Raven R. W. and ~ Roe F. J. C.), pp: 181-186. ButteGorths,‘London. Waller R. E. and Commins B. T. (1967) Studies of the smoke and polycyclic aromatic hydrocarbons content of the air in large urban areas. Enuironm. Res. 1,295-306. Wynder E. L. and Hammond E. C. (1962) A study of air pollution carcinogenesis-I. Analysis of epidemiological evidence. Cancer 15,79-92. Zaghini N., Mangolini S., Cornetti G., Salvatori T. and Rizzi C. (1972) Formation of polycyclic aromatic hydrocarbons in a vertical flow reactor. Combustion Sci. Technol. 5,225-236. Zdghini N., Mangolini S., Arteconi M. and Sezzi F. (1973) Polynuclear Aromatic Hydrocarbons in vehicle exhaust gas. Presented at SAE Milwaukee Congress, l(rl3 September.
Environment
Vol.
8, pp.
693-705.
Pergamon Press 1974.
Printed in Great Britain
CARCINOGENIC AIR POLLUTANTS FROM A EUROPEAN CAR OPERATING
IN THE EXHAUST ON VARIOUS FUELS
A. CANDELI, V. MASTRANDREA,G. MOROZZI and S. TOCCACELI Istituto di Igiene della Facolta di Scienze MM.FF.NN. della Universita di Perugia. Via Dante Alighieri. 06 100 Perugia, Italy (First received 11 October
1973 and injnal
form
25 October 1973)
A&met-The influence of some fuel variables on polycyclic aromatic hydrocarbons (PAH) emission was studied The fuel variables taken into consideration were: tetra-ethyl-lead (TEL) content (0 and 0.63 g 1-l); fuel aromaticity (0, 6 and 48 per cent) and type of aromatic at constant total level of aromatics (benzene, xylene and hydrocarbons with 9 and 10 carbon atoms). Each test consisted of European test cycles on a dynamometer bench, using a European engine of average displacement for European cars (1608 cm’) and without deposits in the combustion chamber. The results obtained show that it is not possible to generalize about the effect of TEL on PAH emission: in two cases the addition of TEL reduced and in one case it increased the PAH emission. Increasing fuel aromaticity increases PAH emission, but these results apply only to the three leaded tested fuels, and not to the same fuels unleaded In three synthetic unleaded fuels at constant total aromatics the type of aromatic present strongly affected the PAH emissions: it seems that simple aromatic hydrocarbons, such as benzene oy xylene, produce less PAH than C, and C,, hydrocarbons INTRODUCTION
The difference between urban and rural mortality from lung cancer represents a real finding, but it has not yet been clearly established which factors should be held responsible (Wynder and Hammond, 1962; Stocks, 1966; Candeli and Mastrandrea, 1971); some workers have suggested that the high urban rates are due to air pollution, since various carcinogenic substances have been shown to be present both in factories and in the general atmosphere in towns (Hueper, 1966; Waller and Commins, 1967; Sawicki, 1967). According to Waller (1967) and Lawther (1970), it is extremely difficult to determine the extent to which general air pollution can be blamed for the production of lung cancer and it is doubtful whether pollution by vehicle exhaust can be shown to be harmful. Nevertheless it is important to minimize the emission of polycyclic aromatic hydrocarbons (PAH) from any sources including motor vehicles as “the available data can neither be said to support, nor to refute the hypothesis of an influence of air pollution on recorded lung cancer rates” (Pedersen et al., 1969). The present investigation relates in particular to the PAH produced by the incomplete combustion of fuels used in internal combustion engines (Kotin et al., 1954; Lyons, 1959; Hoffmann and Wynder, 1962 a,b; Hoffmann et al., 1965; Del Vecchio et al., 1970; Candeli et al., 1970; Morozzi, 1970). In the present work we describe some fuel variables that were studied in order to know better which can affect the quantitative production of the polycyclic aromatic hydrocarbons in European internal combustion engines, and so to contribute to the reduction of the carcinogenicity of the atmosphere, at least as far as a reduction of automative air pollutants is concerned. 693
