A thermometer may be an essential part of any healthcare kit, but this simple instrument could actually make you sick. Break one and the elemental mercury within can evaporate, becoming an indoor air pollutant. Toss one away and the discarded mercury might end up in a waste incinerator’s smokestacks, which will spread the poison over a huge area. Or if it goes to a landfill, it can creep into local water supplies instead.

When airborne mercury eventually settles in a body of water, bacterial metabolic processes take over and transform it into more dangerous methylmercury, a compound that climbs through the food chain, from bacteria to plankton to fish. As larger predators like shark, tuna, and swordfish consume mercury-tainted prey, the poison accumulates in their muscle fiber and gets passed on to people who eat these fish.

The majority of fish-consumption advisories in the United States are issued because of mercury contamination. Forty states warn people against eating fish from their waterways, and Consumer Reports cautions parents not to serve their kids more than one tuna-fish sandwich a week. Pregnant women should also be concerned about the fish in their diet: The Centers for Disease Control found that one in ten women of childbearing age in the United States is at risk of having a newborn with brain damage because of mercury consumption. The federal government has been slow to protect consumers, says Michael Bender, executive director of the Mercury Policy Project, who notes that the Food and Drug Administration’s 20-year-old standards were based on “safe” doses for adults. Stronger rules would protect both children and adults from the potential ill effects of mercury exposure, which include nervous-system damage, sensory impairment, numbness or tingling in the extremities, and loss of coordination.

Can a little thermometer really cause such problems? If evaporated into the atmosphere and precipitated back down, a mere gram of mercury—about as much as in one thermometer—is enough to contaminate a 20-acre lake. While coal-burning power plants are the single-largest source of manmade mercury emissions, the EPA reports that medical- and municipal-waste incinerators together account for one-third of the 150 tons of annual mercury pollution in the United States. (Other mercury-laden refuse from hospitals includes blood-pressure-measuring instruments, gastrointestinal devices, and preservatives. Mercury can also be found in cooking and indoor/outdoor thermometers, not just the medical kind.)

The insidious metallic liquid, which gets its name from the Roman messenger god, is sending us a message: It’s time to quit using mercury, especially since safer options are available. Digital thermometers and alcohol-filled glass models are just as accurate as mercury ones, and many hospitals are already making the switch. (The button-cell batteries in digital thermometers do contain mercury, but only about one percent of the amount in the old-fashioned kind.) Mercury thermometers may still be widespread, but a safe transition is possible. “Sweden phased out mercury thermometers in 1992 without any problems from industry or the medical community,” says Bender.

The economic arguments against using mercury in thermometers are as strong as the environmental ones. While a pound of elemental mercury—enough for about 450 thermometers—can be purchased for one or two dollars, it can cost $5,000 to scrub that same pound from incinerator stacks. A move away from mercury in industrial and consumer uses would especially benefit the developing world, where health and safety regulations are even less stringent. The Mercury Policy Project is working to ban mercury exports and phase out its use worldwide. There’s also plenty of work to do at home, and a handful of local governments are taking good first steps. Michigan has banned mercury in schools; the city of San Francisco and the state of Minnesota have banned the sale of mercury thermometers entirely. Similarly strong limits are currently wending their way through the legislative process in New York, where a bill would restrict sales of mercury products (banning mercury thermometers) and mandate aggressive collection and disposal efforts. At the federal level, Senator Susan Collins (R-Maine) worked with the Mercury Policy Project and other groups to introduce legislation in February that would fund thermometer exchanges and identify safe disposal methods for the nation’s mercury stockpile.

You can act locally by calling your state pollution-control agency or local health department to find out where to responsibly dispose of mercury thermometers and how to safely clean up any spills. (Health Care Without Harm offers cleaning tips on its Web site.) Then pop down to your local drugstore and buy a digital replacement. You’ll be on the road to good health in no time.