This paper studies the impact of environmental quality on the performance of individuals undertaking cognitively demanding tasks under time pressure. We link measures of indoor air quality and thermal conditions to the performance of chess players at official tournaments where players face strong incentives to exert high effort. We use a state-of-the-art chess en- gine to detect erroneous moves and evaluate the quality of the move by comparing the quality of a player’s actual moves with the best moves proposed by the chess computer. The results indicate air pollution (PM2.5) is the deterring factor hindering cognitive performance. We find that an increase of 10 μg/m3 raises the probability of making an error by 1.5 percentage points, and increases the magnitude of the errors by 9.4%. The impact of pollution is exac- erbated by time pressure. When players approach the time control of games, an increase of 10 μg/m3, corresponding to about one standard deviation, increases the probability of mak- ing a meaningful error by 3.2 percentage points, and errors being 17.3% larger. Our results have important implications for high-skilled office workers, in particular, for those executing non-routine cognitive tasks whose share is steadily increasing in developed countries.