Furans and dioxins
 
Dioxins are short for polychlorodibenzodioxins (PCDDs) and furans are short for polychlorodibenzofurans (PCDFs). This group comprises 49 chlorinated dioxins and 87 chlorinated dibenzofurans. The 2,3,7,8- tetrachlorod ibenzod ioxin (2378TCDD) has been the most studied and is the most toxic.Toxicity of dioxins and  furans depends upon the number of chlorine atoms and their position on the ring structures and each has been assigned a TEQ or Toxicological Equivalent to 2,3,7,8-tetrachloro-dioxin. 

sources

Burning chlorine containing material will emit dioxins into the air. The main official UK sources of dioxins (the total is a very small one gram a day; one gram being the same as 1000 000 000 000pg) are fires (accidental and natural e.g. bonfires); clinical waste incineration; cremation; electric arc furnaces; sinter plant; combustion in industry; power stations. Domestic combustion are officially 2.5% of total emissions but are probably greater and are important because of the close proximity of sources to people's breathing zones and to agricultural land.

exposure

Inhaling dioxins can be a serious health hazard but the main biological pathway into human tissue is from eating  food from land and water contaminated by dioxins produced by burning and other processes. Animals then readily accumulate dioxins in their fatty tissue and thus providing a relatively concentrated poison for human consumption. For most people, 90 percent of human dioxin exposure comes from food, particularly meat, fish, poultry, cheese, milk, butter.  Free range eggs and oily fish have been measured as having alarminly high levels. In people, the toxicity of dioxins is due to their high fat-solubility and its ability to bind with the Ah receptor. Dioxin poisoning is long term partially due to the long residence time in human fatty tissue with a half life of roughly ten years whereas in smaller mammals they reside for a much shorter time, weeks in some cases. Dioxin accumulation in the body takes over thirty years to reach a maximum equilibrium state before intake from food and inhalation equals elimination from the body. Upon equilibrium, a single large dioxin intake of 2000pg in a day would only inrease the body burden by typically 1%. Toxicity depends upon the total amount in the body but as this burden is produced by daily intakes of dioxin, a tolerable daily intake (TDI) has now been established by many governmental organisations. A detailed UK study has determined that 2 pg/kg/bw/day to be a safe level which equates to 120pg or 0.12ng for a 60 kg person. The average UK dietary intake is 1.8 pg/kg/bw/day which is much lower than before 1990 but as many as 2 million of the UK population exceed 3 pg/kg/bw/day
Coal smoke typically emits 5000 pg(TEQ)m-3 dioxins. Wood smoke typically emits 1000 pg(TEQ)m-3.  Burning waste materials such as paper, plastic and wrappings can increase domestic fire dioxin emissions to over 100 000 pgm -3. Waste incinerators have a limit of 100pgm-3, much lower in concentration than domestic or waste fires but the quantity they produce is higher. Close proximity to domestic fires may poison through inhalation alone which may account for the alledged dioxin related lung cancer incidence but its main effect is in conjunction with other exposure pathways forcing safe limits to be broken. The main problem with domestic or waste fires is via localised land and water contamination and may be a special concern for farms and the food they produce rather than suburban or urban homes as far as dioxin poisoning is concerned.

External links

• burningissues dioxin page
• USEPA basic factsheet
• USEPA dioxin report
• UK Department of Health dioxin toxicity statement
  
• ATSDR public health statement
  
• Dioxin facts   
  
• Related issue:  UK government statement on dioxin in food:  full report pdf  /  summary pdf