Compact fluorescent light bulbs are highly energy efficient, and contain just five milligrams of mercury on average. Incandescent bulbs release more than twice as much mercury into the environment than CFL bulbs. However, because mercury is a neurotoxin and an environmental contaminant, it is preferable to look for bulbs with lower mercury content. Greener One also encourages you to read Energy Star's tips for disposing CFLs and handling broken bulbs: http://www.energystar.gov/ia/partners/promotions/change_light/downloads/Fact_Sheet_Mercury.pdf