= tetra, pe = penta, hx = hexa, hp = hepta, o = octa) the 2,3,7,8-substituted pcdds the 2,3,7,8-substituted pcdfs dioxin-like pcbs controversy greenpeace and some other environmental groups have called for the chlorine industry to be phased out. however, chlorine industry supporters say that "banning chlorine would mean that millions of people in the third world would die from want of disinfected water". sharon beder and others have argued that the dioxin controversy has been very political and that large companies have tried to play down the seriousness of the problems of dioxin. the companies involved have often said that the campaign against dioxin is based on "fear and emotion" and not on science. human intake and levels most intake of dioxin-like chemicals is from food of animal origin: meat, dairy products, or fish predominate, depending on the country. the daily intake of dioxins and dioxin-like pcbs as teq is of the order of 100 pg/day, i.e. 1-2 pg/kg/day. in many countries both the absolute and relative significance of dairy products and meat have decreased due to strict emission controls, and brought about the decrease of total intake. e.g. in the united kingdom the total intake of pcdd/f in 1982 was 239 pg/day and in 2001 only 21 pg/day (who-teq). since the half-lives are very long (for e.g. tcdd 7–8 years), the body burden will increase almost over the whole lifetime. therefore, the concentrations may increase five to tenfold from age 20 to age 60. for the same reason, short term higher intake such as after food contamination incidents, is not crucial unless it is extremely high or lasts for several months or years. decrease of dioxin concentrations in breast milk in sweden and finland the highest body burdens were found in western europe in the 1970s and early 1980s, and the trends have been similar in the u.s. the most useful measure of time trends is concentration in breast milk measured over decades. in many countries the concentrations have decreased to about one tenth of those in the 1970s, and the total teq concentrations are now of the order of 5-30 pg/g fat (please note the units, pg/g is the same as ng/kg, or the non-standard expression ppt used sometimes in the united states). the decrease is due to strict emission controls and also to the control of concentrations in food. in the u.s. young adult female population (age group 20-39), the concentration was 9.7 pg/g lipid in 2001-2002 (geometric mean). certain professions such as subsistence fishermen in some areas are exposed to exceptionally high amounts of dioxins and related substances. this along with high industrial exposures may be the most valuable source of information on the health risks of dioxins. fate of dioxins in human body dioxins are absorbed well from the digestive tract if they are dissolved in fats or oils (e.g. in fish or meat). on the other hand, dioxins tend to adsorb tightly to soil particles, and absorption may be quite low: 13.8% of the given dose of teqs in contaminated soil was absorbed. the same features causing persistence of dioxins in the environment also cause very slow elimination in humans and animals. because of low water solubility, kidneys cannot excrete them in urine as such. they must first be metabolised to more-water-soluble metabolites, but that metabolism, especially in humans, is extremely slow. this results in biological half-lives of several years for all dioxins. that of tcdd is estimated to be 7 to 8 years, and for other pcdd/fs from 1.4 to 13 years, pcdfs on average slightly shorter than pcdds. in mammals, dioxins are found mostly in fat. concentrations in fat seem to be relatively similar, be it serum fat, adipose tissue fat, or milk fat. this permits measuring dioxin burden by analysing breast milk. initially, however, at least in laboratory animals, after a single dose, high concentrations are found in the liver, but in a few days, adipose tissue will predominate. in rat liver, however, high doses cause induction of cyp1a2 enzyme, and this binds dioxins. thus, depending on the dose, the ratio of fat and liver tissue concentrations may vary considerably in rodents. elimination half-lives in humans of some pcdd/fs. congener half-life, years 2,3,7,8-tcdd 7.2 1,2,3,7,8-pecdd 11.2 1,2,3,4,7,8-hxcdd 9.8 1,2,3,6,7,8-hxcdd 13.1 1,2,3,7,8,9-hxcdd 5.1 1,2,3,4,6,7,8-hpcdd 4.9 ocdd 6.7 2,3,7,8-tcdf 2.1 1,2,3,7,8-pecdf 3.5 2,3,4,7,8-pecdf 7.0 1,2,3,4,7,8-hxcdf 6.4 1,2,3,6,7,8-hxcdf 7.2 1,2,3,7,8,9-hxcdf 7.2 2,3,4,6,7,8-hxcdf 