Once toxicological and/or epidemiological information concerning a chemical is available, a risk assessment analysis can be performed. This analysis tries to answer quantitatively the questions "What are the likely types of toxicity expected for the human population exposed to a chemical?" and "What is the probability of each effect occurring in the population?" Where necessary, risk assessment also tries to determine permissible exposures to the substance in question.
In order to perform a risk assessment on a chemical, it is necessary to know
• hazard evaluation information; i.e., the types of toxicity (acute? cancer? birth defects?) that are expected from it;
• quantitative dose-response information concerning the various possible modes of exposure (oral, dermal, inhalation) for it; and
• an estimation of the potential human exposure to the chemical.
For chronic exposures, threshold or NOEL dose information is normally expressed as milligrams of the chemical per kilogram of body weight per day. In determining the threshold level for the most sensitive members of the human population, it is common to divide the NOEL from animal studies by a safety factor, typically 100. The resulting value is called the maximum acceptable daily intake, ADI, or maximum daily dose; the U.S. EPA instead refers to it as the toxicity reference dose, RfD. Note that the ADI or RfD value does not represent a sharp dividing line separating absolutely safe from absolutely unsafe exposures, since the transferability of toxicity information from animals to humans is not exact, and in most cases the safety factor presumably is quite generous. Some scientists have suggested dividing the NOEL by a further factor of 10 in order to protect very susceptible groups such as children. Indeed, the Food Quality Protection Act in the United States requires that the EPA set limits for residues of pesticides on foods 10 times lower than what is considered to be safe for an adult.
The NOEL for a chemical is found to be 0.010 mg/kg/day. What would its ADI or RfD value for adults be set at? What mass of the compound is the maximum that a 55-kg woman should ingest daily?
As mentioned, in a risk assessment, an attempt is made to estimate the exposure of the affected population. For example, for chemicals whose mode of exposure is primarily through drinking water, regulatory agencies such as the U.S. EPA consider a hypothetical average person who drinks about 2 L of water daily and whose body weight averages 70 kg (154 lb) through life. If the ADI (or RfD) of a substance is 0.0020 mg/kg/day, then for the 70-kg person, the mass of it that can be consumed per day is 0.0020 X 70 = 0.14 mg/day. Thus the maximum allowable concentration of the chemical in water would be 0.14 mg/2 L = 0.07 mg/L = 0.07 ppm. Of course, if there are other significant sources of the substance, they must be taken into account in determining the drinking-water standard. Also, exposure to several chemicals of the same type (e.g., several organochlorines) could produce additive effects, so the standard for each one should presumably be lowered to take this into consideration.
The EPA regulates exposure to carcinogens by assuming that the dose^response relationship has no threshold and can be linearly extrapolated from zero dose to the area for which experimental results are available. The maximum daily doses are then determined by assuming that each person receives the dose every day over his or her lifetime and that this exposure should not increase the likelihood of cancer to more than one person in every million.
In determining regulations to control risk, usually no consideration is given to the economic costs involved. Many economists believe that because regulations cost money to implement, and because society may decide that it has only limited resources that it is willing to spend overall on regulation, a cost-benefit analysis should be used to help decide which substances to regulate. Associated with this line of thinking is the idea that regulations should show a positive payback: The benefits from regulation should exceed the costs. Of course, it is difficult to place a specific monetary value on environmental benefits in many cases. For example, is the $200,000 average cost for saving a life by regulating the chloroform content in water (see Chapter 14) a reasonable amount to pay? If so, would it still be reasonable if the cost instead was 10 or 100 or 1000 times this amount?
Continue reading here: Organophosphate and Carbamate Insecticides
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