Methodological gaps in BTEX emission studies from household firewood combustion: implications for exposure assessment and indoor air pollution

Firewood remains a prevalent source of domestic energy for low- and middle-income countries (LMICs) and a source of benzene, toluene, ethylbenzene, and xylenes (BTEX) emissions, which contribute to indoor air pollution and exposure to hazardous BTEX compounds. Although numerous studies have quantified BTEX emissions from biomass combustion, it remains unclear whether existing study designs adequately reflect real-world household fuel-use practices, particularly the use of mixed-species firewood bundles which is common in LMICs households. This scoping review examined the extent to which literature compares BTEX emissions from the combustion of firewood bundles consisting of single-species and mixed-species. Literature searches were conducted across ScienceDirect, Scopus, Web of Science, and Google Scholar for studies published between 1994 and 2025. Five studies met the inclusion criteria. Results indicate a methodological emphasis on single-species combustion, particularly European hardwoods, under laboratory conditions within high-income countries, failing to represent LMICs combustion practices. Benzene was the most consistently reported compound across all five studies and was generally reported at higher emission factors and concentrations relative to other BTEX compounds, with values in studies reporting emission factors ranging from 108 mg/kg (European beech) to 1500 mg/kg (birch logs), a 13.9-fold difference across single-species combustion alone. However, incomplete BTEX profiling, particularly omissions of ethylbenzene, together with substantial heterogeneity in analytical methods and inconsistent reporting metrics constrained cross-study comparisons. Future research should prioritise context-specific studies that align with the realities of household fuel-use to inform targeted clean cooking interventions, improve alignment with WHO IAQ guidelines, and support progress towards SDG7.