What is the problem with MTBE in gasoline?

If you have ever pumped gas that claimed to "oxygenated" -- something that is common in most urban areas in the winter -- then you have used gasoline containing MTBE. MTBE is the acronym for methyl tertiary butyl ether, a fairly simple molecule that is created from methanol. Click here to see MTBE's chemical structure.

MTBE gets added to gasoline for two reasons:


  • It boosts octane (see this Question of the Day for a discussion of octane).
  • It is an oxygenate, meaning that it adds oxygen to the reaction when it burns (see this Question of the Day for a discussion of oxidizers). Ideally, an oxygenate reduces the amount of unburned hydrocarbons and carbon monoxide in the exhaust.

MTBE started getting added to gasoline in a big way after the Clean Air Act of 1990 went into effect. Gasoline can contain as much as 10% to 15% MTBE.

The main problem with MTBE is that it is thought to be carcinogenic, and it mixes easily with water. If gasoline containing MTBE leaks from an underground tank at a gas station, it can get into groundwater and contaminate wells. Of course, MTBE isn't the only thing getting into the groundwater when a tank leaks -- so is gasoline and a host of other gasoline additives, but in recent years, MTBE has been singled out.

According to this page at the EPA:

Although there is no established drinking-water regulation, USEPA has issued a drinking-water advisory of 20 to 40 micrograms per liter (µg/L) on the basis of taste and odor thresholds. This advisory concentration is intended to provide a large margin of safety for noncancer effects and is in the range of margins typically provided for potential carcinogenic effects.

The most likely thing to replace MTBE in gasoline is ethanol -- normal alcohol. It is somewhat more expensive than MTBE, but it is not a cancer threat.