effects can in fact be interpreted as protective rather than toxic (enzyme induction, aversion to novel foods). human toxicity high doses. toxicity of dioxins at high doses has been well documented after accidents, deliberate poisonings, food contamination episodes, and high industrial exposures. three women in vienna, austria, were poisoned with large doses of tcdd in 1998. the highest concentration of tcdd in fat tissue was 144,000 pg/g, the highest ever reported in human beings. the main feature was chloracne, a serious skin disease. the victim survived, and other symptoms were modest after initial gastrointestinal symptoms and amenorrhea. another acute incident was the deliberate poisoning of victor yushchenko, then presidential candidate of ukraine, in 2004. tcdd concentration in fat was 108,000 pg/g. also in this case the most prominent symptom was chloracne after initial stomach pain indicating hepatitis and pancreatitis. these episodes show that a human being is not as sensitive as the most sensitive animals, since the doses must have been up to 25 μg/kg. two serious food contamination accidents were caused by pcb oils used in heat exchangers. the pcb oil leaked to rice bran oil consumed by thousands of people in japan (yusho disease 1968) and taiwan (yu-cheng disease 1979). the toxic effects have been attributed to dioxin-like pcbs and pcdfs. their daily intake was up to 100,000 times higher than average intake presently. there were many skin problems, chloracne, swelling of eyelids, and hypersecretion of meibomian glands in the eyes. babies born to yusho and yu-cheng mothers were smaller than normal, they had dark pigmentation and sometimes teeth at birth and tooth deformities. foetal deaths and miscarriages were common. perhaps the best known dioxin accident occurred in seveso, italy, in 1976. a tank of chlorophenols released its contents to air including many kilograms of tcdd, and contaminated much of the city. the highest tcdd levels were found in children, up to 56,000 pg/g fat. acute effects were limited to chloracne, although many animals such as rabbits died after eating contaminated grass. dental aberrations were found after 25 years in persons exposed as children, and a slightly increased cancer risk was confirmed 35 years later. in line with animal studies, developmental effects may be much more important than effects in adults. these include disturbances of tooth development, and of sexual development. an example of the variation in responses is clearly seen in a study following the seveso disaster indicating that sperm count and motility were affected in different ways in exposed males, depending on whether they were exposed before, during or after puberty. in occupational settings many symptoms have been seen, but exposures have always been to a multitude of chemicals including chlorophenols, chlorophenoxy acid herbicides, and solvents. therefore, definitive proof of dioxins as causative factors has been difficult to obtain. by far the best proven effect is chloracne. the suspected effects in adults are liver damage, and alterations in heme metabolism, serum lipid levels, thyroid functions, as well as diabetes and immunological effects. low exposures. effects after low exposures such as from food have been difficult to prove. levels of dioxins in contemporary population are 5 to 20 pg/g (teq in fat) and 50 to 100 pg in older people or at least 1000 times lower than those in poisonings (see above). tooth deformities have been considered plausible after long breast-feeding, when the dioxin concentrations were high in 1970s and 1980s. when the concentrations decreased during 1990s and 2000s, the effects were no longer seen. according to a study in russia, sperm counts in 18-19 year old young men were lower when dioxin levels were higher at the age of 8 to 9 years. this was in industrial environments causing relatively high exposures to boys as well as their mothers. the contamination panel of the european food safety agency (efsa) recommended decreasing tolerable weekly intake (twi) levels based on the russian children study. this recommendation can be challenged, because it does not properly consider competing risks following from lost benefits of important and healthy food items such as certain fish. twi levels are not applied for breast feeding, because benefits of breast milk are judged to be far more important than the remote risks of dioxins. a general