Particle-Bound PAHs and Oxidative Potential in Indoor Particulate Matter from Biomass Cookstoves in Rural Rwanda

Traditional cooking with biomass remains a major source of household air pollution. Still, the relationship between fine particulate matter (PM1) composition and toxicological properties, and how that differs between cookstove types, under real-life indoor conditions, is not well understood. This study investigates particle-bound polycyclic aromatic hydrocarbons (PAHs) and oxidative potential (OP) of indoor PM1 associated with cooking, using four cookstoves of different technology levels with wood fuel from Sesbania sesban. Measurements across six households captured variability in cookstove type, kitchen characteristics and user behavior. A unique set of 52 particulate PAH species in indoor air, including 30 parent-, 12 alkylated-, and 10 oxygenated PAHs, is presented. Parent PAHs were the dominant PAH class across all cookstove technologies, with fluoranthene and pyrene being the most abundant individual compounds, while medium- and high-molecular-weight PAHs constituted the major fraction of particle-bound PAHs. The average indoor air concentrations of PM1 ranged from 378 to 20019 μg m–3, and total PAHs from 1.28 to 181 μg m–3. The traditional three-stone fire generally produced the highest PM1 and PAH concentrations, whereas the industry-manufactured natural draft gasifier stove consistently achieved the lowest PM1 (<1000 μg m–3) and PAH concentrations (1.28–3.99 μg m–3), although still substantially elevated from a health-based air-quality perspective. However, performance varied widely among cookstove types and households, with a significant contribution (40–49%) from household-specific factors on total variability. Mass-normalized OP showed no consistent trend across cookstove technologies, whereas volume-normalized OP increased substantially with increased levels of PM1 and total PAHs, i.e., near 1 order of magnitude in some stove-household combinations. Despite detailed PAH speciation, weak associations with OP indicate that bulk PAHs are not reliable predictors of PM related redox activity, and consequent oxidizing activity in humans, pointing to contributions from unmeasured co-emitted species. The study emphasizes the potential for high indoor PAH exposure during cooking events, especially for the three stone fire, but also for the rocket stove and one natural draft gasifier stove. Our findings also indicate that reductions in fine particulate-related PAH exposure, likely depend on the combined effects of stove performance, user behavior and specific household conditions, which highlights the need for integrative metrics and context-specific household air pollution mitigation and clean cooking strategies.