Background: Fine particulate matter (PM2.5) produced by landscape fires is thought to be more toxic than that from non-fire sources. However, the effects of “fire-sourced” PM2.5 on acute respiratory infection (ARI) are unknown.

Methods: We combined Demographic and Health Survey (DHS) data from 48 countries with gridded global estimates of PM2.5 concentrations from 2003 to 2014. The proportions of fire-sourced PM2.5 were assessed by a chemical transport model using a variety of PM2.5 source data. We tested for associations between ARI and short-term exposure to fire- and “non-fire-sourced” PM2.5 using a bidirectional case-crossover analysis. The robustness and homogeneity of the associations were examined by sensitivity analyses. We also established a nonlinear exposure–response relationship between fire- and non-fire-sourced PM2.5 and ARI using a two-dimensional spline function.

Results: The study included 36,432 children under 5 years who reported ARI symptoms. Each 1 µg/m3 increment of fire-sourced PM2.5 was associated with a 3.2% (95% confidence interval [CI] 0.2, 6.2) increment in the risk of ARI. This effect was comparable to that of each 5 µg/m3 increment in PM2.5 from non-fire sources (3.1%; 95% CI 2.4, 3.7). The association between ARI and total PM2.5 concentration was significantly mediated by the proportion of fire-sourced particles. Nonlinear analysis showed that the risk of ARI was increased by both fire- and non-fire-sourced PM2.5, but especially by the former.

Conclusions: PM2.5 produced by landscape fire was more strongly associated to ARI among children under 5 years than that from non-fire sources.