the gaseous emissions, the total role of vehicle exhaust in causing photochemical smog and other types of air pollution cannot be assessed completely until these factors are investigated and their importance in the entire picture is either established or proven to be negligible. The presence of polynuclear aromatic compounds in vehicle exhaust brings up another question related to the lead content. Various suggestions have been made in the past for reducing substantially the lead content of motor fuel, even though, as the discussion at this symposium indicates, there is no clear evidence that the present atmospheric lead levels are detrimental to human health. It is generally accepted that we should maintain a margin of safety with respect to health hazards, and various measures are frequently taken to reduce suspected health hazards before scientists accumulate all of the data required to understand the various potential hazards. In this case, however, there is a compensating difficulty which has not been adequately discussed in connection with these suggestions for limiting the use of lead in motor fuel. The difficulty occurs because of the high compression ratios which have been used in American automobile engines for a number of years. Many of these engines require gasoline of 90 to 100 octane rating for satisfactory operation. With present technology, only two ways are available to produce the necessary quantities of gasoline with this high octane rating. One is by using tetraethyl lead and other lead-containing organic additives. The other is by increasing the aromatic content of the gasoline, which is usually accomplished by using larger amounts of blending stock obtained by catalytic reforming. While other methods of producing high octane gasoline might be developed in the future, no practical methods are in sight at the present time. Therefore, if lead additives were to be drastically limited, it would be impossible to continue to operate the present types of automobile engines without increasing the aromatic content of motor fuel, at least for quite a number of years. An increase in the aromatic content of fuel inevitably leads to an increase in the polynuclear aromatic content of the exhaust, although this relationship is not necessarily on a one-for-one basis (3). The exact role of polynuclear aromatic compounds in causing cancer is not known and may not be completely understood for many years. However, 3,4-benzpyrene and other polynuclear aromatic compounds do induce tumor formation in experimental animals, which at least points a finger of suspicion at these materials as hazards to human health. Epidemiological evidence would indicate that the hazard is likely small, since it appears that smoking is a much more dominant factor in causing lung cancer than air
pollution (4). Also, polynuclear aromatic compounds in the atmosphere come from a variety of sources in addition to motor vehicles, especially in coal-burning cities (5). While there is no reason to suspect that serious hazards exist at this time, any control measures which would cause a considerable increase in the polynuclear aromatic content of the atmosphere should be viewed with suspicion until more information is available. Therefore, cutting down on the lead content of motor fuel to reduce a suspected but unknown and unproved health hazard might increase another health hazard that is suspected but equally unknown and unproved. Trading one unknown hazard for another hardly seems appropriate, especially since lead additives have been used in motor fuel for over 40 years. After that period of time, and after all the experimental work performed, the hazard must not be too great if scientists still debate the question at meetings such as this. No urgency is evident which requires immediate action, and it would seem prudent to wait, in the hope that a better estimate can be obtained in the next year or two concerning the relative hazards of both lead and polynuclear aromatic compounds. Any necessary control measures could then be initiated and avoid the danger that action taken to reduce a presumed hazard might create a separate but possibly greater hazard. In summary, Dr. Habibi’s experimental system appears to be based on the proper concepts and to be well designed to collect valid samples. Extensive use of such a system should aid in providing data that more fully explain the contributions of motor vehicle exhaust to urban air pollution. Presented at American Chemical Society Symposium, “Air Quality and Lead,” Minneapolis, Minnesota, April 14-1.5, 1969.
Bibliography (1) McKee, H. C., McMahon, W. A., Jr., “Automobile Exhaust Particulates-Source and Variation,” J. Air Pollution Control ASSOC.,Vol. 10, December, 1960, pp. 456-62. (2) Hoffman, D., Theisz, E., Wynder, E. L., “Studies on the Carcinogenicity of Gasoline Exhaust,” J. Air Pollution Control Assoc., Vol. 15, No. 4, April, 1965, pp. 162--5. (3) Boubel, R. W., Ripperton, L. A., “Benzo(a)pyrene Production During Controlled Combustion,” J. Air Pollution Control Assoc., Vol. 13, November, 1963, pp. 553-7. (4) Wynder, Ernest L., Hammond, E. C., “A Study of Air Pollution Carcinogenesis,” Cancer, Vol. 15, No. 1, January-February, 1962, pp.. 79-92. ( 5 ) Haneebrauck. R. P.. von Lehmden. D. V.. Meeker. J. E., Sources of Polynuclear Hydrocarbons in the Atmosphere; U S . Public Health Service Publication No. 999-AP-33, National Center for Air Pollution Control. 1967. \
I
Correction
LEAD AND OTHER METAL IONS IN UNITED STATES PRECIPITATION In this article by Allan L. Lazrus, Elizabeth Lorange, and J. P. Lodge, Jr. [ENVIRON. Scr. TECHNOL. 4, 55 (1970)], the complexing reagent indicated in column 2, should read: “diethylammonium salt of diethyldithiocarbamic acid.”
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