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?