2.8 1,2,3,4,6,7,8-hpcdf 3.1 1,2,3,4,7,8,9-hpcdf 4.6 ocdf 1.4 uses dioxins have no common uses. they are manufactured on a small scale for chemical and toxicological research, but mostly exist as by-products of industrial processes such as chlorine bleaching of paper pulp, pesticide manufacture, and combustion processes such as incineration. the defoliant agent orange contained trace amounts of dioxin impurities and caused severe health issues as a result. the wood preservative pentachlorophenol often contained dioxins and dibenzofurans as impurities. the stockholm convention banned the production and use of dioxins in 2001. sources environmental sources pcdd/f-compounds were never synthesized for any purpose, except for small quantities for scientific research. small amounts of pcdd/fs are formed whenever organics, oxygen and chlorine are available at suitable temperatures. this is augmented by metal catalysts such as copper. the optimal temperature range is 400 °c (752 °f) to 700 °c (1,292 °f). this means that formation is highest when organic material is burned in less-than-optimal conditions such as open fires, building fires, domestic fireplaces, and poorly operated and/or designed solid waste incinerators. historically, municipal and medical waste incineration was the most important source of pcdd/fs. pcb-compounds, always containing low concentrations of dioxin-like pcbs and pcdfs, were synthesized for various technical purposes (see polychlorinated biphenyls). they have entered the environment through accidents such as fires or leaks from transformers or heat exchangers, or from pcb-containing products in landfills or during incineration. because pcbs are somewhat volatile, they have also been transported long distances by air leading to global distribution including the arctic. only a minor portion of pcbs in mixtures are dioxin-like. other sources of pcdd/f include: uncontrolled combustion, particularly open burning of waste ("backyard barrel burning"), accidental fires, wildfires. these are presently the most important sources. metal smelting and refining chlorine bleaching of pulp and paper - historically important source of pcdd/fs to waterways. synthesis side products of several chemicals, especially pcbs, chlorophenols, chlorophenoxy acid herbicides, and hexachlorophene. (historical) engines using leaded fuel, which contained the additives 1,2-dichloroethane and 1,2-dibromoethane. in waste incineration improvements and changes have been made to nearly all industrial sources to reduce pcdd/f production. in waste incineration, large amounts of publicity and concern surrounded dioxin-like compounds during the 1980s-1990s continues to pervade the public consciousness, especially when new incineration and waste-to-energy facilities are proposed. as a result of these concerns, incineration processes have been improved with increased combustion temperatures (over 1,000 °c (1,830 °f)), better furnace control, and sufficient residence time allotted to ensure complete oxidation of organic compounds. ideally, an incineration process oxidizes all carbon to co2 and converts all chlorine to hcl or inorganic chlorides prior to the gases passing through the temperature window of 400-700 °c where pcdd/f formation is possible. these substances cannot easily form organic compounds, and hcl is easily and safely neutralized in the scrubber while co2 is vented to the atmosphere. inorganic chlorides are incorporated into the ash. scrubber and particulate removal systems manage to capture some of the pcdd/f which forms even in sophisticated incineration plants. these pcdd/fs are generally not destroyed but moved into the fly ash. catalytic systems have been designed which destroy vapor-phase pcdd/fs at relatively low temperatures. this technology is often combined with the baghouse or scr system at the tail end of an incineration plant. decrease of dioxins in ambient air in different regions (redrawn from dopico and gomez, 2015) the european union limit for concentration of dioxin-like compounds in the discharged flue gas is 0.1 ng/nm³ teq. both in europe and in u.s.a., the emissions have decreased dramatically since the 1980s, by even 90% (see figure). this has also led to decreases in human body burdens, which is neatly demonstrated by the decrease of dioxin concentrations in breast milk. with the substantial decrease of emissions from municipal waste incinerators, other potentially large sources of dioxin-like compounds, for