EPA estimates the U.S. is responsible for the release of 103 metric tons of mercury emissions each year. More than half of these emissions come from coal-fired electrical power. Mercury released into the air is the main way that mercury gets into water and bio-accumulates in fish. (Eating fish contaminated with mercury is the main way for humans to be exposed.)
CFLs contain a very small amount of mercury sealed within the glass tubing - an average of 4 milligrams. Because of this, EPA recommends that consumers take advantage of available local recycling options for CFLs. But if the CFL is not recycled and it ends up in a landfill, EPA estimates that about 11% of the mercury in the CFL is released into air or water, assuming the light bulb is broken. This is because most mercury vapor inside fluorescent light bulbs becomes bound to the inside of the light bulb. Therefore, if all 270 million CFLs sold in 2009 were sent to a landfill (versus recycled, as a worst case) - they would add only 0.12 metric tons, or 0.12%, to U.S. mercury emissions caused by humans.