The toxicity of mercury to humans is dependent upon its form, concentration and exposure pathway.

Mercury emissions into the environment result from both man-made and natural sources. Natural sources of mercury emissions include volcanic activity, forest fires, and the off-gassing of soils, rocks and the oceans. There is no direct scientific evidence of the total amount of naturally-released mercury. However, estimates range from 13 million pounds per year to 36 million pounds per year.

Man-made mercury emissions are estimated to represent less than half of the total mercury emitted into the environment. Estimates vary, but approximately 5 million pounds per year are believed to come from man-made sources.

Man-made sources of mercury emissions include chemical and industrial processes, metal smelting, home heating oil, medical waste incinerators, coal-fired utility boilers, agricultural operations, and solid waste disposal facilities.

Municipal solid waste combustion was once one of the largest known sources of mercury emissions. However, today, because of reduced mercury in consumer products and advanced pollution control devices on municipal waste combustion facilities, these facilities are an insignificant source of mercury emissions into the environment.

Health risk assessments completed over the past several years for new and existing waste-to-energy plants consistently reveal that the levels of mercury emissions result in exposures which are 100 times less than the threshold health effects standard established by federal and state regulatory agencies.

Mercury can be absorbed through inhalation, skin contact or ingestion. Mercury can travel a long way from the point of release, settle in the soil and drain into surface water bodies. Once in water, Mercury is converted to methylmercury by bacteria. Water borne creatures pass this toxin up the food chain. The concentration of methylmercury increases the higher you go in the food chain.

Low exposure to mercury can cause immune system dysfunction, muscle tremors and irritability. High exposure can cause speech and hearing problems, respiratory problems and sometimes death. Fetuses and young children are at greatest risk due to the ongoing development of their nervous systems. They could suffer from developmental problems in learning, walking, talking and birth defects.

Source reduction, product reformulation, and increasingly effective battery recycling programs have reduced mercury in trash by about 90% since 1990, according to the EPA. Pollution control equipment on waste-to-energy plants thereafter remove about 90% of the remaining mercury in the waste stream that is used as a fuel to generate power. Currently, AAA, AA C, D and 9 volt batteries are manufactured mercury-free.

The use of mercury by U.S. manufacturers will decline even further due to the virtual elimination of mercury from alkaline batteries and aggressive recycling and product substitution at hospitals, homes, and businesses.