MTBE

MTBE (methyl tertiary-butyl ether) is a chemical that has been a component of gasoline since 1979, when it was introduced in gasoline to take the place of lead as an octane enhancer. It was not until 1992, however, that the chemical became such a large part of gasoline by volume. In 1992, the amendments to the Clean Air Act mandated that non-attainment areas for the ambient standards set by the Act use reformulated gasoline. The first program implementing this mandate was the Winter Oxyfuel Program, followed by the Year-Round Reformulated Gasoline Program. Reformulated gasoline (RFG) must contain 2% oxygen by weight; the presence of increased oxygen in gasoline helps it to burn cleaner. Often, this additional oxygen is found in gas through the use of an oxygenate, a chemical designed to hold oxygen. Though the first reformulated gasoline regulations did not contain a mandate to include an oxygenate in gasoline, the federal RFG program does mandate the use of this type of chemical. MTBE is just such a chemical. 15% MTBE by volume equals about 2% oxygen by weight. Although the use of MTBE is not specifically mandated by the Clean Air Act, it is the most common oxygenate in use (3).

Almost immediately after the proportion of MTBE in gasoline increased, concern was voiced about the health effects of MTBE inhalation from vapors released from gasoline during vehicle fueling. It was not until 1995, after a major water system in Santa Monica, CA became contaminated, that concern about ingestion of the chemical in drinking water became more pronounced.

MTBE turned out to be such a threat to drinking water because of its chemical properties. It is hydrophilic and fast moving—it is attracted to water, and as such moves more quickly through the ground than other components of gasoline. Since gasoline is stored in underground storage tanks, if such a tank leaks, the MTBE present in the gasoline is attracted to and begins moving towards the nearest water source geologically possible. As underground storage tanks have a finite lifespan and there is much precedent for concern about leaking, there are a host of federal regulations established by the EPA for tank owners to comply with to try to prevent leaks. Even so, tanks do leak and MTBE (along with other gasoline components) is released into groundwater.

Limited studies have tried to elucidate and quantify MTBE's effect on health. At high doses, studies have shown MTBE can cause cancer in laboratory animals. Studies on human health have only been conducted with regard to other symptomatic effects such as headaches, dizziness, and nausea in workers exposed to MTBE fumes (not to MTBE in drinking water) (4). There is disagreement as to the value of these studies. Since 1994, the EPA has listed MTBE as a possible human carcinogen.

Quick MTBE facts:

• gasoline component
• introduced in 1979
• replaced lead as an octane enhancer
• increased amount in gas after 1990 Clean Air Act amendments
• thought to reduce emissions by making engines burn cleaner
• initially supported by many environmental groups because of alleged air quality benefits
• has contaminated private and public water supplies across the country
• concern about potential health effects
• listed as a "possible human carcinogen" by the EPA
• attempted bans by some states, including Maine and California