A REVIEW OF THE TOXICOLOGY OF CYANIDE WITH REFERENCE TO DRINKING WATER
Report No FR0070
S Hunt and J K Fawell
To review the latest information on the toxicity of cyanide with reference to drinking water, and to discuss the setting of a drinking water guide level for the compound.
There is a need to provide water suppliers with sound guidance on the maximum levels of toxic substances that should be permitted in drinking water. The WBO is undertaking a review of its drinking water guidelines for a range of substances. This review is part of the UK water industry's contribution to the process of establishing new guidelines based on a sound scientific assessment of the latest available data.
The previous Guide Level appears to have been set on the basis of: "It is recorded that 4.7 mg cyanide/day is not harmful to man". The Guide Level was set at 100 µg/1.
Studies recently published in rats and pigs do not provide sufficient information for the calculation of no observed adverse effect levels (NOAELs) in these species. However, the data do suggest that the premise of cyanide merely acting as an acute poison is incorrect. Chronic exposure to cyanide may have effects on the nervous system and thyroid. There is evidence that chronic exposure to cyanide may induce clinical or subclinical effects in animals and possibly man at doses lower than previously thought.
The use of rat data in the safety assessment of cyanide in man issuspect as the rat may well detoxify cyanide to thiocyanate more efficiently than other species and hence would be less sensitive to the toxic effects of cyanide. It would appear more appropriate to use the Pig data that are available in the derivation of a Guide Level.
There should be more work on the effects of chronic exposure to relatively low levels of cyanide in drinking water.
V RESUME OF CONTENTS
Cyanide exposure from the diet is considerably greater in the third-world than in developed countries, although data is limited. Cyanide ion is rapidly absorbed from the gastrointestinal tract and is rapidly converted to thiocyanate by the enzyme rhodanese. Much of the data on the pharmacokinetics of cyanide has been obtained in rats whichmay have particularly high levels of rhodanese. Extrapolation of the data to other species should therefore be undertaken with care.
The previous WHO guideline value of 100 µg/1 appears to have been set on the basis that 4.7 mg cyanide/day is not harmful to man. FAO/WHO have previously set an Acceptable Daily Intake of cyanide from food of 50 µg/kg bodyweight/day and the Environmental Protection Agency in the United States have suggested a draft health advisory of 150 µg/1.
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