Firewood Smoke Studies Miss the Mixed-Fuel Reality of Global Kitchens

Billions of people still cook over a wood fire every day, filling their kitchens with a complex cocktail of smoke that includes some of the most hazardous air pollutants known to science. Among the more than 200 organic compounds released when wood burns incompletely, a group of aromatic volatile organic compounds known as BTEX—benzene, toluene, ethylbenzene, and xylenes—stands out for its toxicity. Benzene, the simplest and most abundant of these compounds, is classified by the International Agency for Research on Cancer as a Group 1 carcinogen, and no safe threshold for exposure has ever been established. Yet according to a new scoping review published in Environmental Challenges, the scientific evidence base used to understand and manage these emissions is built on foundations that bear little resemblance to how firewood is actually burned in homes around the world.

The review, conducted by Mahlodi Esther Masekela, systematically mapped the literature on BTEX emissions from household firewood combustion published between 1994 and 2025, following the Arksey and O’Malley scoping framework and adhering to PRISMA-ScR reporting guidelines. Searches across ScienceDirect, Web of Science, Google Scholar, and Scopus identified 583 records, which were screened down to just five eligible peer-reviewed studies. That tiny number is itself a striking finding: after three decades of research, only a handful of investigations have quantitatively characterized BTEX emissions from the specific firewood species burned in domestic cookstoves, and the review’s central concern is what those few studies leave out.

The core methodological problem is the mismatch between study design and real-world fuel use. Four of the five included studies—80 percent—examined only single firewood species, burning one taxonomically distinct wood type at a time. But households in low- and middle-income countries rarely do this. Research in South Africa has documented that families typically use bundles containing up to six different tree species, while studies in Ethiopia have found that mixed fuels, principally wood combined with animal dung, are the most common cooking fuels. Laboratory work has shown that blending fuels fundamentally alters both the total volatile organic compound concentrations and the relative proportions of individual compounds, meaning single-species emission profiles may simply not represent what happens in a real kitchen.

Geography compounds the problem. Four of the five studies were conducted in high-income countries, mostly in Europe—Portugal, Sweden, and Finland—plus one in the United States, while only a single study, from South Africa, represents the low- and middle-income country context. This distribution is starkly inverted relative to the disease burden: firewood accounts for roughly 25 to 60 percent of energy consumption in middle-income countries and up to 60 to 95 percent in developing contexts, while high-income countries derive less than 5 percent of their energy from wood, largely burning it in modern stoves designed to minimize emissions. All five studies were conducted in countries with less than 10 percent primary reliance on polluting fuels and cookstoves, meaning the existing evidence comes almost exclusively from low-exposure settings while the populations facing the highest exposures in Africa and Asia remain critically under-represented. Africa’s air quality monitoring density—just 0.03 monitors per million inhabitants—leaves BTEX emission factors and source profiles largely absent precisely where they are most urgently needed.

Setting matters just as much as species. Most of the reviewed studies were conducted in purpose-built laboratory combustion facilities, and none achieved a full rating for real-world setting representativeness in the review’s quality appraisal. The only study that approached household realism, the South African investigation in Senwabarwana, used a simulated kitchen structure and still could not capture the full variability of actual kitchen geometry, ventilation, and occupant behavior. This matters because field studies of other products of incomplete combustion have repeatedly shown that real-world emissions exceed laboratory measurements and display far greater variability, reflecting inconsistent stove operation, fluctuating fuel quality, and diverse user behaviors that controlled experiments systematically exclude. When laboratory data are used for population-level exposure assessment without field validation, the review warns, health burden estimates risk being systematically biased.

The synthesis also revealed a consistent chemical hierarchy that cuts across geography and methodology. Benzene was the most consistently reported and highest-emitting BTEX compound in every study, with emission profiles generally following the pattern benzene, then toluene, then ethylbenzene, then xylenes. Among studies reporting comparable emission factors in milligrams per kilogram, benzene values ranged from 108 mg/kg for European beech to 1,500 mg/kg for birch logs—a 13.9-fold difference across single-species combustion alone, though the review cautions that differences in adsorbent chemistry between the studies’ sampling methods may account for some of this spread. Benzene’s dominance echoes broader literature on residential wood combustion and suggests it may be a fundamental feature of firewood combustion chemistry rather than a species-specific artifact, arising from the thermal degradation of lignin, the principal aromatic precursor in wood. Still, with only two studies providing complete four-compound profiles, the review frames this pattern as preliminary rather than definitive.

Reporting practices added further obstacles to comparison. Only two of the five studies reported a complete BTEX profile; three omitted ethylbenzene entirely, and one reported benzene only. Ethylbenzene, a Group 2B possible carcinogen that is relatively more abundant in biomass-burning profiles and can help distinguish those emissions, was systematically absent from 60 percent of the studies, potentially due to co-elution with xylene isomers and intermittent detection. The studies also used incompatible metrics: four reported emission factors, which characterize fuel or stove performance, while others reported ambient concentrations relevant to health risk assessment. These quantities are not interchangeable, and the review argues that both are needed simultaneously to serve source characterization and exposure assessment alike. Analytical approaches further fragmented the evidence, spanning adsorbent-based gas chromatography, whole-air canister sampling, and Fourier-transform infrared spectroscopy, each with distinct trade-offs in detection limits, sample stability, and susceptibility to interference.

The health stakes are considerable. Households in low- and middle-income countries typically cook three times a day, four to six hours per session—roughly 21 meals per week, far above the global average—implying chronic exposure far exceeding the 365-day threshold used in toxicology. Benzene targets the hematopoietic system, with prolonged exposure linked to aplastic anemia and leukemia, while toluene is associated with cognitive impairment and cardiac sensitization, and xylenes with headaches and memory deficits. Simultaneous co-exposure complicates matters further: BTEX compounds compete for shared metabolic pathways involving the enzyme CYP2E1, producing less-than-additive metabolism but potentially greater-than-additive neurological effects as unmetabolized parent compounds persist in the bloodstream. With benzene concentrations measured at combustion sources running thousands of times above the World Health Organization’s most stringent risk-based reference level, and BTEX vapors persisting indoors for one to fourteen days, poorly ventilated kitchens may never fully clear between cooking episodes.

The review concludes that the effect of mixed-species combustion on BTEX emissions remains an unresolved gap in the literature, and it calls for future studies designed around the mixed fuel bundles and fuel stacking practices—wood co-burned with coal, charcoal, crop residues, and dung—that actually characterize household energy use in low- and middle-income settings, conducted within real homes rather than laboratories. Standardized full-profile BTEX reporting, integration of combustion frequency and ventilation data, and field-based exposure measurements are identified as priorities. Until the evidence base aligns with the conditions under which exposure actually occurs, the review warns, the populations bearing the greatest burden from firewood smoke will remain the least represented in the science meant to protect them.

Subject of Research: Methodological gaps in BTEX emission studies from household firewood combustion and their implications for indoor air pollution exposure assessment.

Article Title: Methodological Gaps in BTEX Emission Studies from Household Firewood Combustion: Implications for Exposure Assessment and Indoor Air Pollution

Article References: Masekela, M. E. (2026). Methodological Gaps in BTEX Emission Studies from Household Firewood Combustion: Implications for Exposure Assessment and Indoor Air Pollution. Environmental Challenges, Article 101665. https://doi.org/10.1016/j.envc.2026.101665

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101665

Keywords: BTEX, benzene, firewood combustion, indoor air pollution, household energy, exposure assessment, scoping review, LMICs, volatile organic compounds, clean cooking, wood smoke, public health