Dioxins are a group of chemicals characterized by their ability to cause a variety of genetic and carcinogenic effects on humans and animals. The effects vary widely between different dioxin compounds. They are therefore assigned different toxic equivalency factors, TEFs. Emissions and exposure levels of dioxins are based on these factors and expressed as toxicity equivalent quantities (TEQs), either according to WHO (WHO-TEQs) or an international standard with NATO origin (i-TEQs).

Among the more notable dioxins are polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), dioxin-like polychlorinated biphenyls (PCBs), and analogues where the chlorine atoms in the compounds are replaced by bromide atoms. The named compound groups are all highly toxic, classified as persistent organic pollutants (POPs) accumulating in the food chain. Their use is either banned or severely restricted in many countries. 

PCDDs are occasionally referred to simply as “dioxins” although they are actually just a subset of the dioxins group of compounds. A notable PCDD is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) which was the active agent in the Seveso disaster in 1976.

PCDDs, PDCFs, and dioxin-like PCBs mostly occur in solid form (crystals) at room temperature but can take liquid or gaseous forms at higher temperatures.

There are some natural sources of dioxins such as volcanos and forest fires, but dioxins are mostly man-made for example through the incineration of chlorine-containing substances such as PVC. 

Accordingly, regulations on emissions from waste incinerators often include restrictions on dioxins. Specifically within the European Union, the waste incineration BAT conclusions (WI-BATC) calls for sampling of PCDDs, PCDFs, dioxin-line PCBs, and states maximum concentrations (TEQ weights per volume) obtained either during short-term or long-term sampling.