694
A. CANUELI rtul
Many studies dealing with this problem have been published, and the results obtained, both from laboratory burner tests and from engine tests, indicate that the type of hydrocarbons used as fuels can influence the formation of PAN. The study carried out by Boubel and Ripperton (1963) on the production of benzo-(a)-pyrene (BaP; also known as 3: 4 benz-pyrene) during the combustion of four fuels of different molecular structure, but with the same carbon number (benzene, cyclohexane, hexene-1 and hexane), in a laboratory burner, showed that the amount of BaP during the combustion of benzene ranged from 10 to 30 times as much as that produced with any of the other fuels. Likewise Begeman (1962), Hoffmann and Wynder (1962a) and Hoffmann rt al. (1965) using an American V-8 engine operating on a city driving schedule on a dynamometer and burning various fuels(50 per cent benzene + 50 per cent xylene; 50 per cent xylene + 50 per cent aliphatic hydrocarbons; standard petrol with 36 per cent aromatics; isoctane; di-iso-butylene) showed that benzene and its derivatives produced more PAH than did aliphatic hydrocarbons. According to Begeman (1962) the BaP emission rate with the aromatic fuel was 37 times that with the di-iso-butylene fuel and 2.7 times that with standard petrol. McKee and McMahon (1967) have surveyed the relevant literature up to 1967. More recently Begeman and Colucci (1970), Griffing et al. (1971) and Gross (1972 a,b), have confirmed these findings and Griffing et al. found that the BaP levels in exhaust gas increased linearly with the percentage of aromatic constituents (from 12 to 48 per cent). According to Begeman and Colucci (1970) and Rinehart et al. (1971) the average emission of PAW from “Indolene” petrol, which is rich in aromatics, was higher than from commercial premium petrol. The second relevant factor is lead anti-knock. For a given octane number however, anti-knock and aromatic contents of the fuel are not independent of one another, since the aromatic hydrocarbons exert a markedly anti-knock effect, and these two factors have therefore often been studied together. In the work already cited, Griffing et al. (1971), who studied several leaded and unleaded fuels at different levels of aromaticity, pointed out that the addition of tetra-ethyl-lead (TEL) does not affect BaP emission. Similar results were reported by Begeman and Colucci (1970) although slightly higher emissions of BaP were found with leaded “lndolene” petrol than with the same petrol unleaded. Rinehart et al. (1970) experimented both with commercial premium fuel and with “Indolene”, with and without lead, and their results showed that lead did not appreciably affect BaP emission when the comparison was made between fuels at constant aromaticity. Gross (1972a) obtained similar results since, according to the author, “the addition of anti-knock lead to gasoline had variable effects on PNA emission; both increases and decreases were observed.” Other factors that have been studied and connected with the PAH emission are: the presence of the PAH themselves in unburnt fuel (Commins, 1962; Begeman and Colucci, 1968, 1970; Gross, 1972 a,b); the presence of deposits rich in PAH in the combustion chamber (Gross, 1972 a,b); the carry-over of the PAH from one engine test to another (Griffing et al., 1971; Gross, 1972b);and the oil consumption (Hoffmann et al., 1965 ; Begeman and Colucci, 1970; Gross, f972b). EXPERIMENTAL
This study was designed to determine the concentration of PAH in the exhaust from a European car operating with each of 9 fuels. These were chosen in order to study the
Carcinogenic air pollutants in car exhaust
influence of tetra-ethyl-lead anti-knock, aromaticity level of aromaticity, on PAH emission.
