|a (P) — Provisional guideline value. This term is used for
constituents for which there is some evidence of a potential hazard but
where the available information on health effects is limited; or where
an uncertainty factor greater than 1000 has been used in the derivation
of the tolerable daily intake (TDI). Provisional guideline values are also
recommended: (1) for substances for which the calculated guideline value
would be below the practical quantification level, or below the level that
can be achieved through practical treatment methods; or (2) where disinfection
is likely to result in the guideline value being exceeded.
b For substances that are considered to be carcinogenic,
the guideline value is the concentration in drinking-water associated with
an excess lifetime cancer risk of 10-5 (one additional cancer
per 100 000 of the population ingesting drinking-water containing the substance
at the guideline value for 70 years). Concentrations associated with estimated
excess lifetime cancer risks of 10-4 and 10-6 can
be calculated by multiplying and dividing, respectively, the guideline
value by 10.
In cases in which the concentration associated with an excess lifetime
cancer risk of 10-5 is not feasible as a result of inadequate
analytical or treatment technology, a provisional guideline value is recommended
at a practicable level and the estimated associated excess lifetime cancer
It should be emphasized that the guideline values for carcinogenic substances
have been computed from hypothetical mathematical models that cannot be
verified experimentally and that the values should be interpreted differently
from TDI-based values because of the lack of precision of the models. At
best, these values must be regarded as rough estimates of cancer risk.
However, the models used are conservative and probably err on the side
of caution. Moderate short-term exposure to levels exceeding the guideline
value for carcinogens does not significantly affect the risk.
c NAD — No adequate data to permit recommendation of a health-based
d ATO — Concentrations of the substance at or below the health-based
guideline value may affect the appearance, taste, or odour of the water.