Lead-free but not problem-free: human health risks from gasolines. (Science Selections).

Since 1973, many countries around the world have begun phasing out the use of leaded gasoline in automobiles. The purpose of phasing out leaded gas is to decrease the amount of lead in the atmosphere, thereby eventually decreasing the amount of lead in human blood, which has well-documented adverse effects on health. Unleaded gasolines are largely replacing the leaded formulation, but there is some question as to whether these fuels pose new and different hazards to human health. In this month's issue, a group of Taiwanese researchers led by Hsiao-Hsuan Mi find that certain unleaded gasoline formulations can produce more toxic polycyclic aromatic hydrocarbons (PAHs) than unleaded formulations [EHP 109: 1285-1290]. The group studied emissions from three types of gasoline in use in the Taiwanese market--premium leaded gasoline (PLG), 92-octane lead-free gasoline (92-LFG), and 95-octane lead-free gasoline (95-LFG).

Many PAHs have been classified as probable or possible carcinogens by the International Agency for Research on Cancer. Since different PAH compounds have different levels of carcinogenic potency, or likelihood of inducing tumor growth, engine exhaust components were analyzed in terms of both their raw PAH content and their potential health risk using a formula based on the carcinogenic potency of the very toxic PAH benzo[a]pyrene (BaP).

The toxic equivalency factor--or relative carcinogenic potency--of each of the 21 PAH compounds found in the test emissions was compared to that of BaP. This yielded a measurement of each compound's BaP equivalent concentration or Ba[P.sub.eq]. PAH and Ba[P.sub.eq] concentrations were then summed to arrive at total concentrations for each of the three gasoline formulations. After running tests using the same engine, running speeds (idling, 40 km/hr, 80 km/hr, and 110 km/hr), and sampling and analysis methods, the authors then extrapolated their results to annual consumption figures for the three formulations in Taiwan for the years 1994-1999.

Significant differences were found in the amounts of PAHs and Ba[P.sub.eq] concentrations in the exhaust emissions. The 95-LFG contained the highest PAH levels and Ba[P.sub.eq] concentrations, followed by the PLG, and then the 92-LFG. As the authors state, logically this would imply that when PLG is replaced with 95-LFG, PAH and Ba[P.sub.eq] emissions would increase, whereas when PLG is replaced with 92-LFG, those emissions would decrease.

That conclusion was borne out when the laboratory data were applied to annual Taiwanese consumption rates. During the six-year period examined, PLG consumption dropped substantially, 92-LFG consumption rose steadily, and 95-LFG consumption rose sharply to nearly three times the consumption of the less hazardous 92-LFG. The authors' calculations showed that Taiwan's total PAH and Ba[P.sub.eq] emission rates consistently increased during the same period. (Whether health effects increased as well was not addressed in this study.)

Although Mi and his colleagues are careful to limit their conclusions to the situation in Taiwan and suggest further studies in other areas with other gasoline formulations, the implication of their work is clear. The threat to human health posed by leaded gasoline may be receding into the past, but the unleaded formulations that have replaced it bring with them new health risks. The good news, however, is that these risks can be mitigated through this type of analysis and appropriate subsequent policy adjustment.