Human exposure assesment 1999

Human exposure assesment 1999 - 6 HUMAN EXPOSURE ASSESSMENT...

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P M S c i e n c e A s s e s s m e n t 6 - 1 6 HUMAN EXPOSURE ASSESSMENT The likelihood of an adverse response to particles is influenced by the degree of exposure, defined as any contact between the chemical at a specified concen- tration and the outer (e.g., skin) or inner (e.g., respi- ratory tract epithelium) surface of the human body (Sexton and Ryan, 1988; Lioy, 1990). Exposure im- plies the simultaneous occurrence of these two events in time and space (Ott, 1982). Changes in the degree of exposure are influenced by the duration (how long you are exposed), magnitude (concentra- tion) and frequency (how often you are exposed) of exposure. Inhalation is the only exposure pathway considered in the assessment. This chapter will briefly: review the factors influencing exposure and the methods of determining air pollution exposure, review monitoring studies of personal and micro- environmental particle concentrations, present the results of a probabilistic exposure model, and dis- cuss the relationship between current ambient levels of particles and human exposure in Canada. 6.1 CONCEPTS Ambient particles are typically collected over a 24 h sampling period, although hourly average measure- ments of particle mass help illustrate the diurnal, weekly and seasonal variability in particle concentra- tions. Until more hourly data become available, expressing exposure over 24 h facilitates compari- son of individual and population exposures with ambient particle levels. Over a 24 h period, a person spends their time in many locations or microenvironments. US residents spend on average 21½ hours indoors, one hour out- doors, and 1½ hours in a vehicle. The data vary by age, gender and day of the week (US EPA, 1995b). For an individual, microenvironmental exposures lasting minutes to hours can be summed over one day and expressed by the following equation: E c t l ij ij j J = = 1 (6.1) where E i equals the exposure to individual I , t i j denotes the time the I-th individual spent in the j-th microenvironment, and c i j denotes the average pollutant concentration for the I-th individual in the j-th microenvironment (Ott, 1982; Duan, 1991). This equation is based upon the concept that concentra- tions of pollutants measured in locations where people are, multiplied by the time spent in each place, will approximate personal exposure (Lioy, 1990). Therefore, individual and population exposures will vary, depending on the time spent in various micro- environments (frequency and duration of exposure) and the particle concentrations in those micro- environments (magnitude of exposure). With people spending, on average, less than 10% of their time outdoors (Özkaynak et al., 1995; U.S. EPA, 1995b), it is important to understand what influences the concentrations of particles in other microenvironments.
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