Environment and Asthma in California

Milestone 6

Problem Statement

In California, asthma remains a significant public health challenge, where cities rank among the most polluted in the nation (American Lung Association [ALA], 2025). Outdoor air pollutants, including ozone and fine particulate matter (PM₂.₅), trigger asthma symptoms and emergency department (ED) utilization (Fan et al., 2016; Zheng et al., 2015). These air pollutants disproportionately burden communities already facing health and socioeconomic disadvantages (ALA, 2023; Perez & Coutinho, 2021). To examine population vulnerabilities, the CalEnviroScreen (CES) tool aggregates multiple environmental indicators - including ambient air pollutants, traffic density, and fine particulate matter exposures - to compare across census tracts (Office of Environmental Health Hazard Assessment, 2021). This report will analyze CES data and county ED rates to (1) evaluate associations between environmental indicators and asthma morbidity, and (2) determine whether specific environmental measures warrant further investigation to inform target environmental health interventions.

Methods

CalEnviroScreen 4.0

The CalEnviroScreen (CES) 4.0 datasets (CES Measures & CES Demographics datasets) were obtained from the 2021 California Office of Environmental Health Hazard Assessment (COEHHA). They include environmental measures (e.g., PM2.5, ozone, drinking water contamination, CES 4.0 score) and demographics (e.g., population, race/ethnicity, age group) at the census tract level. Census tract designations are from the 2010 Census, and demographic data is from the 2015–2019 American Community Survey. Column names were cleaned, and “County” was removed from the county name column to standardize datasets in preparation for joins. CES 4.0 score, PM2.5, and ozone were selected to address the research question. Measures were then aggregated to the county level, using population weighting to allow for a more precise view of county-level exposure.

Asthma Emergency Department (ED) rates

Crude and stratified asthma ED visits by age and race/ethnicity were obtained through 2015–2020 California Health and Human Services data. Columns were cleaned and values in the strata column were renamed. The dataset was converted to wide format and overlapping race/ethnicity strata values were aggregated into “other/multiple”. ED visit and age-adjusted ED visit rates were aggregated. Total ED visit rates from aggregation were then used to calculate the percentage of race/ethnicity-specific ED visits per county. Then, filtered to only show data for 2020. A regression analysis was utilized to examine the relationship between race/ethnicity and asthma ED visits.

Results

Summary Table: Asthma ED Rates, CES 4.0 Scores and Air Quality Indicators by County

This table summarizes the distribution of county-level asthma morbidity (asthma ED visit rate per 100,000), environmental burden and social vulnerability (population-weighted CES 4.0 score), and asthma-related air quality indicators (population-weighted PM2.5 and ozone concentrations) across California counties. “Risk” levels were defined for each indicator based on state averages and guidelines from WHO and the EPA. Progressive cell shading indicates potential risk of environmental and/or health burden to a population. WHO guidelines were included for comparison to EPA standards due to WHO’s more conservative thresholds for environmental exposure. Note, for ozone, no county levels exceeded the EPA standards, so average county ozone concentration was added as the second risk level.

Average CES 4.0 vs Asthma ED Visits per CA County

This is a high-level view of the relationship between Average CES 4.0 scores per California county, and their age-adjusted asthma ED visit rate. A line of best fit shows increasing rates of ED visits among counties with higher CES 4.0 scores. The color gradient indicates the percent of the county population living below the federal poverty line (aggregated from the census tract level using population weights). Notable outliers with higher asthma ED rates, like Modoc and Lake Counties, have a high percentage of individuals living below the poverty rate. On the other side with a low poverty rate, Marin County has notably low asthma ED rates. This suggests that socioeconomic status is an important variable to consider in the relationship between CES 4.0 and asthma ED visits.

Summary: Asthma ED Visits vs CES 4.0 Scores by Race/Ethnicity per County

This visual describes the relationship between the percentage of race/ethnicity-specific asthma ED visits (aggregated from total ED visits in 2020 by race/ethnicity-strata) and average CES 4.0 scores (aggregated from total CES 4.0 scores) across all California counties. Counties with higher percentages of Black and Hispanic asthma ED visits had higher average CES 4.0 scores. Meanwhile, counties with higher percentages of White and Asian asthma ED visits had lower CES 4.0 scores. However, in some cases, higher percentages of American Indian/Alaska Native and “Other/Multiple” race/ethnicity groups indicated no clear correlation.

Discussion

Our analysis highlights the complex relationship between environmental exposures, asthma morbidity, and sociodemographic factors. Through our data, we observed a relationship between CalEnviroScreen (CES) 4.0 scores and asthma ED visit rates at the county level, with ED rates trending higher as CES 4.0 scores increase. Importantly, socioeconomic status affected this relationship, as counties with more residents below the poverty line experienced higher asthma ED rates regardless of their CES 4.0 scores.

Although no clear correlation between race/ethnicity-specific asthma ED visit rates and CES 4.0 scores could be established, differences in race/ethnicity-specific asthma ED visit rates across California revealed that counties with higher Black and Hispanic asthma ED visit rates often experienced higher CES 4.0 scores. This suggests potential racial/ethnic disparities throughout California that are exacerbating asthma rates. 

Lastly, through a cumulative risk analysis conducted using Summary Table data, we identified counties experiencing the highest combined burden of asthma morbidity, cumulative environmental and demographic vulnerability (CES 4.0 score), and air quality exposures. Fresno and Tulare were the highest-risk counties, each meeting three high-risk criteria, while several Central Valley counties - including Merced, Kern, Kings, and Madera—also exhibited elevated cumulative risk. Based on our findings, interventions must consider geographic, racial/ethnic, and socioeconomic disparities to reduce California asthma rates.

Sources

American Lung Association. (2025). State of the air 2025 report. https://www.lung.org/getmedia/5d8035e5-4e86-4205-b408-865550860783/State-of-the-Air-2025.pdf

American Lung Association. (2023, November 2). Disparities in the impact of air pollution. https://www.lung.org/clean-air/outdoors/who-is-at-risk/disparities

Office of Environmental Health Hazard Assessment. (2021). CalEnviroScreen 4.0 report. https://oehha.ca.gov/sites/default/files/media/downloads/calenviroscreen/report/calenviroscreen40reportf2021.pdf

Fan, J., Li, S., Fan, C., Bai, Z., & Yang, K. (2016). The impact of PM₂.₅ on asthma emergency department visits: a systematic review and meta-analysis. Environmental Science and Pollution Research, 23(1), 843–850.

Perez, M. F., & Coutinho, M. T. (2021). An Overview of Health Disparities in Asthma. The Yale journal of biology and medicine, 94(3), 497–507.

Zheng, X-Y., Ding, H., Jiang, L-N., et al. (2015). Association between air pollutants and asthma emergency room visits and hospital admissions: A systematic review and meta-analysis of time series studies. PLoS ONE, 10(9), e0138146.