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value. The result represents the background concentration at a particular monitor
site from all contributing sources except major industrial sources. A background
concentration value was then assigned to each of the 1,520 receptor locations
through extrapolating the background concentrations of the three monitor sites using
a nearest neighbor function. Finally, values of these background concentrations were
then added to the air pollution data generated by the dispersion model for the 1,520
receptor locations for the 192 time points.
16.4.3
Exposure Assessment
To assess individual exposure to air pollution based on the particulate concentrations
generated by the above steps, participants of the Mid-Ohio Area Household Travel
Survey whose assigned survey days (days on which they provided activity-travel
data) matched the days of the particulate matter concentrations, whose trips were
undertaken entirely within the boundary of Franklin County, and whose travel
locations were successfully geocoded were selected from the dataset. There were
292 such participants and they were used as the subsample for assessing individual
exposure to particulate matters in this study.
Two exposure measures were evaluated in this study: static exposure and
dynamic exposure. Static exposure assumes that an individual's exposure is deter-
mined only by the level of pollution concentrations at the residential location and
thus assesses individual exposure based only on the home location. This value was
obtained by a simple summation of particulate concentrations at the home location
of each selected participant for the 192 time points (24 h for the 8 survey days).
Dynamic exposure, on the other hand, takes into account the effects of people's
travel on their exposure. To obtain this value, people's location at different times
of a day was first identified and their exposure at a particular time point was then
determined based on where they were at that time. Finally all exposure values thus
identified for the 192 time points were summed up to obtain the person's dynamic
exposure.
Since particulate concentrations in the study area were estimated on an hourly
basis, locations of the 292 selected participants at the beginning of each hour for
the entire 24-h period of the eight survey days were first identified. If an individual
was undertaking an activity at a stationary location (i.e., home, workplace, etc.) at
the start of the hour, then the concentration value associated with that location was
applied to his or her 24-h summation of particulate matter exposure. On the other
hand, an individual may be traveling at the start of a given hour. For this kind of
cases, location of the individual along the route at the beginning of a given hour was
estimated using information on the origin and destination locations, the departure
and arrival times of each trip, as well as the travel speeds of the road segments
traversed. Because the activity-travel diary dataset does not provide information
about the actual routes taken, this estimation was based on the assumption that the
shortest path between an original and destination location was used for all the trips
by the participants.
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