Small amounts of PCDFs can enter the environment from a number of sources. Accidental fires or breakdowns involving capacitors, transformers, and other electrical equipment (e.g., fluorescent light fixtures) that contain polychlorinated biphenyls (PCBs) are known to release high levels of furans formed by thermal degradation. PCDFs are also produced as unwanted compounds during the manufacture of several chlorinated chemicals and consumer products, such as wood treatment chemicals, some metals, and paper products. When the waste water, sludge, or solids from these processes are released into waterways or soil in dumpsites, they become contaminated with PCDFs.
Furans also enter into the environment from burning municipal and industrial waste in incinerators. The exhaust from cars that use leaded gasoline, which contains chlorine, releases small amounts of these compounds in the environment. Small amounts of PCDFs may also enter into the environment from burning of coal, wood, or oil for home heating and production of electricity. Many of these chemicals or processes that produce PCDFs in the environment are either being slowly phased out or strictly controlled.
Furans in air are present mostly as solid particles and to a much lesser extent as vapour. Some of the PCDFs present in air return to the land and water by settling, snow, and rainwater. An amount of PCDFs in the vapour phase is destroyed by reacting with certain chemical agents, called hydroxyl radicals, naturally present in the atmosphere. Depending on the congener, furans may remain in air for an average of more than 10 days. Once in the air, they can be carried long distances.
Polychlorinated dibenzofurans tend to stick to suspended particles and settled particles in lakes and rivers and can remain at the bottom of lakes and rivers for several years. PCDFs can build up in fish, and the amount of the compounds in fish can be tens of thousands times higher than the levels in water. The PCDFs in water can get into birds or other animals and humans that eat fish containing PCDFs.
Furans bind strongly to soil and are not likely to move from the surface soil into groundwater. In some instances, PCDFs from some waste landfills may reach underground water. PCDFs are more likely to move from soil to water or other soils by soil erosion and flooding. The breakdown or loss of PCDFs in soil occurs over years, so PCDFs remain in soil for years. Most PCDFs found in plants are probably deposited by air. Cattle that eat plants contaminated by PCDFs will build up some of the PCDFs in their bodies. Some will enter the milk and meat of cattle.