The environmental benefits of switching to compact fluorescent lights are substantial.
For example, using a single 20-watt CFL instead of a 75-watt incandescent bulb saves over 550 kWh of electricity over its lifetime. If your electricity is produced from burning coal, then using a CFL would save 250 kg of coal that would release 650 kg of carbon dioxide and 10 kg of sulphur dioxide, not to mention a variety of other harmful pollutants.
Although containing mercury, CFLs present an opportunity to prevent mercury from entering the air. The highest source of mercury to the environment comes from burning fossil fuels such as coal. CFL use 80% less energy than incandescent bulb and lasts about 10 times longer. A power plant will emit 14 mg of mercury to produce the electricity to run an incandescent bulb compared to only 2.4 mg of mercury to run a CFL for the same time.