Associations between Short-Term Exposure to Ambient Air Pollution and Influenza: An Individual-Level Case-Crossover...

Influenza imposes a heavy burden on public health. Little is known, however, of the associations between detailed measures of exposure to ambient air pollution and influenza at an individual level.

We examined individual-level associations between six criteria air pollutants and influenza using case-crossover design.

In this individual-level time-stratified case-crossover study, we linked influenza cases collected by the Guangzhou Center for Disease Control and Prevention from 1 January 2013 to 31 December 2019 with individual residence-level exposure to particulate matter (${\mathrm{PM}}_{2.5}$ and ${\mathrm{PM}}_{10}$), sulfur dioxide (${\mathrm{SO}}_2$), nitrogen dioxide (${\mathrm{NO}}_2$), ozone ($O_3$) and carbon monoxide (CO). The exposures were estimated for the day of onset of influenza symptoms (lag 0), 1-7 d before the onset (lags 1-7), as well as an 8-d moving average (lag07), using a random forest model and linked to study participants' home addresses. Conditional logistic regression was developed to investigate the associations between short-term exposure to air pollution and influenza, adjusting for mean temperature, relative humidity, public holidays, population mobility, and community influenza susceptibility.

$N=\mathrm{108,479}$ eligible cases were identified in our study. Every $10{\text{-}\mu g/m}^3$ increase in exposure to ${\mathrm{PM}}_{2.5}$, ${\mathrm{PM}}_{10}$, ${\mathrm{NO}}_2$, and CO and every ${5\text{-}\mu g/m}^3$ increase in ${\mathrm{SO}}_2$ over 8-d moving average (lag07) was associated with higher risk of influenza with a relative risk (RR) of 1.028 (95% CI: 1.018, 1.038), 1.041 (95% CI: 1.032, 1.049), 1.169 (95% CI: 1.151, 1.188), 1.004 (95% CI: 1.003, 1.006), and 1.134 (95% CI: 1.107, 1.163), respectively. There was a negative association between $O_3$ and influenza with a RR of 0.878 (95% CI: 0.866, 0.890).

Our findings suggest that short-term exposure to air pollution, except for $O_3$, is associated with greater risk for influenza. Further studies are necessary to decipher underlying mechanisms and design preventive interventions and policies. https://doi.org/10.1289/EHP12145.