Compact florescent lights or CFLs require mercury to operate. The mercury gas contained within the tube gets excited when electricity is passed through it and emits UV light which excites the coating of phosphor lining the tube, generating light. Though manufacturers have been making steady progress on the amount of mercury required for CFL bulbs, most on the market still contain the deadly toxin. Because of this, it might be easy to favor the incandescent bulb which contains no mercury. But consider this: the source of most of America's electricity, burning coal, releases lots of mercury into the atmosphere. The mercury generated from the extra electricity needed over the lifetime of an incandescent bulb is far more than the amount found in your average CFL bulb.