Long-term exposure to biomass fuel smoke indoors is causally associated with chronic obstructive pulmonary disease (COPD) 1 but the effect of acute exposures has not been studied. Should acute effects occur, this might increase the risk of an exacerbation of pre-existing lung disease. We studied acute changes in lung function and oxygen saturation in individuals exposed regularly to biomass smoke in Nepal. Spirometry (EasyOne spirometer using American Thoracic Society (ATS) criteria) and oxygen saturation data were collected from 26, randomly selected, life-long non-smoking women (mean6SD age 38.0611. 7 years) before and after cooking (morning or evening). Smoke concentrations were measured using a DustTrak (TSI, Shoreview, Minnesota, USA) during cooking events and expressed as PM2. 5 (particulate matter of< 2.5 mm in diameter).