I did some quick and dirty calculations to find out which is better for the environment in regards to mercury emissions. Incandescent, CFL, or LED.
First of all, we know that the average electricity production in the US is 57% coal. Also, coal power plants emit on average 0.06 milligrams of mercury per kWh. Finally, CFLs contain about 4.5 milligrams of mercury and last about 1000 hours.
Therefore, using 60 watt incandescent bulbs over a period of 1000 hours would emit 2.05
milligrams of mercury.
A 20 watt CFL would emit only 0.68 mg but we have to add the 4.5 mg contained in the bulb, so a total of 5.18
milligrams of mercury over the same time period.
Currently, LED bulbs are roughly the same efficiency as CFLs but contain no mercury, so an LED bulb would emit only 0.68
This is assuming the power source is 57% coal and the remainder of the power mix is mercury-free. This is also assuming the bulbs are disposed of in the landfill (CFLs are supposed to be disposed of in hazardous waste but nobody bothers to do so).
The result: In terms of mercury emissions the LED bulb comes out ahead of the pack with the least mercury emissions by far. LEDs currently seem very expensive until you consider that they last up to 10 times longer than even CFLs. Second is the incandescent, and the CFL is worst. The solution: Upgrade to the latest lighting technology: LEDs
Sources: http://www.powerscorecard.org/tech_deta ... ource_id=2 & http://www.myledlightingguide.com/Artic ... ticleID=19