2. Relevance to Public Health 2.1 Background and Environmental Exposures to Aldrin and Dieldrin in the United States

Aldrin (C12H8Cl6) and dieldrin (C12H8Cl6O) are two organochlorine insecticides that were used for agricultural and public health purposes from the early 1950s until 1989, when their manufacture in the United States was discontinued. Aldrin and dieldrin were popular pesticides for corn and cotton crops, and were used as a prophylactic and for treatment of timber against termite infestation. Consistent with their intended use on insects in soil, aldrin and dieldrin are not very water soluble, but readily bind to sediment and are rarely leached into deeper soil layers and groundwater. As they take decades to break down in the environment, past agricultural uses of aldrin and dieldrin have resulted in persisting soil residues and uptake in a wide range of crops. In biological systems of soils, plants, and animals, aldrin converts rapidly to dieldrin by a microsomal oxidation reaction (epoxidation). The half-life of dieldrin in temperate soils is about 5 years, while it disappears more quickly (up to 90% in 1 month) from tropical soils. Organochlorine pesticides, including dieldrin, continue to enter streams in the United States from atmospheric deposition and erosion of soils contaminated from past use. Aldrin and dieldrin may be volatilized from sediment and redistributed by air currents, contaminating areas far from their sources. Nationally, levels of aldrin and dieldrin have declined since their agricultural uses were discontinued. Aldrin bioconcentrates in mollusks and fish, and high levels of dieldrin have been found concentrated in fish, snails, and lake trout. Detectable dieldrin concentrations in fish have shown a strong association with corn production acreage.