Impacts of travel activity and urbanicity on exposures to ambient oxides of nitrogen and on exposure disparities

Abstract

Daily exposures to ambient oxides of nitrogen were estimated here for residents of Hillsborough County, FL. The 2009 National Household Travel Survey provided geocoded data on fixed activity locations during each person-day sampled. Routes between activity locations were calculated from transportation network data, assuming the quickest travel path. To estimate daily exposure concentrations for each person-day, the exposure locations were matched with diurnally and spatially varying ambient pollutant concentrations derived from CALPUFF dispersion model results. The social distribution of exposures was analyzed by comparing frequency distributions of grouped daily exposure concentrations and by regression modeling. To investigate exposure error, the activity-based exposure estimates were also compared with estimates derived using residence location alone. The mean daily activity-based exposure concentration for the study sample was 17 μg/m³, with values for individual person-day records ranging from 7.0 to 43 μg/m³. The highest mean exposure concentrations were found for the following groups: black (20 μg/m³), below poverty (18 μg/m³), and urban residence location (22 μg/m³). Urban versus rural residence was associated with the largest increase in exposure concentration in the regression (8.3 μg/m³). Time in nonresidential activities, including travel, was associated with an increase of 0.2 μg/m³ per hour. Time spent travelling and at nonresidential locations contributed an average of 6 and 24 %, respectively, to the daily estimate. A mean error of 3.6 %, with range from -64 to 58 %, was found to result from using residence location alone. Exposure error was highest for those who travel most, but lowest for the sociodemographic subgroups with higher mean exposure concentrations (including blacks and those from below poverty households). This work indicates the importance of urbanicity to social disparities in activity-based air pollution exposures. It also suggests that exposure error due to using residence location may be smaller for more exposed groups.

Keywords: Environmental inequality; Exposure error; Human activity patterns; Traffic pollution; Urban form.

Fig. 1

The study area of Hillsborough County, FL. The inset provides the location of the study area within the state of Florida

Fig. 2

The spatial distribution of sample population activity time (% of time spent) and urbanicity in the study area. a % of total time spent in all activity types within the block group, b % of total time spent in nonresidential activities within the block group, c difference (%) between residential and nonresidential activity times spent in each block group, and d urbanicity category of the block group

Fig. 3

The estimated diurnal cycle of hourly average ambient NO_X concentrations (μg/m³) in the study area, from dispersion modeling results

Fig. 4

Cumulative distributions for the activity-based daily exposure concentration (left side), residence-based daily exposure concentration (middle), and daily exposure error between the two as a percent difference, (C _A−C _R) / C _A (right side). The box plot whiskers indicate the 5th and 95th percentile values, while cross indicates the mean value. Summary statistics are provided below each box plot; 95 % confidence intervals around each mean are in parentheses

Fig. 5

Cumulative distributions of daily activity-based exposure concentration for population subgroups related to a personal attributes and b urban characteristics. Category definitions are provided in the text. Note that the racioethnic subgroup populations are not exclusive, populations have overlapping individuals

Fig. 6

Cumulative distributions of time-weighted NO_X concentration (μg/m³) (left-side) and NO_X exposure (μg/m3)·h (right-side) by activity type location for all sampled daily records including some activity away from residence. Summary statistics are provided below each box plot; 95 % confidence intervals around each mean are in parentheses

Fig. 7

Cumulative distributions of exposure error for population subgroups related to a personal attributes and b urban characteristics. Middle income refers to households with income above the poverty threshold but with incomes less than $75 thousand (k). Note that the racioethnic subgroup populations are not exclusive, and populations have overlapping individuals