How should you dispose of an old thermometer?

It depends on the thermometer. If the thermometer is made of glass and filled with a silver liquid, that liquid is Mercury. Your local environmental protection organization might be able to tell you where you could safely dispose of it. Otherwise, you might check with companies that deal with scrapping out old cars, which contain mercury switches, or fluorescent bulbs, which contain mercury.

If the thermometer does not have a silver liquid in it, it doesn't contain much or any mercury, and it should be possible to toss it with your normal household trash. When a glass thermometer has a red or blue liquid, the liquid is alcohol or, rarely, some other organic liquid such as kerosene. It's not that much of a threat. Metal coil thermometers with dial pointers are just scrap metal when you need to dispose of them. Electronic thermometers have batteries, which might have some mercury but might not. If in doubt, take the battery out and put it in a battery recycling bin. If you must throw it out in the trash, at least the amount of mercury in it (if any) will be small. The circuits themselves contain the same contaminants and heavy metals that any piece of electronics would, but since the amount of circuitry in an electronic thermometer is small, the amount of waste in one is small also.