Polychlorinated dibenzofurans (PCDFs) are a family of chemicals known as polychlorinated dibenzofurans or simply furans. These chemicals contain one to eight chlorine atoms attached to the carbon atoms of the parent chemical, dibenzofuran. The PCDF family contains 135 individual compounds (known as congeners) with varying harmful health and environmental effects. Of these 135 compounds, those that contain chlorine atoms at the 2,3,7,8-positions of the parent dibenzofuran molecule are especially harmful. Other than for laboratory use of small amounts of PCDFs for research and development purposes, these chemicals are not deliberately produced by industry. Most PCDFs are produced in very small amounts as unwanted impurities of certain products and processes utilizing chlorinated compounds. Only a few of the 135 PCDF compounds have been produced in large enough quantities so that their properties, such as colour, smell, taste, and toxicity could be studied. The few PCDF compounds that have been produced in those quantities are colourless solids. They do not dissolve in water very easily. There is no known use for these chemicals.