Quantifying the wildfire contribution to PAH concentrations and health effects

Wildfires are an increasingly important source of toxic air pollutants, including carcinogenic polycyclic aromatic hydrocarbons (PAHs), with implications for human health under a changing climate. Here, we quantify the influence of fire events on PAH concentrations, chemistry, and associated incremental lifetime cancer risk (ILCR) using a global three-dimensional chemical transport model. Fires contribute up to 42% of local PAH concentrations in fire-prone regions, including Australia, sub-Saharan Africa, Siberia, and Canada, and account for approximately 4.6% of global total PAH concentrations. Fire emissions also enhance fine particulate matter (PM_2.5) levels, promoting partitioning of PAHs into the particle phase. Additionally, fires alter atmospheric oxidant levels, increasing the formation of highly toxic PAH degradation products, which contribute up to 55% of fire-sourced PAH cancer risk. As a result, fire-derived PAH mixtures are 51% more carcinogenic per unit mass than non-fire mixtures. Globally, fires contribute between 4 and 6% of the total all-source PAH ILCR, with some regions exceeding the recommended risk threshold of $1.0\times {10}^{-6}$ due solely to fire-related pollution. Sensitivity analyses indicate that even when fire emissions are dominated by less toxic lower molecular weight PAHs, fire-sourced PAHs are more toxic than anthropogenic-sourced PAHs, highlighting the critical role of atmospheric chemistry in modulating health impacts. These findings demonstrate that wildfires not only elevate PAH concentrations but also increase their per-mass toxicity through chemical transformations. With projections of increasing wildfire frequency and intensity under climate change, our results underscore the need for comprehensive monitoring, emission characterization, and mitigation strategies to address the growing public health risks of fire-related air pollution, particularly in regions frequently affected by wildfires.