Cartography

Objective

The aim of this study is to determine if there is a spatial relationship between air pollution and race at the census tract level in Pierce County, Washington. The Asian and Black populations were selected as the focus due to having similar total populations in the county overall. Exposure to air pollution, which can pose health risks such as respiratory and heart problems, cancer, and premature death, is not equal all populations. Income, education, gender, race, and ethnicity can all be factors influencing health equity (Tacoma-Pierce County Health Department, 2016). 

Background

Pierce County is located in the Puget Sound region in Washington (Figure 1). With a population of about 910,000, it is the second most populous county in the state after King County to the north. It contains Mount Rainier, which is the tallest mountain in the state and the entire Cascade Range. Much of the population resides in the lower elevation areas along the Interstate 5 corridor in the major cities of Tacoma, Lakewood, and Puyallup. The local economy includes lumber and paper mills, shipping through the Port of Tacoma, healthcare services, military services at the Joint Base Lewis-McChord, and growing half of the nation's rhubarb supply (Pierce County, 2021).

Data for this project include American Community Survey 5-Year Estimates (ACS) from the U.S. Census Bureau from 2017 through 2021. Air pollution data is from the Environmental Protection Agency's Toxic Release Inventory (TRI) for 2022. This includes the amounts of toxic chemicals that industrial facilities released into the air as the sum of fugitive and stack emissions. The projected coordinate system used for all static maps is the NAD 1983 HARN State Plane Washington South FIPS 4602 (U.S. feet).

Demographics

From the ACS data, almost 70% of the Pierce County population identifies as White alone. The percent Asian population and the percent Black population of each census tract are shown in Figure 2 as choropleth maps. For both populations, the census tracts with higher percentages tend to be in the more urban area along Interstate 5 with lower percentages in the more rural areas in the southeast toward Mount Rainier.

For the Asian population, the county average is 6.5% while the census tract with the highest percentage of 39% is in the city of Puyallup. The general trend is that census tracts with the highest percent Asian populations are to the north and east in the more urban area. The county average for the Black population is 6.9% and the the census tract with the highest percentage of 31% is in southwest Tacoma. Within the urban area, census tracts with the highest percent Black populations tend to be in the south and west.

Air Pollution

The total pounds of air emissions in 2022 for each TRI facility are shown in Figure 3. All 24 of these industrial facilities are within the more urban area of the county. The distribution of air releases are positively skewed towards higher pollution amounts. There are more facilities with lower amounts of emissions than there are facilities with significantly higher amounts.

Approximately 93% of the total pounds of emissions occurred from facilities in the north of the county around the Port of Tacoma on the Puget Sound. These facilities mostly operate in the paper and wood industries or in other industries associated with shipping. The second largest group of polluting facilities are to the south near Frederickson and work with transportation and electrical equipment as well as construction materials.

Comparison

To determine how the variables of race and air pollution may be related at the census tract level, the total air pollution for each census tract was found by aggregating the emission amounts from all TRI facilities within it. An interactive visualization was created to make comparisons between air pollution and each population (Figure 4). In these bivariate choropleth maps, more saturated blue colors are used to indicate census tracts with higher percentages of that population and more saturated reds show census tracts with higher air pollution amounts. Combinations of the two variables are created by overlapping colors, so that the census tracts with the highest percentages of the population and highest air pollution display as darker purple colors.

Analysis

With only ten of the 193 census tracts containing TRI facilities, it is difficult to make comparisons between the two populations across the individual dashboards. For each census tract, the percent Asian population was subtracted from the percent Black population. A census tract was considered to have a higher population if the difference between populations was at least 3% of the total census tract population. If the difference between the two populations was less than 3%, then the populations were considered to be similar. This allowed for the relationship between race and air pollution to be explored in a single bivariate choropleth map (Figure 5).

