THE RISK ASSESSMENT OF ORGANIC COMPOUNDS IN DRINKING WATER
Report No FR0401
As the toxic mechanisms for organic contaminants are better understood, less uncertainty is involved in the risk assessment process. This may lead to drinking water standards that are less conservative than previously established, but which will still a fford adequate protection to consumers. A knowledge of the way in which such standards are calculated will help in understanding why small differences in the numerical values are not usually significant from a health point of view. This in turn will be of value when dealing with enquiries from customers and the media in relation to chemical contamination of drinking water.
It is important that drinking water standards are based on sound scientific judgment. A conservative approach is usually taken when standards for organic chemicals are derived, especially for chemicals that have been found, or are suspected to be carcinogenic. This approach reflects uncertainties in the data and our knowledge of toxic mechanisms. As new toxicity data become available and the toxic mechanisms are better understood, less uncertainty is involved in the risk assessment process, and this may lead to revised numerical values for some standards. The calculated values necessarily contain a number of assumptions and a substantial allowance for uncertainties in the extrapolation process. This will of course impact significantly on the final numerical value. Small changes in the values may not be significant from a toxicological or health point of view, but this is difficult to convey to the consumer and, in terms of compliance, such changes could make a large difference in the costs for water treatment in achieving given standards. There are also instances when water suppliers need to know how a standard has been derived. This is necessary to understand the health significance if a standard is approached or exceeded, and will also be of value in dealing with enquiries from consumers and the media on chemical contamination of supplies.
V RESUME OF CONTENTS
The number of organic chemicals currently regulated are few, and the limits for some of these need updating so that more recent toxicity data are taken into account. For guidance, water suppliers may use WHO guidelines, US EPA standards (MCLs) and health advisories (HAs) or, if these are not available, WRc SNARLs (suggested no adverse response levels) for the many organic chemicals that are not regulated. These organisations can adopt different approaches to standard setting resulting in different evaluations. It is important that the different values produced by each organisation are interpreted correctly and, to do this, an understanding of the methods used to derive standards is necessary.
Animal toxicity data are usually used to derive a safe dose of a chemical for man. The method used to extrapolate animal data to man will depend on the chemical and its mechanism of toxicity. For mohere are new data that enable a better understanding of their mechanisms of toxicity. The new WHO (World Health Organization) guideline values for these chemicals have incorporated the more recent data.
Once a safe dose of a chemical for man has been estimated, a drinking water standard is calculated by deciding how much of the chemical could come from drinking water, compared to other sources, such as air and the diet. Assumptions also have to be made, regarding the average daily drinking water consumption. A 60 kg adult drinking 2 litres of water per day is normally used to calculate standards for organic chemicals, although a child or bottle fed infant may be used, if it is considered that the latter are particularly susceptible to the toxic effects of a chemical.
The methods used to derive health-based standards do not consider the organoleptic properties of chemicals. Some chemicals can, however, cause taste or odour at concentrations much lower than a health-based limit. It should be made clear where this is the case, and it is useful if water suppliers have access to both health-based standards and the organoleptic threshold concentrations.
Much uncertainty is involved in the methods used to calculate standards and it is not always clear, which method should be used for a given chemical. This uncertainty, together with the assumptions made in the calculations, lead to a standard that can v ary by many orders of magnitude, depending on the criteria used. This uncertainty means that a very conservative approach is usually taken during standard setting. Most drinking water standards include substantial margins of safety and have been calculated, assuming that a person would be exposed to that concentration of chemical, every day, for a lifetime. For this reason, short-term exceedances of drinking water standards do not usually mean that exposure will result in any adverse health effects.
Copies of the Report are available from FWR, price £15.00 less 20% to FWR Members