695
and type of aromatics at a constant
Fuels
In Tables 1,2 and 3 respectively, the distillation curves, the physical constants and the composition of the fuels used in our experiments are reported. The fuels A, B, C and the same fuels, plus 0.63 g 1-l of TEL, which we have called A-TEL, B-TEL and C-TEL, are special blends prepared to provide increasing concentrations of aromatics: 0, 6, 48 per cent, and different levels of aliphatics: 100,47,52 per cent respectively; only fuel B contains olefins (47 per cent). The distillation curves of these fuels are typical of commercially available fuels. Fuels D, E and F are synthetic unleaded fuels all having the same aromatic content as fuel C (48 per cent), but containing different types of aromatics: benzene in fuel D, mostly xylenes in fuel E and mostly hydrocarbons with 9 and 10 carbon atoms in fuel F. The aromatic component of fuel C is a blend characteristic of catalytic reforming. Engine and complete tar collection system
The tests were done on a typical European car engine (1608 cm3) without emission control, following the standard European city driving schedule on a dynamometer bench. The combustion chamber was cleaned after each test in order to remove deposits which could contain PAH (Gross, 1972 a,b), as the same engine was used for all our tests. In each test the European cycle of 195 s duration was repeated until 50 1. of petrol had been used; the average simulated speed was 19 km h-l. Prior to experimental use, the vehicle was operated on rollers by a driver, following the European cycle. During the tests the engine throttle position and the engine speed in rev min- 1 were recorded continuously. These parameters were then imposed on the same engine at the dynamometer bench. To Table 1. Distillation curves of fuels A, B and C ASTM D 36
A
B
C
IBP”C* (% vol. evap.) 5 10 20 30 40 50 60 70 80 90 95
37
39
39
57 65 77 86 94 100 103 106 111 124 169
50 53 57 61 67 73 82 91 102 116 125
53 58 67 74 87 101 118 133 144 158 169
FBP”Ct
183
142
188
98 1 1
98 1 1
(%Ivol.) Recovery Residue Loss * Initial boiling point. t Final boiling point.
97 1.5 1.5
696
Yia
i
-
-
--
697
Carcinogenic air pollutants in car exhaust Table 3. Composition of the aromatic component of test fuels (C, D, E and F) at constant total aromatic@
Compounds Benzene Toluene Ethyl-benzene para-Xylene meta-Xylene ortho-Xylene iso-Propyl-benzene n-Propyl-benzene 1: 3- plus 1: Cmethyi-benzene 1:3.5Trimethyl-benzene 1 :ZMethyl-ethyl-benzene 1:2:4-T~methyl-benzene 1: 2: 3-Trimethyl-benzene Aromatics C,,,
C*
Et
Ft
(% vol.)
(% vol.)
(% vol.)
0.39 0.28 9.98 7.47 19.58 9.14 0.11 0.06 0.27 0.08 0.06 0.24 0.06 0.28
0.42 0.14 0.08 0.12 0.36 0.55 0.12 1.36 9.14 3.27 2.47 12.35 5.31 12.31
3.04 12.46 2.59 3.76 6.66 3.57 0.12 0.65 4.09 1.53 0.84 4.44 1.47 2.47
47.91 0.09
* Fuel with commercially typical distillation curve. t Synthetic fuels. $ Aromatics 48 per cent, paraffin 52 per cent
avoid variation in the emissions due to different equivalence ratios* (Commins, 1969; Begeman and Colucci, 1970) and thus to obtain comparable data for fuels differing in physical properties, engine carburation was adjusted to the same equivalence ratio (1.10) for all tests. The engine was operated strictly according to the European cycle by experienced motor engineers at the Petroleum Products Laboratory of Snam Progetti. All particulate matter and condensable vapours were collected by a total condensation system similar to that described by Begeman and Colucci (1962) and described in detail in our previous paper (Candeli et al., 1970). The benzene-soluble material and the methanol-soluble material were recovered from the various parts of the collecting system as follows: (a) from the condensed water by extraction with benzene in a Scheibel countercurrent liquid-liquid extraction column: (b) from the filter medium by extraction with a mixture of benzene and method (4: 1, v/v); and (c) from the condenser, accumulator, transfer drum, and other parts of the exhaust system by rinsing with benzene. Most of the solvent from tar solutions was stripped from the tar by distillation through an Oldershaw distillation column, operated at 2: 1 reflux ratio. The remaining solvent was removed under vacuum with a “‘rotavapor” rotary evaporator. These three fractions of tar were combined, and a dried sample of about 5 g, to which a known quantity of tritiated BaP had previously been added as an internal standard, was used for the analysis. Analytical procedure
Column chromatography on silica gel deactivated with 5 per cent of water and eluted with hexane, was adopted for a preliminary purification of the tar sample, mostly from paraffins. The residue of the eluate of this first column was fractionated by chro~tography on alumina, activated at 120” for 8 h and deactivated with 2 per cent of water. Analysis by * Equivafence ratio = [air to fuef (stoichiome~ic)]/[air A.1’. X/7--a
to fuel (actual)].
