Average Daily Exposure equation

The oral reference dose (Oral RfD) is an estimate of the daily exposure of a person to a contaminant that is likely to be without appreciable risk of a deleterious non-carcinogenic effect during a lifetime (USEPA http //www.epa.gov/iris/). Oral RfD values for POP concentrations in seafood types are presented in Table 16.5, together with the daily intake of POPs from seafood consumed in Singapore. Daily intakes of POPs from seafood are below the oral RfD. The cancer benchmark concentration (Dougherty et al., 2000) represents the exposure concentration at which a lifetime cancer risk equates to one excess cancer death in one million persons. This level is defined as the public health protective concentration in the Congressional House Report to the Food Quality Protection Act of 1996 in the USA. Cancer benchmark concentrations were exceeded for DDTs, heptachlor, and PCBs (See Table 16.5). The cancer hazard ratio is the ratio of the MDI for a specific contaminant relative to the cancer benchmark concentration. The cancer hazard ratio represents the extent to which average daily exposure exceeds the benchmark concentration. The cancer hazard ratio of seafood consumption... 

The general population can be exposed to chemical substances in indoor as well as in outdoor (ambient) air via inhalation of vapors, aerosols, and dusts in the air. The term inhalation exposure is defined as the concentration of a substance in inhaled air at the boundary of the body, and is expressed as an average concentration per unit time (e.g., mg/m per day). In order to estimate a daily dose of a substance from the exposure concentration of the substance in the air, the inhalation rate is used. According to US-EPA (1997), the average daily dose (ADD) can be estimated from the exposure concentration by using the following equation ... 

For any case-study built around Equation 1, we have to consider, for model input, parameters that provide emissions or environmental concentrations, intermedia transfer factors, ingestion (or other intake) rates, body weight, exposure frequency and exposure duration. For our specific case-study below, we are interested in concentrations in surface waters due to deposition from the atmosphere. The relevant intermedia transfer factor is the bioconcentration factor for fish concentration from surface water concentrations. The intake data we need are the magnitude and range of fish ingestion in our exposed population. Because PBLx is a persistent compound that accumulates in fat tissues, we will focus for this case not on exposure frequency and duration but on long-term average daily consumption. 

In the Monte Carlo approach, there are no inherent limitations on the complexity of the exposure equation, the number of component variables, the probability distributions for the variable components, or the number of iterations. This freedom from limitations is especially useful in simulating the distributions of a lifetime average daily dose (LADD) for the different exposure scenarios considered herein. As its name suggests, a LADD is the average over all of the days in an individual s lifetime of the dose of a chemical (e.g. atrazine, simazine, or both) received by that individual in those days as a result of his or her exposure from one or more exposure pathways (e.g. water, diet and herbicide handling). Because the exposure equation can explicitly consider each day individually, the values of the equation s variable components can vary from day to day and have different distributions for different ages. The length of an individual s lifetime can also vary from individual to individual. 

Once there is a measure of the concentration of the pesticide in the exposure medium (air, water, food, etc.) in contact with the body or the actual concentration that comes into contact with the body, a daily dose metric can be calculated (e.g. maximum, average, geometric mean, etc.). This typically involves developing a mathematical equation that expresses dose as a function of pesticide concentration and other important parameters referred to as human exposure factors (USEPA, 1999a). In the context of this discussion, the term human exposure factor refers specifically to (a) human characteristics, such as body weight, surface area, life expectancy, inhalation rates for air and consumption rates for food, drinking water and soil (b) human behaviors, such as activity patterns, occupational and residential mobility and consumer product use, which are used by exposure assessors to calculate potential dose. 

Many commercial units, like the Samplair shown in Figure E53-3, in Experiment 53 in Appendix A, come with the volume calibrated. However, an additional calculation usually is needed. The number desired when doing particulate air sampling is the number of particles per cubic meter per 8 hours or 24 hours of exposure. Because the air is sampled for only a few minutes and only a small volume of air actually is sampled, a calculation is needed to convert to the desired value. In addition, the sampling often is done several times daily, so an average value per sample time is used. Equation 37-1 shows how each factor is obtained and how the count/mVday is calculated. The first factor corrects for the small volume, and the second factor corrects for the short time of sampling. The 1440 min would be changed to 370 min for an 8-hour exposure. 

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