Dioxin is an organic solid of white crystalline needles. Dioxin is not produced or used commercially in the US. It is a contaminant formed in the production of some chlorinated organic compounds. It may also be formed during combustion of a variety of chlorinated organic compounds.

In 1974, Congress passed the Safe Drinking Water Act. This law requires the Environmental Protection Agency (EPA) to determine safe levels of chemicals in drinking water which do or may cause health problems. These non-enforceable levels, based solely on possible health risks and exposure, are called Maximum Contaminant Level Goals (MCLG).

The MCLG for dioxin has been set at zero because EPA believes this level of protection would not cause any of the potential health problems described below. Based on this MCLG, EPA has set an enforceable standard called a Maximum Contaminant Level (MCL). MCLs are set as close to the MCLGs as possible, considering the ability of public water systems to detect and remove contaminants using suitable treatment technologies.

The MCL has been set at 0.00003 parts per billion (ppb) because EPA believes, given present technology and resources, this is the lowest level to which water systems can reasonably be required to remove this contaminant should it occur in drinking water. These drinking water standards and the regulations for ensuring these standards are met, are called National Primary Drinking Water Regulations. All public water supplies must abide by these regulations.

Dioxin is released to the environment in emissions from the incineration of municipal refuse and certain chemical wastes, in exhaust from automobiles powered by leaded gasoline, in emissions from wood burning in the presence of chlorine, in accidental fires involving transformers containing PCBs and chlorinated benzenes, and from the improper disposal of certain chlorinated chemical wastes. It has been released to the environment as a low level impurity in various pesticides.

Dioxin is one of the most toxic and environmentally stable tricyclic aromatic compounds of its structural class. Due to its very low water solubility, most of the dioxin occurring in water will adhere to sediments and suspended silts. Similarly, it tends to adhere to soil if released to land, and is not likely to leach to ground water. Two processes which may be able to remove dioxin from water and soil are evaporation and breakdown by sunlight. Dioxin is generally resistant to microbial breakdown. Dioxin has a very great tendency to accumulate in aquatic life, from algae to fish.

EPA in cooperation with the Food and Drug Administration (FDA) and the Food Safety and Inspection Service (FSIS) of the U.S. Department of Agriculture (USDA) continue to study dioxin with the goal of significantly reducing public exposure to the chemical. Although the agencies continue to explore exposure to the chemical, little has been done to demonstrate practically how to eradicate dioxin from the environment.
Dioxin in DepthChlorine Chemistry Division, American Chemistry Council
Dioxin and Related CompoundsEnvironmental Protection Agency
DioxinsNational Institute of Environmental Health Sciences, National Institutes of Health
Q&A About DioxinsCenter for Food Safety and Applied Nutrition, U.S. Food & Drug Administration