Compound Risk of Air Pollution and Heat Days and the Influence of Wildfire by SES across California, 2018-2020: Implications for Environmental Justice in the Context of Climate Change

Abstract

Major wildfires and heatwaves have begun to increase in frequency throughout much of the United States, particularly in western states such as California, causing increased risk to public health. Air pollution is exacerbated by both wildfires and warmer temperatures, thus adding to such risk. With climate change and the continued increase in global average temperatures, the frequency of major wildfires, heat days, and unhealthy air pollution episodes is projected to increase, resulting in the potential for compounding risks. Risks will likely vary by region and may disproportionately impact low-income communities and communities of color. In this study, we processed daily particulate matter (PM) data from over 18,000 low-cost PurpleAir sensors, along with gridMET daily maximum temperature data and government-compiled wildfire perimeter data from 2018-2020 in order to examine the occurrence of compound risk (CR) days (characterized by high temperature and high PM_2.5) at the census tract level in California, and to understand how such days have been impacted by the occurrence of wildfires. Using American Community Survey data, we also examined the extent to which CR days were correlated with household income, race/ethnicity, education, and other socioeconomic factors at the census tract level. Results showed census tracts with a higher frequency of CR days to have statistically higher rates of poverty and unemployment, along with high proportions of child residents and households without computers. The frequency of CR days and elevated daily PM_2.5 concentrations appeared to be strongly related to the occurrence of nearby wildfires, with over 20% of days with sensor-measured average PM_2.5 > 35 μg/m³ showing a wildfire within a 100 km radius and over two-thirds of estimated CR days falling on such days with a nearby wildfire. Findings from this study are important to policymakers and government agencies who preside over the allocation of state resources as well as organizations seeking to empower residents and establish climate resilient communities.

Keywords: PM2.5; air pollution; climate change; compound risk; heatwaves; purpleair.

Figure 1.

Wildfire burn areas and the distribution of unique PurpleAir sensors for the years 2018–2020 across California.

Figure 2.

Monthly average PM_2.5 concentrations across entire state of California by year, based on aggregation of sensor measurements and spatial interpolation.

Figure 3.

Percent of (a) minority populations, (b) temperature and air pollution vulnerability indicators and (c) economic indicators averaged across census tracts grouped by frequency of compound-risk (CR) days per year. Trendlines are presented to illustrate the direction of the slope and should not be interpreted as indicating a linear relationship.

Figure 4.

Percent of economic indicators averaged across census tracts grouped by frequency of compound-risk (CR) days per year. Trendlines are presented to illustrate the direction of the slope and should not be interpreted as indicating a linear relationship.

Figure 5.

Scatter plots of total wildfire burn area across California compared to the percent of (a) high temperature days (max daily temp. > 35 °C), high PM_2.5 days (mean daily PM_2.5 > 50 μg/m³), and compound risk (CR) days (both high temp. and high PM_2.5) within all California census tracts and (b) mean annual PM_2.5 concentrations within both urban and rural census tracts in California from 2018 to 2020.

Figure 6.

Number of sensor-based PM_2.5 observation days where average daily measurements exceeded various concentration thresholds (blue line) and the percent of those measurements (orange bars) where a wildfire was reported within a 100 km radius.

Figure 7.

Scatter plots of average daily PM_2.5 concentrations as measured by PurpleAir sensors grouped according to the (a) number of wildfires reported within 100 km of a sensor on the same day of PM_2.5 measurement, (b) the distance of a sensor to the nearest wildfire on the same day of PM_2.5 measurement, and (c) the cumulative area of wildfires that burned across California during the same week of measurement.

Figure 8.

Compound risk days overlaid with heat days at the census tract level.