Ambient Air Pollution and COVID-19 in California

As of December 2023, more than 6.9 million people globally had died from COVID-19, including more than 1.165 million deaths in the United States. It is estimated that approximately 18.8 million people in the United States have experienced post-acute COVID-19 conditions, also known as post-acute sequelae of SARS-CoV-2 (PASC) or long COVID, in the first 3 years after the pandemic. Although some initial cases of long COVID have resolved, with the ongoing incidence of COVID-19, roughly 17.8 million persons in the United States continue to suffer from long COVID at the time of this writing.^1-3 Preliminary evidence early in the COVID-19 pandemic suggested that exposure to air pollution increased the likelihood of contracting COVID-19 and worsened outcomes for those who became ill. The validity of these findings was uncertain, however, as few studies used highly accurate exposure models incorporating individual-level data on patient characteristics and risk factors. Although the COVID-19 public health emergency has ended, the disease continues to pose substantial risks to individual and population health. At the time of this writing, nearly 35,000 individuals per week are hospitalized with COVID-19 in the United States, and the weekly number of COVID-19-related deaths ranges from 900 to 1,400.⁴.

In this study, we investigated relationships between ambient air pollution and COVID-19-related outcomes, including incidence, severity, mortality, and long COVID conditions. We used advanced models to estimate exposures, incorporating numerous air pollutants, particle species, and wildfire emissions. We used administrative COVID-19 data and several cohorts of patients from a large health system, and each was formed to evaluate different hypotheses.

Daily air pollution exposures for Southern California were estimated with high spatial and chemical resolution, using a combination of land use regression and chemical transport models for the years 2016, 2019, and 2020. Exposure variables included ozone (O₃), nitrogen dioxide (NO₂), fine particulate matter (PM) ≤2.5 μm in aerodynamic diameter (PM_2.5mass), ultrafine PM ≤0.1 μm in aerodynamic diameter (PM_0.1), and major sources or chemical components of PM in each size fraction. Exposures for multiple study populations were investigated using statistical analysis methods to test for associations with COVID-19-related outcomes, including the following.

• COVID-19 cases (N = 773,374) and deaths (N = 14,311), by age, race, and sex, for 308 ZIP codes in Los Angeles County between June 19 and January 3, 2021. A negative binomial regression was performed for both individual and multiple ambient air pollutants to evaluate their associations with COVID-19 incidence and mortality.

• Patients with COVID-19 who were admitted to Kaiser Permanente Southern California (KPSC) hospitals between June 1, 2020, and January 30, 2021 (N = 21,415). Cox proportional hazards models were used to evaluate associations between ambient air pollutant exposure and COVID-19 mortality. A subset was of KPSC patients with COVID-19 who received care exclusively in KPSC hospitals (N = 15,978). A multistate survival model was used to examine how air pollution affects the transition to recovery or deterioration to more severe COVID-19 states (e.g., intensive care admission or death). A subset was of KPSC patients with COVID-19 who maintained membership with KPSC for 1 year after hospital discharge (N = 12,634). We combined a set of 45 diagnoses of post-acute sequelae of SARS-CoV-2 (PASC) into categories based on organ systems and then studied a subset of these PASC categories that could be affected by air pollution, including cardiac, cardiometabolic, pulmonary, and neurological conditions. Logistic regression was used to evaluate associations between 30-day air pollution exposure before hospital admission and PASC conditions diagnosed at 3 months and 12 months post-discharge.

PM_0.1, O₃, NO₂, and PM_2.5 elemental carbon exposures were identified as risk factors for COVID-19 incidence and mortality in the general population of Los Angeles County. Air pollution exposures were also significantly associated with COVID-19 mortality in the cohort of hospitalized KPSC patients, controlling for other individual health risks. Incremental increases equivalent to the interquartile range for several pollution exposure concentrations were significantly associated with increased mortality, including PM_2.5 mass (hazard ratio [HR], 1.12), PM_0.1(HR, 1.06), PM_2.5 nitrate (HR, 1.12), PM_2.5 elemental carbon (HR, 1.07), PM_2.5 on-road diesel (HR, 1.06), and PM_2.5 on-road gasoline (HR, 1.07). Humidity and temperature in the month of diagnosis were significant negative predictors of COVID-19 mortality and negative modifiers of the air pollution effects. Results of the multistate analysis were consistent with these findings and further suggested that O₃, NO₂, and PM_2.5 each were associated with deteriorating health states. Increased PM_2.5 concentration was associated with increased risk of deterioration to both intensive care admission (HR, 1.16) and death (HR = 1.11). Effects of O₃ were similar to those of PM_2.5, but O₃ also affected the transition from recovery to death (HR, 1.24). Several air pollutants - particularly O₃, PM_0.1, and PM_2.5 nitrate - were significantly associated with several long COVID outcomes, including cardiac, cardiometabolic, and pulmonary conditions.

Broadly, we concluded that several common air pollutants are associated with COVID-19 incidence, mortality, and progression to more severe states of illness, including long COVID conditions. Air pollution is a modifiable environmental risk factor that could be altered to improve the prognosis of COVID-19, thereby also reducing the public health impacts of coronaviruses now and in the future. This is particularly important for preventing long COVID, as evidence suggests that PASC conditions can occur even in vaccinated individuals. Given that 10% to 30% of individuals with COVID-19 will experience some form of PASC, which can have lifelong debilitating effects,⁵ the importance of addressing modifiable environmental risk factors, such as air pollution, cannot be underestimated. A recent Lancet editorial noted that societal investment in understanding the pathogenesis of long COVID and preventive measures has lagged well behind the levels needed to effectively treat and mitigate this complex disease.⁶ Our research focused mostly on hospitalized patients, but it also included one study on the general population effects. The results of both analyses were generally concordant, although our most important findings likely apply only to patients hospitalized with COVID-19.