Toxic Air Contaminant Emissions from Residential and Commercial Fossil Gas Appliances: A Systematic Review.

PURPOSE OF REVIEW: Gas appliances are increasingly recognized as sources of indoor toxic air contaminant (TAC) exposure, yet no prior review has systematically synthesized emission factor (EF) data across appliance types and fuels. We reviewed the literature to characterize TAC emissions from residential and commercial gas appliances, identify data gaps, and assess implications for regulation and public health. RECENT FINDINGS: Twenty-five studies met inclusion criteria. Evidence of nonzero emissions was identified for at least 41 TACs released through gas leakage and combustion pathways. Formaldehyde and benzene were the only compounds with sufficient data for quantitative characterization, with EFs varying by up to two orders of magnitude across studies and operating conditions. The literature was heavily concentrated on cooking stoves, while heaters, water heaters, and dryers were rarely studied. Emerging evidence suggests methane and carbon monoxide measurements may support inference-based estimation of leakage- and combustion-related TAC emissions. Gas appliances emit multiple carcinogenic and respiratory toxicants through both leakage and combustion. Expanded monitoring, standardized EF methodologies, and targeted measurement campaigns for understudied appliances are needed to support appliance standards, emissions inventories, and health-protective building decarbonization policies.