conclusion may be that safety margins are not very great concerning developmental effects, but toxic effects are not likely at the present population levels of dioxins. a number of cross-sectional studies have shown associations between type 2 diabetes and several pop compounds including dioxins. such observational studies cannot prove causality, i.e. there may be an association which does not prove that one is the cause of the other. the main problem is that similar associations can be found with many quite different pops, which have only long half-lives and tendency to accumulate in lipids in common. this suggests that they may all be related to diet and obesity which are by far the most common causes of type 2 diabetes. over the years there have been speculations on various effects of dioxins on endometriosis, sexual development, liver function, thyroid hormone levels, white blood cell levels, immune functions, and even learning and intelligence. while some of these effects might be possible after heavy exposures (like in the seveso disaster), these claims are only based on potential exposures of population, not supported by actual measurements of dioxin concentrations. e.g. absorption from bleached tampons claimed to be associated with endometriosis is insignificant compared with daily dioxin intake from food. carcinogenicity dioxins are well established carcinogens in animal studies, although the precise mechanism is not clear. dioxins are not mutagenic or genotoxic. the united states environmental protection agency has categorised dioxin, and the mixture of substances associated with sources of dioxin toxicity as a "likely human carcinogen". the international agency for research on cancer has classified tcdd as a human carcinogen (class 1) on the basis of clear animal carcinogenicity and limited human data, and subsequently also 2,3,4,7,8-pcdf and pcb 126 as class 1 carcinogens. the mechanism is thought to be mainly promotion, i.e. dioxins can accelerate the formation of tumours caused by other factors, and adversely affect the normal mechanisms for inhibiting tumour growth. some researchers have also proposed that dioxin induces cancer progression through a very different mitochondrial pathway. as with many toxic endpoints of dioxin, a clear dose–response relationship is difficult to establish. after accidental or high occupational exposures there is evidence on human carcinogenicity. increases in cancer have been modest, in fact reaching statistical significance has been difficult even after high accidental or occupational exposures like in yusho and yucheng poisonings, seveso accident, and combined occupational cohorts. therefore controversies on cancer risk at low population levels of dioxins are understandable. the problem with iarc evaluations is that they only assess hazard, i.e. carcinogenicity at any dose. it is likely that there is a practical safe threshold for the non-genotoxic dioxins, and the present population levels do not possess any risk of cancer. there is thus some agreement on that cancer risk is taken care of as well, if daily intake limits are set to protect from developmental effects. among fishermen with high dioxin concentrations in their bodies, cancer deaths were decreased rather than increased. all this means that in case of important beneficial food items and breast feeding a thorough benefit/risk analysis is needed before setting limits, in order to avoid increased other risks or lost benefits. risk assessment the uncertainty and variability in the dose–response relationship of dioxins in terms of their toxicity, as well as the ability of dioxins to bioaccumulate, have led who experts to recommending very low tolerable daily intake (tdi) of dioxin, 1-4 pg/kg body weight per day, i.e. 7x10−11 to 2.8x10−10g per 70-kg person per day, to allow for this uncertainty and ensure public safety in all instances. authorities have then set weekly or monthly intake levels that equal to tdis around 2 pg/kg. because dioxins are eliminated very slowly, the body burden accumulated during the whole lifetime is high compared with daily doses, and occasional modest exceedances of limit values do not change it much. therefore long-term intake is much more important than daily intake. specifically, the tdi has been assessed to guarantee the safety of children born to mothers exposed to such a daily intake of dioxins all their lifetime prior to pregnancy. it is likely that