We estimate the acute effect of air pollution oncognitive performance and how the effect varies with task proficiency usingdata of over 25,000 individuals and 927,000 plays of a popular onlinearithmetic training game. To isolate exogenous pollution fluctuation, weinteract spatial variation in NOx emissions with thermal inversion episodes.This exogenous variation negatively affects cognitive performance of proficientplayers, but not of beginners. Instrumenting NO2 pollution with the emissions ´ inversion interaction, we find that a 1 ppb concentration increaselowers the number of correct answers of proficient individuals by 0.6 percentand by 0.3 percent overall.