A. CANDELI
69X
e?nl.
U.V.spectrophotometry of each fraction eluted with cyclohexane was used for identification and quantification of PAHs; when it was necessary, in order to isolate polycyclic hydrocarbons with absorption spectra which closely resembled those of pure reference compounds, further separation was achieved by ~hro~tography on acetylated paper (~offmann and Wynder, 1962 ; Candeli et al., 1967). To determine the absolute amount of BaP in the examined tar samples, we employed the tracer technique (Hoffmann and Wynder, 1962b) using tritiated BaP(400 mCi mM- ‘) supplied by the Radiochemical Centre at Amersham (Candeli and Morozzi, 1969). RESULTS
The amounts of tar residue collected from 50 1, of each of the 9 fuels together with the weights per litre of fuel are summarized in Table 4. Table 4. Tar recovered from the exhaust gas of test fuels Fuel A*
A-TEL* B*
B-TEL* c* C-TEL* Dt Et Ft
Total tar (g)
Tar (g I-’ fuel)
35.26 27.10 28.64 28.05 44.60 34.00 47.17 68.59 I195.73
0.71 0.54 0.57 0.56 0.89 0.68 0.95 1.37 3.91
* Fuels with commercially typical distillation curves. t Synthetic fuels.
Table 5 lists the 13 PAH’s identified and shows the quantities of each that were collected per litre of fuel. The data concerning the three leaded fuels, A-TEL, B-TEL and C-TEL, and the same unleaded fuels A, B and C are also displayed in Figs. I,2 and 3, which show the ratios of the quantities of PAH emitted by leaded fuels to those emitted by unleaded, expressed as percentages in order better to demonstrate the differences between leaded and unleaded fuels. The data showing the effects of increasing aromaticity and of different types of aromatics are shown in Figs. 4-6. These data indicate that unleaded fuels give more tar than leaded fuels, the percent differences between fuels with the same composition being: 23.1 per cent (between A and A-TEL); 2.8 per cent (between B and B-TEL) and 23.8 per cent (between C and C-TEL). Likewise, and with only two exceptions, the amounts of PAHs are higher for fuels C and A without lead antiknock than for the leaded fuels, whereas exactly the reverse happens when fuel B is considered (Table 5 ; Figs. i-3). The percentage differences between the amounts of PAH found in fuel C-TEL and fuel A-TEL emissions (Fig. 4) are of the same order of magnitude as the percent differences found in the case of the two unleaded fuels, C and A (Fig. 5). With leaded fuels, when aro~ti~ity increases from 0 to 6 per cent and then to 48 per cent, the amounts of tar and of PAH emission (with the exception of chrysene) increase too, but for unleaded fuels the findings are no longer consistent (Table 5, Figs. 4 and 5). In fact the PAH emissions are always higher in fuel C, with the highest concentration of
1.6 (3.2)t 2.2 0.5 3.6 2.0 1.4
3:CBenzpyrene 1.1 (1.9)t * 0.2 2.7 0.7 1.6
18.2 28.2 17.8 37.1 7.3 31.6 *
Fuel A-TEL
0.5 5.1 1.9 4.7
(&
41.7 38.6 24.0 45.7 11.1 29.8 3.6
B
Fuel
4.1 (6.5)t 0.3 6.1 31.2 10.4 7.0
50.6 85.1 52.0 52.9 13.6 50.9 4.6
Fuel E-TEL
4.9 3.6 111.8 25.5 52.0
(26.0)t
16.6
392.5 570.5 235.5 251.5 32.4 64.9 38.9
Fuel C
* The quantity could not be established owing to impurities and to low concentration of the compounds. t Values corrected by the trace technique using tritiated 3:4-benzpyrene.
