Gasoline Additives: How do the ethanol and nitrogen in your gas affect the environment?

Illuminating answers to environmental questions.
March 8 2011 6:59 AM

Ethanol, Nitrogen, MTBE, Lead

Are gasoline additives getting any better for the environment?

Gas pump. Click image to expand.

Gasoline companies have been pushing engine-cleaning formulations lately. Setting aside the benefits for my car, are these additives good for the environment?

Gasoline is a complex mixture of, well ... stuff. The thick crude oil that comes out of the earth bears little resemblance to the gasoline powering your Porsche. In order to make crude safe and useful for cars, refiners must add, alter, and remove hundreds of chemicals. These exotic molecules include long chains of carbons, detergents, and chemical enhancers that supposedly make gasoline cheaper, greener, and more efficient—values that are often at odds with each other.

To appreciate the role these additives play, however, we need a basic understanding of hydrocarbons, the main ingredient of both crude oil and gasoline. Hydrocarbons consist of hydrogen and carbon atoms only, but in a dizzying variety of permutations, which scientists name after the number of carbon atoms each has. Crude oil contains lots of different forms—from methane, the simple one-carbon molecule, to 85-carbon chains with complicated branching patterns. Refiners have to break up the longer chains and eliminate some of the shorter chains to get a mixture that consists mostly of four- to 12-carbon molecules.

Refiners particularly prize octane, an eight-carbon molecule. Internal combustion engines work by compressing a mixture of air and gasoline in a closed cylinder, then igniting the mixture to create a controlled explosion. But if you compress the mixture too much, it can combust on its own before the spark comes along. This phenomenon is called knocking, and it's a quick way to destroy the engine.

Octane is highly compressible, which helps avoid knocking. That's why higher octane ratings at the pump are better for high-performance cars, which need higher compression to generate more power. Eighty-seven octane, for example, means the gas is 87 percent octane or at least behaves like gasoline with that much octane.

That second clause is crucial, because, in reality, 87 octane gas—or 89, 91, and 93 octane gas, for that matter—never contains that precise percentage of octane. It's too challenging and expensive for refiners to reach that composition consistently. Instead, they're allowed to add foreign chemicals to the gasoline to get lower-quality fuel to behave like 87 octane.

One of the first gasoline additives used for this purpose was tetraethyl lead, which turned out to be an environmental disaster. Everywhere it went, the original octane enhancer left a nasty cloud of lead—which, as you might have heard, is really, really bad for our health and the environment. In the 1970s, the Environmental Protection Agency began forcing refiners and importers to reduce the amount of lead in their gasoline; by 1996, the EPA had banned lead from automobile gasoline entirely. Over those two decades, the regulations reduced lead levels in American children's blood by 70 percent. (Refiners can still add lead to jet fuel and boat engines.)

As the EPA began phasing out lead, refiners replaced it with a chemical called methyl tertiary-butyl ether. MTBE presents a fascinating environmental quandary. On the one hand, it helps gas burner cleaner. In 1989, with several U.S. cities gripped by smog, the EPA mandated that gasoline producers find a way to solve the problem, and MTBE proved to be the most effective chemical. In some areas, gasoline was 15 percent MTBE. Demand for the chemical nearly doubled between 1990 and 1994, and it became the second-most manufactured chemical by volume in the United States.

But while MTBE was great for urban air, it was bad for the water supply. When gasoline leaked from tanks, the MBTE mixed easily with water and soil and biodegraded much slower than gasoline's other ingredients. The EPA found that MTBE had contaminated  five to 10 percent of drinking water samples where the chemical was being used heavily.

Toxicologists aren't sure how dangerous MTBE is at these concentrations. While some people can reportedly taste its turpentine-like flavor in drinking water at very low levels and claim that it causes nausea and dizziness, researchers haven't been able to definitely link MTBE to any particular symptoms. Though the chemical causes cancer in laboratory animals and is classified as a "potential human carcinogen" by the EPA, it might take more MTBE than you'd get from drinking water to cause cancer in humans. Most states have banned or sever e l y restricted MTBE use, and public water utilities have successfully sued oil companies to remove it from soil and groundwater.

Today's most common alternative to MTBE is ethanol. Like MTBE, ethanol increases the oxygen content of gasoline, which is supposed to lead to cleaner burning. But ethanol raises its own environmental hazards. While some claim increasing the amount of ethanol in gas could reduce our reliance on foreign oil and trim greenhouse gas emissions, it would also release higher levels of certain volatile organic compounds known to harm human health, such as acetaldehyde and benzene. A 2005 literature review suggested that E10, a gasoline mixture including 10 percent ethanol, actually produces more smog than ordinary gas.

Nitrogen is another key additive. A few years ago, some companies started pushing nitrogen-enriched gasoline, emphasizing its ability to clean a car's engine. It's not really a new idea—the EPA has required that a certain level of detergent, such as nitrogen, be added to a fuel for years. It's also not clear whether the new gasoline makes a significant difference to an engine's performance or longevity. While there's little research on the subject, the added nitrogen probably doesn't significantly alter gasoline's environmental impact. Internal combustion engines do release nitrogen oxide, a smog-forming gas. But the small amounts of nitrogen being added to gasoline make a negligible contribution—most of the nitrogen in that nitrogen oxide comes from ambient air, not the gasoline itself.

But let's try to keep some perspective. Though gasoline additives are worrisome, fretting over nitrogen in your gas tank is a bit like cutting cookies from your diet because of the preservatives. Why worry about additives, when there are so many other reasons to cut down on gasoline use?

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Brian Palmer is Slate's chief explainer. He also writes How and Why and Ecologic for the Washington Post. Email him at explainerbrian@gmail.com. Follow him on Twitter.