The next step in the Health Impact Assessment is to assess the exposure, which is calculated as the so called average daily dose (ADD) for non-cancer risk or threshold chemicals, or the so called lifetime average daily dose (LADD) for cancer risk or non-threshold chemicals (WHO ECEH 2004). Chemicals causing toxic effects are often classified as threshold chemicals which means that they show a dose threshold below which adverse effects are assumed not to occur. Only after exceeding a certain threshold concentration will the probability of an adverse effect increase. Chemicals causing carcinogenic effects are often classified as non-threshold chemicals which means that there is no threshold concentration below which exposure is relatively harmless. As the concentration increases so does the probability of an adverse effect (Van Leeuwen & Vermeire 2007). For classifying chemicals with respect to their potential carcinogenic risk the website of the International Agency for Research on Cancer (IARC) could be consulted (http://monographs.iarc.fr/). The IARC categorizes chemicals into the following groups:
Group 1: the chemical is carcinogenic to humans
Group 2A: the chemical is probably carcinogenic to humans
Group 2B: the chemical is possibly carcinogenic to humans
Group 3: the chemical is not classifiable as to its carcinogenicity to humans
Group 4: the chemical is probably not carcinogenic to humans.
In this, chemicals that fall in group 1, 2A and 2B are often considered to be carcinogenic and chemicals that fall in group 3 and 4 as toxic. However, it should be taken into account that some chemicals can have both carcinogenic and toxic effects (Van Leeuwen & Vermeire 2007). It is therefore important to determine which type(s) of health effect(s) can be caused by the priority chemical by consulting additional epidemiological and experimental evidence.
When a chemical is being assessed which has both carcinogenic and toxic effects, the LADD or ADD should be calculated separately for each type of effect.
The aim of the exposure assessment is thus to estimate the internal exposure dose in individuals of a population resulting from contact with contaminated water. The internal exposure dose is defined as the amount of a chemical absorbed and available for interaction with biologically significant receptors within an individual (expressed in mg/kg/d) (United States Environmental Protection Agency 1992). This is calculated separately for every potential exposure pathway by using equations that include variables for among others the exposure concentration, contact rate, exposure frequency, exposure duration, body weight, and exposure averaging time (United States Environmental Protection Agency 1989). For the input of these variables site-specific exposure information or environmental guidelines could be used (United States Environmental Protection Agency 1989). Statistical tools, such as the Monte Carlo analysis, could be used to enter the values as frequency distributions, which results in a frequency distribution for the ADD and LADD. Or discrete values could be selected or estimated from the ranges of each of the variables and subsequently used to solve the ADD and LADD equations. The values of some of these variables depend among others on the characteristics of the potentially exposed population (United States Environmental Protection Agency 1989). In addition, some subpopulations may be at increased risk from chemical exposures due to increased sensitivity, behavior patterns that may result in high exposure, and/or current or past exposures from other sources (United States Environmental Protection Agency 1989; Agency for Toxic Substances and Disease Registry 2005). It is therefore important to determine the population characteristics (i.e. identify who is exposed) before calculating the internal exposure dose(s) so that the right values will be used in the calculation of the (L)ADD for the entire population or different subpopulations (such as different age groups and/or genders). The total exposure of individuals is determined by summing up the estimated internal exposure doses of the different relevant exposure routes.
The assessment of human exposure makes a distinction between potential direct exposure routes and potential indirect exposure routes. The latter refers to the consumption of organisms by humans. These organisms have taken up chemicals through earlier exposure to contaminated water. For the exposure of humans to contaminated water the following directs routes of exposure may play a role (United States Environmental Protection Agency 1989; Agency for Toxic Substances and Disease Registry 2005; Harezlak & Oste 2010):
– Ingestion of chemicals in water
– Dermal uptake of chemicals in water
– Inhalation of chemicals in air (volatilized or otherwise emitted from water).
In addition, the following indirect routes of exposure may play a role (United States Environmental Protection Agency 1989; Agency for Toxic Substances and Disease Registry 2005; Harezlak & Oste 2010):
– Consumption of contaminated fish and shellfish
– Consumption of contaminated crops, meat, eggs and dairy products.
For these indirect exposure routes the chemical content in organisms should be calculated first before the internal exposure dose can be calculated (Harezlak & Oste 2010).
A specific population can be exposed to a chemical through multiple exposure routes. Whether or not a certain exposure route is involved depends on the forms of use on or around a particular location. In the remainder of this section, the equations by which the human exposure can be quantified via the above mentioned routes of exposure will be discussed.
These equations are derived by combining reports of the United States Environmental Protection Agency (United States Environmental Protection Agency 1989; United States Environmental Protection Agency 1992), Agency for Toxic Substances and Disease Registry (Agency for Toxic Substances and Disease Registry 2005) and Deltares (Harezlak & Oste 2010).