Both groups of chemicals may have up to eight chlorine atoms attached at carbon atoms 1 to 4 and 6 to 9. Only one chlorine can attach to each position. In all there are 75 possible PCDDs and 135 possible PCDFs and individual members of these families are referred to as congeners. Once all 4 of these positions are occupied by chlorine atoms, the presence of additional chlorine atoms generally progressively reduces the toxicity of the congeners. They become increasingly insoluble in water with increasing level of chlorination.
Fs are amongst the highest reported for any environmental organic contaminants.
Fs is through the ingestion of food, and to a lesser extent through inhalation of air and skin contact with contaminated materials. This is broadly comparable with the estimated dietary intakes of people living in other industrialised countries. The remainder of the estimated dietary intake comes from eggs, cereal products, fruit and vegetables. Obviously there will be great variations in exposure, depending on the dietary habits of the consumer.