Rationale: Nearly 60% of U.S. children live in counties with particulate matter less than or equal to 2.5 mm in aerodynamic diameter (PM2.5) concentrations above air quality standards. Understanding the relationship between ambient air pollution exposure and health outcomes informs actions to reduce exposure and disease risk.

Objectives: To evaluate the association between ambient PM2.5 levels and healthcare encounters for acute lower respiratory infection (ALRI).

Methods: Using an observational case-crossover design, subjects (n = 146,397) were studied if they had an ALRI diagnosis and resided on Utah’s Wasatch Front. PM2.5 air pollution concentrations were measured using community-based air quality monitors between 1999 and 2016. Odds ratios for ALRI healthcare encounters were calculated after stratification by ages 0–2, 3–17, and 18 or more years.

Measurements and Main Results: Approximately 77% (n = 112,467) of subjects were 0–2 years of age. The odds of ALRI encounter for these young children increased within 1 week of elevated PM2.5 and peaked after 3 weeks with a cumulative 28-day odds ratio of 1.15 per 110 mg/m3 (95% confidence interval, 1.12–1.19). ALRI encounters with diagnosed and laboratoryconfirmed respiratory syncytial virus and influenza increased following elevated ambient PM2.5 levels. Similar elevated odds for ALRI were also observed for older children, although the number of events and precision of estimates were much lower.

Conclusions: In this large sample of urban/suburban patients, short-term exposure to elevated PM2.5 air pollution was associated with greater healthcare use for ALRI in young children, older children, and adults. Further exploration is needed of causal interactions between PM2.5 and ALRI.