Abstract Air pollutants were liberated in a laboratory, and the decay of concentration was measured by standard methods during the return to equilibrium. Half-lives corrected for air leakage were as follows: hydrogen chloride 7 min; sulphur dioxide 40 to 60 min; smoke 145 to 300 min. The half-life and equilibrium concentration of sulphur dioxide could be reduced by changing the conditions. The results are compared with published work on pollution in dwellings.