10: 1 1-Benzfluoranthene 11: 12-Benztluoranthene 1: 12-Benzperylene Indeno(l,2,3-cd)pyrene Coronene
25.9 61.4 57.1 70.2 8.6 64.2 *
A
Fuel
Anthracene Pyrene Ffuoranthene 2: 3-Benztluorene 1: ZBenzanthracene Chrysene 1: 2-Benzpyrene
PAH
10.2 (18S)t 9.2 18.7 54.5 21.7 41.8
268.1 192.1 183.8 153.5 29.7 27.9 16.9
Fuel C-TEL
Table 5. Emission of polycyclic aromatic hydrocarbons (pg 1- ’ fuel burnt)
(i$ 0.3 14.3 4.0 9.3
91.1 58.4 37.4 ’ * 10.6 38.7 4.7
Fuel D
0.74 27.3 11.7 15.9
3.5 (7;6)+
161.5 123.6 109.5 116.5 10.4 102.5 15.9
Fuel E
48.1 (85.4)+ 40.5 1.9 266.7 102.1 46.4
1577.6 1564.3 1557.7 2708.6 233.0 202.9 90.8
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of some PAHs from engine burning various unleaded fuels, relative from the fuel containing the most aromatics (fuel C).
to those
aromatics, than in fuels B and A; but comparing these two latter fuels without TEL (Fig. 5) it is clear that smaller quantities of some polycyclic hydrocarbons, and greater quantities of others, are emitted from fuel A (0 per cent aromatics), than from fuel B (6 per cent aromatics). The data conerning the influence of the type of aromatics in unleaded fuels at a constant level of aromaticity (48 per cent), on the amount of tar and on the amount of PAH emitted, are shown in Tables 4 and 5 and in Fig. 6. Fuel F (C, and C,, aromatics) gives higher emission values than fuel E (xylenes) and the latter gives higher emission values than those found in fuel D (benzene), concerning both residue and PAH; BaP corrected values, for instance, are respectively: 85.4; 7.6; 6.7 ,ugl- ‘. The data found for fuel C, containing a mixture of aromatics including C9 and C,,, (Table 3), are situated between those found for fuels E and F, with the exception of chrysene and coronone. DISCUSSION
Engine emissions are sensitive to slight changes in operating conditions and we would have liked to duplicate our experiments in order to ascertain how reproducible the results were. Unfortunately this was not possible. Nevertheless the figures obtained show a consistent pattern which may be summed up as follows: (1) Of the three fuels tested with and without TEL the two which did not contain olefines
Carcinogenic air pollutants in car exhaust
703
•ll E xylenes 00 benzene
Fig. 6. Emission of PAH from engine burning fuels of the same aromatic content (48 per cent) but with different aromatic components.
produced less PAH when TEL was added It is known that PAH formation involves free radical reactions and this is therefore in accordance with the theory that TEL acts as a free radical inhibitor (Griffing et al., 1971; Zaghini et al., 1972): for the fuel B containing olefinic hydrocarbons (47 per cent olefins, 6 per cent aromatics and 47 per cent of paraffinic hydrocarbons), the presence of TEL had the opposite effect. This lack of a consistent effect is in line with the findings of Rinehart et al. (1970), of Begeman and Colucci (1970), of Griffing et al. (1971) and of Gross (1972a). Our data, relating to the weights of the tar in the exhaust from leaded and unleaded fuels, are not in agreement with those reported by Ter Haar et al. (1972), but it must be pointed out that whereas our results relate to both condensable and particulate material the data of Ter Haar et al. deal only with particulate matter. (2) The increasing emission of PAH with the increasing level of fuel aromaticity (0, 6 and 48 per cent) verified for our leaded fuels, is in agreement with the data reported by Hoffmann and Wynder (1962b), Hoffmann et al. (1965), Begeman (1962), Begeman and Colucci (1970), Griffing et al. (1971) and Gross (1972a), all of whom worked with American engines operating on California cycle; they are also in agreement with the data referred by Boubel and Ripperton (1963), who experimented with a laboratory burner. In his latest paper (1972b) Gross plays down the effect of aromaticity on PAH emissions, emphasizing the role of the PAH content of the unburnt fuels. Research on this problem