example from forest and wild fires, have increased relative to industrial sources. they are however not included in the total inventory due to uncertainties in available data. a more recent study on the environmental effects of accidental fires, including forest fires, estimated the emissions from dioxins (pcdd/fs) to be about equivalent to those from traffic and municipal waste combustion. open burning of waste (backyard barrel burning) has not decreased effectively, and in the u.s. it is now the most important source of dioxins. total u.s. annual emissions decreased from 14 kilograms (31 lb) in 1987 to 1.4 kilograms (3.1 lb) in 2000. however, backyard barrel burning decreased only modestly from 0.6 kilograms (1.3 lb) to 0.5 kilograms (1.1 lb), resulting in over one third of all dioxins in the year 2000 from backyard burning alone. other sources low concentrations of dioxins have been found in some soils without any anthropogenic contamination. a puzzling case of milk contamination was detected in germany. the source was found to be kaolin added to animal feed. dioxins have been repeatedly detected in clays from europe and usa since 1996, with contamination of clay assumed to be the result of ancient forest fires or similar natural events with concentration of the pcdd/f during clay sedimentation. dioxins and biomass in sugarcane cultivation, the remaining bagasse after extraction of sugar is used in large amounts for energy production (both process heat and electric energy for use both in the sugar factory itself and for other consumers), and locally it has been thought to be a remarkable source of dioxins. this basically indicates that burning biomass produces dioxins and it should be done at high enough temperatures and there should be proper filtering of flue gases. for the treatment of gases and pollutants, sugarcane industries often use wet gas scrubbers, such as the venturi type. in addition, other treatment systems also used are electrostatic precipitators and bag filters. these methods may be insufficient improperly burned biomass, which undergoes only low temperature incomplete combustion, is responsible for much of the negative health effects of indoor air pollution and is a particular problem in the global south where biomass like wood or cowdung are often the only commonly available fuels for cooking and home heating. in addition to dioxins, other harmful products of incomplete combustion - such as carbon monoxide - are also released when biomass is burned in low oxygen conditions. using plastic, particularly chlorine-containing plastics such as polyvinyl chloride, as a fuel or firestarter further increases dioxin emissions. environmental persistence and bioaccumulation all groups of dioxin-like compounds are persistent in the environment. very few soil microbes nor animals can break down the pcdd/fs with lateral chlorines (positions 2,3,7, and 8). lipophilicity (tendency to seek for fat-like environments) and very poor water solubility make these compounds move from water environment to living organisms having lipid cell structures. this is called bioaccumulation. increase in chlorination increases both stability and lipophilicity. the compounds with the very highest chlorine numbers (e.g. octachlorodibenzo-p-dioxin) are, however, so poorly soluble that this hinders their bioaccumulation. bioaccumulation is followed by biomagnification. lipid-soluble compounds are first accumulated to microscopic organisms such as phytoplankton (plankton of plant character, e.g. algae). phytoplankton is consumed by animal plankton, this by invertebrates such as insects, these by small fish, and further by large fish and seals. at every stage or trophic level, the concentration is higher, because the persistent chemicals are not "burned off" when the higher organism uses the fat of the prey organism to produce energy. due to bioaccumulation and biomagnification, the species at the top of the trophic pyramid are most vulnerable to dioxin-like compounds. in europe, the white-tailed eagle and some species of seals have approached extinction due to poisoning by persistent organic pollutants. likewise, in america, the population of bald eagles declined because of pops causing thinning of eggshells and other reproductive problems. usually, the failure has been attributed mostly to ddt, but dioxins are also a possible cause of reproductive effects. both in america and in europe, many waterfowl have high concentrations of dioxins, but usually