Effects of winter gasoline additives on air quality and health.

A chemical added to gasoline to reduce carbon monoxide pollution appears not to pose a substantial human health risk, a National Research Council committee said in a review of a draft report from the federal government. More definitive data are needed to assess short-term health effects and to determine if this additive is effective in reducing carbon monoxide pollution in cold temperatures, the committee said.

In areas of the country that have not met air quality standards for carbon monoxide, federal law requires the use of additives that increase the level of oxygen in gasoline during winter months, when lower temperatures tend to cause vehicles to emit more carbon monoxide. A higher level of oxygen can cause the fuel to burn more cleanly. Available data indicate that the extent to which oxygenated fuels reduce winter air concentrations of carbon monoxide has been zero to about 10 percent, the committee said.

At the request of the Environmental Protection Agency (EPA), the committee reviewed a draft of a federal report that assesses the effects of oxygenated fuels on public health, air quality, fuel economy, engine performance, and water quality. The committee determined that much of the federal report adequately represents what is known about the effects of methyl-tertiary-butyl ether (MTBE) - the most commonly used additive in the federal oxygenated fuels program on health, the environment, and motor vehicles. In addition to evaluating the report's scientific basis, the committee identified research needed to better understand the impacts of oxygenated fuels.

"The effects of MTBE and other oxygenates are still relatively unknown, even though they now are used widely in the United States," said committee chair Bailus Walker, professor of environmental and occupational medicine, Howard University Cancer Center, Washington, D.C. "Specific, well-targeted research is needed to answer the questions about potential trade-offs in using these chemicals."

Because of limited data, there was less emphasis placed on other oxygenated fuels in the government's draft report. Other compounds used as oxygenates are ethanol, tertiary-butyl alcohol, ethyl-tertiary-butyl ether, and tertiary-amyl-methyl ether.

Health Effects Research Needed

The federal government's oxygenated fuels program, which was mandated in the Clean Air Act Amendments of 1990, took effect in 1992. MTBE has since been implicated in complaints about short-term health effects - such as headaches, coughs, and nausea - by residents of Alaska, Montana, New Jersey, and Wisconsin. EPA and other organizations have conducted research in recent years in an attempt to address these complaints, ass well as concerns about fuel economy, engine performance, and water quality.

The government's draft report says, "limited epidemiological studies and controlled exposure studies conducted to date do not support the contention that MTBE as used in the winter oxygenated fuels program is causing significant increases over background in acute symptoms or illnesses in the general public or workers." The committee expressed concern that these studies of short-term health effects attributed to MTBE exposure suffer from poor design, inadequate assessment of exposure, insufficient numbers of people studied, subjective interpretation of test results, and the possibility that test subjects were not typical of the general population.

The committee also said there are no data to indicate that the reported short-term health effects are confined to a sensitive subpopulation, noting that the federal report's conclusion that a small percentage of the population may be sensitive to MTBE alone or in gasoline appears to ignore consistent data showing an increase in short-term health problems among workers exposed to MTBE on the job. Workers for whom there is consistent evidence of an increase in symptoms due to MTBE exposure should be studied in more detail, the committee said.

The committee agreed with the federal report that MTBE exposures are not likely to be linked to serious health risks, such as cancer. However, some of the animal studies used to reach that conclusion have deficiencies and may not be relevant to human health. Cancer estimates based on animal models should be questioned until investigations recommended by the committee can be undertaken. The committee concurred with the government report that adverse reproductive and developmental effects are not expected to result from typical exposures to MTBE.

While it agreed with the government that fuels containing MTBE do not appear to pose health risks substantially greater than those associated with conventional fuels, the committee noted "important deficiencies" in the analysis used in the government's report to assess human health risk from exposure to MTBE. The report's failure to provide some indication of the magnitude of comparative risk associated with each type of fuel "is a serious deficiency and should be corrected," the committee said.

Cold-Weather Air Pollution Effects Unclear

Carbon monoxide levels have been reduced in the past 20 years, and automobile emission controls have been a contributing factor. However, it is important to determine to what extent oxygenated fuels have reduced carbon monoxide, the committee said. These fuels decrease emissions at warmer temperatures, but the current data are unclear about the fuels' effectiveness in reducing carbon monoxide in colder temperatures, especially under 20 [degrees] F. Available data also suggest that the use of these fuels might increase emissions of another pollutant, nitrogen oxides. A carefully controlled, statistically well-designed field study should be performed in cold temperature conditions to clarify the effectiveness of oxygenated fuels at reducing pollutants in cold weather. Differences in the design, operation, and maintenance of motor vehicles also can affect the amount of carbon monoxide emitted and should be considered in such research.

Mainly caused by the incomplete burning of motor-vehicle fuels, carbon monoxide pollution is a particular concern for people with cardiovascular disease. The committee agreed with the government's draft report that not enough data exist to determine whether the oxygenated fuels program has been successful in reducing carbon monoxide pollution to a level where cardiovascular-related disease and death rates are not affected.

The large majority of states do not monitor the presence of MTBE or other fuel oxygenates in storm-water runoff, ground water, or drinking water. Where some data are available, MTBE has been detected in less than five percent of the ground-water samples analyzed. Although this suggests that much of the population is not exposed to MTBE-contaminated drinking water, the lack of monitoring data prevents an adequate assessment of human exposure to MTBE, the committee said.

The committee supported the government report's contention that the use of oxygenated fuels causes a two percent to three percent loss in fuel efficiency, and typically does not adversely affect engine performance.

Despite uncertainties in making estimations, the committee encouraged the government to address and document the costs and benefits of the winter oxygenated fuels program, at least at a broad level.

The study was funded by the Environmental Protection Agency. The National Research Council is the operating arm of the National Academy of Sciences and National Academy of Engineering. It is a private, non-profit organization that provides science and technology advice under a congressional charter.