The one topic of concern when it comes to CFLs is the mercury they contain and how to dispose of them.
FACT: Using CFLs instead of standard incandescent lights actually results in a net reduction of mercury releases through less coal use. This is because 87% of US mercury emissions are from coal burning power plants & incinerators. By using a CFL which requires less energy from the coal burning power plants and incinerators, you are reducing the amount of mercury released into the atmosphere. The trace amount of mercury in the CFL plus the mercury emitted form power plant to light the CFL is less than the amount of mercury released from coal burning power plants to light an incandescent.
FACT: Mercury is a hazardous material that can cause damage to the nervous system and kidneys among other ailments. While the minimal amount of mercury found in a CFL is not dangerous in and of itself, it is the accumulation of mercury that may become dangerous. This is why it is important to dispose of CFLs properly. CFLs that are thrown in the garbage rather than recycled or disposed of as hazardous waste may leach into the groundwater and bioaccumulate within fish. Mercury levels found in fish has been on the rise and has spurred concerns of mercury poisoning among fish-centric cultures.
Check with your local solid waste agency for proper disposal suggestions. Many locations will have you dispose of your CFLs in the same manner you would unused paint while others may suggest local recycling centers. Additionally, some local utilities and lighting suppliers and retailers offer recycling options.