Water Quality
Standards
Summary information extracted from: Guidelines for drinking-water quality, 2nd ed. Addendum to Vol. 1. - Recommendations. - Geneva, World Health Organization, 1998. pp. 15-16.
PAHs form a class of diverse organic compounds each containing two or more fused aromatic rings of carbon and hydrogen atoms. Most PAHs enter the environment via the atmosphere from a variety of combustion processes and pyrolysis sources. Owing to their low solubility and high affinity for particulate matter, they are not usually found in water in notable concentrations. The main source of PAH contamination in drinking-water is usually the coal-tar coating of drinking-water distribution pipes, used to protect the pipes from corrosion.
PAHs have been detected in a variety of foods as a result of the deposition of airborne PAHs, and in fish from contaminated waters. PAHs are also formed during some methods of food preparation, such as char-broiling, grilling, roasting, frying, or baking. For the general population, the major routes of exposure to PAHs are from food and ambient and indoor air. The use of open fires for heating and cooking may increase PAH exposure, especially in developing countries. Where there are elevated levels of contamination by coal-tar coatings of water pipes, PAH intake from drinking-water could be equal to or even exceed that from food.
Evidence that mixtures of PAHs are carcinogenic to humans comes primarily from occupational studies of workers following inhalation and dermal exposure. No data are available for humans for the oral route of exposure.
The guideline value for benzo[a]pyrene (BaP), one of the most carcinogenic PAHs, corresponding to an excess lifetime cancer risk of 10-5, was estimated as 0.7 µg/litre in the Guidelines for drinking-water quality. This was based on an oral carcinogenicity study in mice and calculated using a two-stage birth–death mutation model, which incorporates variable dosing patterns and time of killing. Quantification of dose–response for tumours, on the basis of new studies in which the carcinogenicity of BaP was examined following oral administration in mice, but for which the number of dose groups was smaller, confirms this value. The guideline value of 0.7 µg/litre is therefore retained.
There are few data on the oral toxicity of PAHs other than BaP, particularly in drinking-water. Relative potencies of carcinogenic PAHs have been determined by comparison of data from dermal and other studies. The order of potencies is consistent, and this scheme therefore provides a useful indicator of PAH potency relative to BaP.
Fluoranthene (FA) is the most commonly detected PAH in drinking-water and is associated primarily with coal-tar linings of cast iron or ductile iron distribution pipes. In a 13-week gavage study in mice, a NOAEL of 125 mg FA per kg of body weight per day was identified, based on increased serum glutamate pyruvate transaminase levels, kidney and liver pathology, and clinical and haematological changes. A conservative uncertainty factor of 10 000 (100 for interspecies and intraspecies variation, 10 for the use of a subchronic study and inadequate database, and 10 because of clear evidence of co-carcinogenicity with BaP in mouse skin-painting studies) gives a TDI of 0.0125 mg/kg of body weight per day. Assuming a 60-kg adult drinking 2 litres of water per day with an allocation of 1% of the TDI to water (because there is significant exposure from food), a health-based value of 4 µg/litre (rounded figure) can be calculated.
This health-based value is significantly above the concentrations normally found in drinking-water. Under usual conditions, therefore, the presence of FA in drinking-water does not represent a hazard to human health. For this reason, the establishment of a numerical guideline value for FA is not deemed necessary.
The presence of significant concentrations of BaP in drinking-water in the absence of very high concentrations of FA indicates the presence of coal-tar particles, which may arise from seriously deteriorating coal-tar pipe linings.
It is recommended that the use of coal-tar-based and similar materials for pipe linings and coatings on storage tanks be discontinued.
