Fluorescent lamps use less electricity than conventional incandescent lights, which is beneficial to the environment. All fluorescent lamps, however, contain mercury. Most compact fluorescent lamps (CFLs) contain 3 to 5 mg of mercury, with some brands, such as Earthmates, containing as little as 1 mg.

Mercury
Mercury is a toxic metal associated with contamination of water, fish, and food supplies. While compact fluorescent lamps contain low levels of mercury, because electricity production from fossil fuel power plants results in atmospheric emissions of mercury, reducing electricity consumption through use of CFLs should result in a net reduction of mercury into the environment, especially true if the CFL is kept out of the waste stream when the lamp expires.

Disposal
Products containing mercury, such as CFLs, should be disposed of in accord with local laws and regulations and not disposed of with regular trash. Take expired compact fluorescents to a local recycling center, your community's hazardous waste collection event, or recycle them through a recycling service such as Veolia ES.

Additional Information: Energy Star Mercury Fact Sheet (pdf)
Additional Information: Earth911.com
Additional Information: LampRecycle.org
Additional Information : How to Dispose of Compact Fluorescent Lamps (pdf)
Additional Information : Maine Compact Fluorescent Lamp Breakage Study