Are cleaner cooking solutions clean enough? A systematic review and meta-analysis of particulate and carbon monoxide concentrations and exposures

Globally, approximately 3 billion primarily cook using inefficient and poorly vented combustion devices, leading to unsafe levels of household air pollution (HAP) in and around the home. Such exposures contribute to nearly 4 million deaths annually (WHO 2018a, 2018b ). Characterizing the effectiveness of interventions for reducing HAP concentration and exposure is critical for informing policy and programmatic decision-making on which cooking solutions yield the greatest health benefits. This review synthesizes evidence of in-field measurements from four cleaner cooking technologies and three clean fuels, using field studies aimed at reducing HAP concentration and personal exposure to health damaging pollutants (particulate matter (PM2.5) and carbon monoxide (CO)). Fifty studies from Africa, Asia, South and Latin America, provided 168 estimates synthesized through meta-analysis. For PM2.5 kitchen concentrations, burning biomass more cleanly through improved combustion stoves (ICS) with (n = 29; 63% reduction) or without (n = 12; 52%) venting (through flue or chimney) and through forced-draft combustion (n = 9; 50%) was less effective than cooking with clean fuels including ethanol (n = 4; 83%), liquefied petroleum gas (LPG) (n = 11; 83%) and electricity (n = 6; 86%). Only studies of clean fuels consistently achieved post-intervention kitchen PM2.5 levels at or below the health-based WHO interim target level 1 (WHO-IT1) of 35 mu g m(-3). None of the advanced combustion stoves (gasifiers) achieved WHO-IT1, although no evidence was available for pellet fuelled stoves. For personal exposure to PM2.5, none of the ICS (n = 11) were close to WHO-IT1 whereas 75% (n = 6 of 8) of LPG interventions were at or below WHO-IT1. Similar patterns were observed for CO, although most post-intervention levels achieved the WHO 24 h guideline level. While clean cooking fuel interventions (LPG, electric) significantly reduce kitchen concentrations and personal exposure to PM2.5 in household settings, stove stacking and background levels of ambient air pollution, have likely prevented most clean fuel interventions from approaching WHO-IT1. In order to maximize health gains, a wholistic approach jointly targeting ambient and HAP should be followed in lower-and-middle income countries.