the tdi for other population groups could be higher. one important cause for differences in different assessments has been carcinogenicity. if the dose-response of tcdd in causing cancer is linear, it might be a true risk. if the dose-response is of a threshold-type or j-shape, there is little or no risk at the present concentrations. understanding the mechanisms of toxicity better is hoped to increase the reliability of risk assessment. recently also developmental effects have been reassessed by the contamination panel of the european food safety agency (efsa). they propose decreasing the tolerable weekly intake (twi) from 14 pg/kg to 2 pg/kg. this is likely to cause another controversy before being accepted by european countries. dioxin intake and levels in breast milk in 1970s and 1980s were 5 to 10 times higher than presently, and very few effects have been found, possibly mild developmental effects on teeth. toxicity equivalents all dioxin-like compounds share a common mechanism of action via the aryl hydrocarbon receptor (ahr), but their potencies are very different. this means that similar effects are caused by all of them, but much larger doses of some of them are needed than of tcdd. binding to the ahr as well as persistence in the environment and in the organism depends on the presence of so-called "lateral chlorines", in case of dioxins and furans, chlorine substitutes in positions 2,3,7, and 8. each additional non-lateral chlorine decreases the potency, but qualitatively the effects remain similar. therefore, a simple sum of different dioxin congeners is not a meaningful measure of toxicity. to compare the toxicities of various congeners and to render it possible to make a toxicologically meaningful sum of a mixture, a toxicity equivalency (teq) concept was created. each congener has been given a toxicity equivalence factor (tef). this indicates its relative toxicity as compared with tcdd. most tefs have been extracted from in vivo toxicity data on animals, but if these are missing (e.g. in case of some pcbs), less reliable in vitro data have been used. after multiplying the actual amount or concentration of a congener by its tef, the product is the virtual amount or concentration of tcdd having effects of the same magnitude as the compound in question. this multiplication is done for all compounds in a mixture, and these "equivalents of tcdd" can then simply be added, resulting in teq, the amount or concentration of tcdd toxicologically equivalent to the mixture. the teq conversion makes it possible to use all studies on the best studied tcdd to assess the toxicity of a mixture. this is most useful in regulatory work, but it can also be used in scientific studies. this resembles the common measure of all alcoholic drinks: beer, wine and whiskey can be added together as absolute alcohol, and this sum gives the toxicologically meaningful measure of the total impact. the teq only applies to dioxin-like effects mediated by the ahr. some toxic effects (especially of pcbs) may be independent of the ahr, and those are not taken into account by using teqs. tefs are also approximations with certain amount of scientific judgement rather than scientific facts. therefore, they may be re-evaluated from time to time. there have been several tef versions since the 1980s. the most recent re-assessment was by an expert group of the world health organization in 2005. the skeletal formula and substituent numbering scheme of the parent compound dibenzo-p-dioxin who toxic equivalence factors (who-tef) for the dioxin-like congeners of concern class congener toxic equivalence factor polychlorinated dioxins 2,3,7,8-tcdd 1 1,2,3,7,8-pecdd 1 1,2,3,4,7,8-hxcdd 0.1 1,2,3,6,7,8-hxcdd 0.1 1,2,3,7,8,9-hxcdd 0.1 1,2,3,4,6,7,8-hpcdd 0.01 ocdd 0.0003 polychlorinated dibenzofurans 2,3,7,8-tcdf 0.1 1,2,3,7,8-pecdf 0.03 2,3,4,7,8-pecdf 0.3 1,2,3,4,7,8-hxcdf 0.1 1,2,3,6,7,8-hxcdf 0.1 1,2,3,7,8,9-hxcdf 0.1 2,3,4,6,7,8-hxcdf 0.1 1,2,3,4,6,7,8-hpcdf 0.01 1,2,3,4,7,8,9-hpcdf 0.01 ocdf 0.0003 non-ortho-substituted pcbs 3,3’,4,4’-tcb (77) 0.0001 3,4,4’,5-tcb (81) 0.0003 3,3’,4,4’,5-pecb (126) 0.1 3,3’,4,4’,5,5’-hxcb (169) 0.03 mono-ortho-substituted pcbs 2,3,3’,4,4’-pecb (105) 0.00003 2,3,4,4’,5-pecb (114) 0.00003 2,3’,4,4’,5-pecb (118) 0.00003 2’,3,4,4’,5-pecb (123) 0.00003 2,3,3’,4,4’,5-hxcb (156) 0.00003 2,3,3’,4,4’,5’-hxcb (157) 0.00003 2,3’,4,4’,5,5’-hxcb (167) 0.00003 2,3,3’,4,4’,5,5’-hpcb (189) 0.00003 (t