Furans are by-products of natural and industrial processes and are considered environmental pollutants. They are chemical substances found in small amounts in the environment , including air, water and soil . They are also present in some foods. Although the amounts are small, they are persistent and remain in the environment for long periods of time, also accumulating in the food chain. The U.S. Environmental Protection Agency's (EPA) Persistent Bioaccumulative and Toxic (PBT) Chemical Program classifies furans as priority PBTs.
Furans belong to a class of organic compounds known as heterocyclic aromatic hydrocarbons . The basic furan structure is a five-membered ring consisting of four atoms of carbon and one oxygen. Various types of furans have additional atoms and rings attached to the basic furan structure. Some furans are used as solvents or as raw materials for synthesizing chemicals .
Polychlorinated dibenzofurans (PCDFs) are of particular concern as environmental pollutants. These are three-ringed structures, with two rings of six carbon atoms each (benzene rings) attached to the furan. Between one and eight chlorine atoms are attached to the rings. There are 135 types of PCDFs, whose properties are determined by the number and position of the chlorine atoms. PCDFs are closely related to polychlorinated dioxins (PCDDs) and polychlorinated biphenyls (PCBs). These three types of toxic compounds often occur together and PCDFs are major contaminants of manufactured PCBs. In fact, the term dioxin commonly refers to a subset of these compounds that have similar chemical structures and toxic mechanisms. This subset includes 10 of the PCDFs, as well as seven of the PCDDs and 12 of the PCBs. Less frequently, the term dioxin is used to refer to all 210 structurally-related PCDFs and PCDDs, regardless of their toxicities.
Furans are present as impurities in various industrial chemicals. PCDFs are trace byproducts of most types of combustion , including the incineration of chemical, industrial, medical, and municipal waste, the burning of wood, coal , and peat, and automobile emissions . Thus most PCDFs are released into the environment through smokestacks. However the backyard burning of common household trash in barrels has been identified as potentially one of the largest sources of dioxin and furan emissions in the United States. Because of the lower temperatures and inefficient combustion in burn barrels, they release more PCDFs than municipal incinerators. Some industrial chemical processes, including chlorine bleaching in pulp and paper mills , also produce PCDFs.
PCDFs that are released into the air can be carried by currents to all parts of the globe. Eventually they fall to earth and are deposited in soil, sediments, and surface water. Although furans are slow to volatilize and have a low solubility in water, they can wash from soils into bodies of water, evaporate, and be re-deposited elsewhere. Furans have been detected in soils, surface waters, sediments, plants, and animals throughout the world, even in arctic organisms. They are very resistant to both chemical breakdown and biological degradation by microorganisms .
Most people have low but detectable levels of PCDDs and PCDFs in their tissues. Furans enter the food chain from soil, water, and plants. They accumulate at the higher levels of the food chain, particularly in fish and animal fat. The concentrations of PCDDs and PCDFs may be hundreds or thousands of times higher in aquatic organisms than in the surrounding waters. Most humans are exposed to furans through animal fat, milk, eggs, and fish. Some of the highest levels of furans are found in human breast milk. The presence of dioxins and furans in breast milk can lead to the development of soft, discolored molars in young children. Industrial workers can be exposed to furans while handling chemicals or during industrial accidents.
Furans bind to aromatic hydrocarbon receptors in cells throughout the body, causing a wide range of deleterious effects, including developmental defects in fetuses, infants, and children. Furans also may adversely affect the reproductive and immune systems. At high exposure levels, furans can cause chloracne, a serious acne-like skin condition. Furan itself, as well as PCDFs, are potential cancer-causing agents.
The switch from leaded to unleaded gasoline , the halting of PCB production in 1977, changes in paper manufacturing processes, and new air and water pollution controls, have reduced the emissions of furans. In 1999 the United States, Canada, and Mexico agreed to cooperate to further reduce the release of dioxins and furans.
On May 23, 2001, EPA Administrator Christie Whitman, along with representatives from more than 90 other countries, signed the global treaty on Persistent Organic Pollutants . The treaty phases out the manufacture and use of 12 toxic chemicals, the so-called "dirty dozen," that includes furans. The United States opposed a complete ban on furans and dioxins; thus, unlike eight of the other chemicals that were banned outright, the treaty calls for the use of dioxins and furans to be minimized and eliminated where feasible.
[Margaret Alic Ph.D. ]
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