704
A. CANDELI etat.
is in progress in our Institute on the same fuels as were tested in the experiments described above. The ratio between BaP values in the 48 and 0 per cent aromatic leaded fuel emissions is lQ’1.9 = 9.7; the corresponding ratio calculated from Griffing’s data, for the Federal Test procedure* is 548,/142 = 3.9; the figure 142, for BaP emission from fuel containing 0 per cent aromatics, was obtained from those for six observations with fuels containing from 10 up to 48 per cent aromatics. The same ratio for unleaded fuels tested in our experiments is 26.0/3.2 = 8.1. For the unleaded fuels PAH emission no longer increases in a consistent manner with fuel aromati~ity as the olefinic fuel B, with 6 per cent of aromatics, gives higher values for some PAHs, and lower values for other PAHs, than those found in the emission of the paraffinic fuel A with 0 per cent aromatics. Nevertheless the quantities of PAH emitted by the unleaded fuel with the highest percentage of aromatics (48 per cent) are always higher than those found for the other two fuels with 6 and 0 per cent of aromatics. (3) As a third and final point we have found, al~ough in only a few tests, that at a constant level of aromatics the type of aromatic compound in the fuel strongly affects the PAH emission. With unleaded fuels containing aromatics the quantity of PAH’s emitted increased with increasing number of carbon atoms (6 in benzene; 8 in xylenes; 9 and 10 in straight line substituted benzene). The amount of BaP, for instance, was 11 times higher in the emission of fuel at 48 per cent aromatics of the type C, and C, 0, than in the emission when the engine was burning fuel confining xylene, and 13 times higher than when the fuel contained benzene. These findings are consistent with Badger’s theory (1962) according to which C6-C4 fragments are intermediate compounds which are closely involved in the formation of 3 : 4benzpyrene which occurs in pyrolysis and in incomplete combustion (Zaghini et al., 1973). ~C~now~e~ge~en~s-The authors wish to thank the Petroleum Products Laboratory of Snam Progetti for providing the samples of tar produced from a European engine, and the Ente Nazionale Idrocarburi (ENI) for supporting the whole research. REFERENCES Badger G. M. (1962) Mode of formation of carcinogens in human environment. Natl. Cancer Instit. ~onogrup~ 9, i-16. Begeman C. R. (1962) Carcinogenic Aromatic Hydrocarbons in Automobile ESJIuents. SAE Technical Progress Series, Vol. 6. McMillan, New York 1964. (Presented to Automotive Engineering Congress Detroit, Michigan, January 1962). Begeman C. R. and Colucci .l. M. (1962) Apparatus for determining the contribution of the automobile to the benzene-soluble organic matter in air. Natl. Cancer Instit. Monograph 9, 17-57. Begeman C. R. and Colucci J. M. (1968) Benzo(a)pyrene in gasoline partially persists in automobile exhaust. Science 161,271.
Begeman C. R. and Colucci J. M. (1970) Polynuclear aromatic hydrocarbons emissions from automotive engines. SAE Trans. 79,1682-1698.
Boubel R. M. and Ripperton L. A. (1963) Benzo(a)pyrene production during controlled combustion. J. Air Potlut. Control. Ass. 13,553-557.
Candeli A., Mastrandrea V. and Savino A. (1967) Il problema della cancerogeniciti dell’aria inquinata: II-Analisi dell’atmosfera della galleria di Perugia. Riu. Ital. Ig. 27,165-174. Candeli A. and Morozzi G. (1969) IJ problema della cancerogenicita dell’aria inquinata: III-Tecnica dei traccianti radioattivi per la determ~n~ione quanti~t~~ degli idrocarburi aromatici policiclici. Giorn. lg. Med. Prev. l&3-17.
Candeli A., Mastrandrea V. and Morozzi G. (1970) idrocarburi aromatici policiclici nei gas di scarico di un motore a combustione interna alimentato con diverse benzine de1 commercio. Riu. Ital. Ig. 29/30, 190-204. * Federal Register, Department of Health, Education and Welfare. 33, No. 108, Part II (4 June 1968).
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