Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
irrigation sprays. During other water-based activities (e.g.,
swimming, showering and bathing), other contaminants may be absorbed
via a dermal (percutaneous) route. Although the contribution of
non-oral routes is usually much less than that of oral routes, these
pathways should not be overlooked in the selection of measurement
methods to assess exposure. Methods for modelling exposure through
these pathways are discussed in Chapter 6.
7.3.1 Factors influencing water quality
In order to select appropriate measurement and monitoring
methods, it is important to understand the following factors that
influence the quality of the water being sampled, and the resultant
exposure:
* treatment systems
* distribution networks
* storage practices
* spatial and temporal variations
* climatic and seasonal changes.
Water treatment encompasses a variety of processes, ranging from
simple screening and filtration to multi-step purification. The latter
includes methods for coagulation, aeration, de-aeration, colour
removal, softening, disinfection, fluoridation, stabilization and
demineralization. Some of these steps constitute "removal", and others
involve the "addition" of treatment chemicals to mitigate the hazards
of contaminants in water. A list of chemical additives typically used
in water treatment systems is shown in Table 26. The reaction of
treatment chemicals with other substances present in raw (untreated)
water often results in the generation of intermediate reaction
products with adverse health significance. For instance, chlorine,
accepted worldwide for disinfection and oxidation, results in the
formation of disinfection by-products such as trihalomethanes (e.g.,
chloroform).
Table 26. Water treatment chemicals
Activated alumina Sodium bicarbonate
Aluminum sulfate Sodium calcium magnesium polyphosphate
Ammonia (glassy)
Ammonium hydroxide Sodium carbonate
Bentonite clay Sodium chlorite
Calcium hydroxide Sodium fluoride
Calcium hypochlorite Sodium hydroxide
Calcium oxide Sodium metabisulfite
Carbon (activated, granular, Sodium polyphosphate (glassy)
and powder) Sodium silicate
Carbon dioxide Sodium siliconfluoride
Chlorine Sodium tripolyphosphate
Ferric chloride Sodium zinc polyphosphate (glassy)
Ferric sulfate Sodium zinc potassium polyphosphate
Ferrous sulfate (glassy)
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