For chemicals exhibiting a threshold for toxic effects, the GVs were derived using the following formula:

The no-observed-adverse-effect level (NOAEL) is the greatest dose of the chemical, found by animal experiments or human observation, that causes no detestable adverse health effects. lf an NOAEL is not available, the lowest observed-adverse-effect level (LOAEL) is used with a corresponding increase in the uncertainty factor (UF). The LOAEL is the lowest dose of a substance at which there is a detestable adverse health effect.

Basis of the uncertainty factor (Table 2)

The uncertainty factor (UF) is a value by which the NOAEL (or LOAEL) is reduced to derive a tolerable daily intake (TDI). The value of the uncertainty factor depends on the nature of the toxic effect, the size and type of population lo be protected, the quality of the toxicological information, and is determined on a case-by-case basis. Uncertainty factors ranging from 1 to 10,000 are generally used (Table 2). lf the risk assessment is so uncertain that a UF higher than 10,000 is required, the resulting TDI would be so imprecise as to lack meaning. Such a situation merely indicates a need for additional data.

The tolerable daily intake (TDI) is an estimate of the amount of a substance in drinking water, expressed on a body weight basis (mg or lig/kg of body weight) that can be ingested daily over a lifetime without appreciable health risk.

Body weight (bw) in kg is usually the body weight of an adult (60 kg). Where infants and children are especially at high risk. body weights of 5 kg and 10 kg have been used.

Percent (P) is the percentage of the TDI allocated lo drinking water. Values ranging from 1 to 100% have been used depending on the estimated exposure from other sources-food and air When limited information is available on sources of exposure, an arbitrary (default) value of 10% of the TDI is used.

Consumption (C) is the volume of consumption of drinking water for each day and is estimated lo be 2 L for adults, 1 L for a 10-kg body weight child, or 0.75 L for a 5-kg body weight infant.

The Guideline value (GV) is expressed in mg or pg/L of drinking water.

The percentage of the TDI allocated to drinking water assumes a typical exposure scenario or is based on a default value that may not be appropriate in all countries. In countries where relevant data on exposure are available, authorities are encouraged to develop context-specific GVs that are tailored lo local circumstances and conditions.

Where a UF greater than 1000 is used, the GV is terined "provisional" lo indicate the incompleteness of available data. Provisional GVs are also recommended when the health-based value is below the limit of quantification, when such a value cannot be attained through practical treatment techniques, or when disinfection is likely to result in the GV being exceeded.

Certain principles developed by JECFA and JMPR were adopted in developing the TDls and GVs:

• The proposed TDls are regarded as tolerable throughout life; they are not established with such precision that they cannot be exceeded for short periods of time. Short-term exposure to levels exceeding the TDI is not a cause for concern, provided that the individual's intake averaged over a long period of time does not exceed this level.
   
• lt is not possible to make generalizations concerning the length of time during which intakes in excess of the TDI would be toxicologically detrimental. The induction of detrimental effects will depend on factors that vary from contaminant to contaminant. The biological half-life of the contaminant, the nature of the toxicity, and the amount by which the exposure exceeds the TDI are all crucial. The final task group meeting on the revision of the Guidelines recommended that WHO also develop health-based GVs for short-term exposure of a few days rather than lifetime exposure.
   
• The large uncertainty factors generally involved in establishing a TDI also provide assurance that exposure exceeding the TDI for short time periods is unlikely to result in any deleterious effects on health. However, consideration should be given to any potential acute toxic effects that may occur if the TDI is substantially exceeded for short periods of time.
   
• The GV was generally rounded to one significant figure to reflect the uncertainty in animal toxicity data and exposure assumptions made. More than one significant figure was used only where extensive information on toxicity and exposure provided greater certainty.