When a thermometer breaks, the amount of liquid mercury it contains can contaminate all the fish in a 20-acre-lake as well as cause neurological damage to people. Many states have banned the sale of mercury thermometers. Using digital thermometer is a better alternative. The problem though is that people are just throwing out their mercury thermometers in with their normal trash. And that can lead to contamination.
So check out to do if you have any breakage for safe clean up here and then check out this link for how to dispose of your mercury thermometers safely. If you can't find one in the information, contact your local hazardous waste hauler for information on where to dispose of mercury thermometers safely.