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Dioxin is an unintentional by-product of incineration and combustion processes, and can form through natural or manmade processes. Dioxin is emitted naturally through forest fires, composting, and volcanic eruptions. In the 1930s, dioxin levels in the environment began to rise due to the increase in industrial processes. The following incineration processes have historically released dioxin through incinerator ash and gases:

• trash incinerators
• hospital waste incinerators
• hazardous waste incinerators
• cement kilns burning hazardous wastes
• sewage sludge incinerators

Polyvinyl chloride (PVC) is the least recyclable type of plastic. Because it is often contaminated with biohazardous waste or is often improperly sorted, it is a large part of the waste stream going into medical waste incinerators. The chlorine contained in PVC and oher wastes in the medical waste stream aid in the formation of dioxins. Another source of dioxin is the chlorinated bleaching process used by pulp and paper mills to produce paper. This process has led to dioxin contamination of air, solid wastes, and bodies of water downstream from the mills. Improvements in controls in recent years have greatly decreased releases of dioxin from medical waste incinerators and pulp and paper processes.

According to the Environmental Protection Agency (EPA), backyard open trash burning is now a major source of dioxin emissions. Backyard burning occurs at low temperatures, in poor smoldering conditions, and in favorable surroundings such as high carbon monoxide levels, all of which contribute to the generation of dioxin emissions.

Source: cfpub.epa.gov/ncea/cfm/dioxindb.cfm?ActType=default

Pesticides, herbicides, and insecticides that use chlorine in their manufacturing processes have all been shown to have traces of dioxins. Some of these products include Agent Orange (banned from military use in 1970), 2,4,5-trichlorophenol (2,4,5-T; banned from use in 1985), Silvex, 2,4-D (commonly used in weed and feed applications for lawn care), and pentachlorophenol. Pentachlorophenols, which are used to preserve wood, can contain relatively high levels of dioxins. Wood treatment facilities and sawmills have been significant sources of contamination, and wastes from these facilities are now treated as hazardous.

Wood burning in industrial facilities (used for electricity and heat) produces dioxin emissions to the air. Facilities that burn wood containing high levels of chlorine have significantly greater levels of dioxin emissions. Power plants, smelters, steel mills, oil and gas refineries, cement kilns, and oil and wood stoves and furnaces all emit dioxins.

Dioxin Emission Sources Over Time (g-TEQ)

Category	1987	& Total	1995	% Total	2002/4	% Total
1-Incineration of:	--	--	--	--	--	--
Municipal solid waste	8,877	77	1,250	71	12	32
Medical waste	2,590	22	488	27	7	18
Sewage sludge	6	0.05	14	0.84	14	39
Hazardous waste	5	0.04	5	0.33	3	9
Total incineration	11,478	82	1,758	54	37	3
2-Backyard barrel burning	604	4	628	19	628	56
3-Metal smelting	955	6	301	9	35	3
4-Cement kilns	131	0.94	173	5	25	2
5-Land-appl'd sewage sludge	76	0.55	76	2	76	6
6-Pulp and paper	372	2.67	23	0.71	15	1
7-Coal-fired utilities	50	0.36	60	1	60	5
8-Industrial wood burning	26	0.19	27	0.85	27	2
9-Residential wood burning	89	0.64	62	1	62	5
10-Diesel trucks	27	0.20	35	1	35	3
11-Other	137	0.98	103	3	100	9
TOTAL	13,949	100	3,252	100	1,106	100

Source: cfpub.epa.gov/ncea/cfm/dioxindb.cfm?ActType=default