The differences between populations seem to align with the trends observed in the population choropleth maps in Figure 2. Census tracts in the north and east of the urban area tend to have higher Asian populations, while those in the south and west tend to have higher Black populations. The ten census tracts with air pollution data have a similar distribution across the three population groups and are summarized in Figure 6. Although there are more census tracts that have a higher Black population and high air pollution amounts, it is difficult to define any trend with the overall lack of air pollution data.

Conclusion

A spatial relationship between industrial air pollution and the Asian or Black populations in Pierce County could not be ascertained. As the Port of Tacoma is such a dominant contributor of air pollution, a more sound determination can not be made in the absence of other polluting facilities. Any other visual trends present seem to be a result of the the distributions of the population alone. Statistical regression would be a typical next step for analysis, but with so few data points any correlations found may not be realistic. Instead, the study could be expanded to include other sources of air pollution such as fossil fuel use for energy production, transportation related emissions, and smoke from wildfires. Beyond just the visual trends are how this data could be further implemented. A more robust equity analysis that includes other socioeconomic factors such as income, education, the cost of housing, and access to healthcare could lead to more meaningful actions towards environmental justice.

References

Pierce County. (2021, June). About Pierce County. Pierce County. https://www.piercecountywa.gov/193/About-Pierce-County

Tacoma-Pierce County Health Department. (2016). Fairness Across Places? Your Health in Pierce County: 2015 Health Equity Assessment. Tacoma-Pierce County Health Department. https://tpchd.org/wp-content/uploads/2023/12/Full-Report-Your-Health-in-Pierce-County-2016.pdf

Data Sources

Pierce County. (2019). Parks polygons [Shapefile]. Pierce County Open Geospatial Data Portal. https://gisdata-piercecowa.opendata.arcgis.com/datasets/piercecowa::parks-polygons-1/about

U.S. Census Bureau. (2021). 2021 TIGER/Line Shapefiles [Geodatabase]. U.S. Census Bureau. https://www.census.gov/geographies/mapping-files/time-series/geo/tiger-data.html

U.S. Environmental Protection Agency. (2022). Toxic Release Inventory Basic Data File [CSV]. U.S. Environmental Protection Agency. https://www.epa.gov/toxics-release-inventory-tri-program/tri-basic-data-files-calendar-years-1987-present

U.S. Geological Survey. (2023). National Hydrography Dataset Best Resolution (NHD) – Washington [Geodatabase]. U.S. Geological Survey. https://www.sciencebase.gov/catalog/item/61f8b8ebd34e622189c32939

U.S. Geological Survey. (2014). Shuttle Radar Topography Mission 1 Arc-Second Global: SRTM1N46W122V3 [GeoTIFF]. Earth Resources Observation and Science Center. https://earthexplorer.usgs.gov

U.S. Geological Survey. (2014). Shuttle Radar Topography Mission 1 Arc-Second Global: SRTM1N46W123V3 [GeoTIFF]. Earth Resources Observation and Science Center. https://earthexplorer.usgs.gov

U.S. Geological Survey. (2014). Shuttle Radar Topography Mission 1 Arc-Second Global: SRTM1N47W122V3 [GeoTIFF]. Earth Resources Observation and Science Center. https://earthexplorer.usgs.gov

U.S. Geological Survey. (2014). Shuttle Radar Topography Mission 1 Arc-Second Global: SRTM1N47W123V3 [GeoTIFF]. Earth Resources Observation and Science Center. https://earthexplorer.usgs.gov

Washington State Department of Transportation. (2010). WSDOT - City Points [Shapefile]. Washington Geospatial Open Data Portal. https://geo.wa.gov/datasets/WSDOT::wsdot-city-points/about

Washington State Department of Transportation. (2024). WSDOT - Highway Urban and Urbanized Area (2023) [Shapefile]. Washington Geospatial Open Data Portal. https://geo.wa.gov/datasets/WSDOT::wsdot-highway-urban-and-urbanized-area-2023/about

Washington State Department of Transportation. (2024). WSDOT - State Route (1:500K) Current [Shapefile]. Washington Geospatial Open Data Portal. https://geo.wa.gov/datasets/WSDOT::wsdot-state-route-1500k-current/about