In cars, it makes gasoline burn cleaner and reduces air pollution. In people, it causes illness and may cause cancer.

A lot of cars run on gas with MTBE in their tanks - an estimated 70 percent. And in Florida, too many people have dangerous levels of MTBE in their wells and drinking water.

Oil companies developed the chemical in the '70s to replace lead in gasoline. It works by adding oxygen to the gas and increasing octane, which reduces tailpipe emissions by 40 percent. The Environmental Protection Agency embraced MTBE about five years ago as an effective way to reduce air pollution, but it was slow to look at what would happen if the chemical leaked from underground storage tanks into water supplies. Now, that's exactly what's happening.

Traces of MTBE have been found in more than 2,400 locations in Florida in the past decade. Levels of MTBE were high enough to smell in 767 well or underground water samples. Hillsborough County heads the MTBE list with 96 contaminated wells, nearly a third of which exceeded federal limits. In those cases, workers had to install special filters or disconnect the well line and hook homeowners to city water lines. Health officials have described some water samples that smell like raw gasoline.

Volusia County has reported only scattered instances of MTBE contamination. So far, there is no indication the gas additive has seeped into any municipal water systems. But the state is potentially awash in MTBE.

More than 2,000 public water supply wells in the state are within about a half mile of leaking underground storage tanks, reaching from one end of Florida to the other. Volusia County has cleaned up about 100 gas stations, but has another 200 stations on its waiting list.

Underground tanks, however, aren't the only source of MTBE. Any place with gasoline is a potential threat to water supplies - landfills, junkyards, farms and auto repair shops. MTBE has two especially dangerous characteristics. Unlike benzene, MTBE degrades slowly, which allows it to stay in the ground a long time. MTBE also dissolves easily in water, quickly traveling underground and contaminating wells.

State health officials aren't doing nearly enough to monitor MTBE contamination in private wells or guard the purity of municipal systems. After years of stalling, the state also must adequately fund its program to clean up old and leaking underground gas station tanks.

Even if the EPA finally gets around to replacing MTBE, which the Clinton administration now backs, Florida could face another dangerous decade before it's MTBE-free. Until then, it's up to Florida's health officials to protect residents and their water supplies from MTBE poisoning.