Long-Term Air Pollution Exposure, Plant-based Diet, and Asthma Exacerbations in the Nurses' Health Study II

Short-term ambient air pollution exposure may worsen asthma health. Effects of longer-term air pollution exposures on asthma exacerbations and risk mitigation by dietary factors are unknown.

To examine associations between 48-month air pollution exposure and asthma exacerbations and whether a plant-based diet modifies these relationships.

Women with asthma in the Nurses' Health Study II were followed from 1997 to 2014. We estimated 48-month time-varying average residential ambient fine particulate matter ≤2.5 μm in aerodynamic diameter (PM2.5), nitrogen dioxide (NO2), and ozone (O3) exposures using nationwide spatiotemporal models. Plant-based diet index (PDI) scores were calculated on the basis of food frequency questionnaires administered every 4 years. Air pollution and diet assessments were repeated measures within individuals, whereas asthma exacerbations in the past year were captured in 1998 and 2014. Average air pollutant exposure was assessed in the 48 months before each outcome assessment year. Single-pollutant and multipollutant logistic regression models with generalized estimating equations to account for repeated measures within participants were used to assess the effects of each air pollutant on asthma exacerbation risk. We also evaluated effect measure modification by PDI scores on the effects of each air pollutant on asthma exacerbation risk using two-way interaction terms.

Of 4,326 participants, median 48-month PM2.5, NO2, and O3 concentrations were 13.7 μg/m3, 12.0 ppb, and 25.5 ppb, respectively, from July 1993 to June 1997 and 8.9 μg/m3, 6.6 ppb, and 27.8 ppb, respectively, from July 2009 to June 2013. In adjusted single-pollutant models, greater exposures to both PM2.5 and NO2 were associated with higher odds of asthma exacerbation (odds ratio [OR], 1.43; 95% confidence interval [CI], 1.14-1.80; and OR, 1.25; 95% CI, 1.12-1.38, respectively). In multipollutant models, greater exposure to NO2 was associated with higher odds of asthma exacerbation (OR, 1.23; 95% CI, 1.06-1.42). There were no statistically significant interactions between pollutants and PDI score on asthma exacerbations.

Long-term exposure to ambient NO2 and PM2.5, even at low concentrations, may increase asthma exacerbation risk in women but is not attenuated by a plant-based diet as measured herein. Further research is needed on the long-term effects of inhaled pollutants on asthma health and personal, modifiable strategies to reduce risk.