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The chemistry of the troposphere is substantially influenced by a wide range of chemical processes which are primarily driven by the action of solar ultraviolet radiation of wavelengths shorter than about 310 nm on ozone and water vapour. This leads to the formation of hydroxyl (OH) radicals which, despite very low tropospheric concentrations, remove most gases that are emitted into the atmosphere by natural and anthropogenic processes. Although tropospheric ozone only makes up about 10% of all atmospheric ozone, through the formation of OH, it determines the oxidation efficiency of the atmosphere and it is, therefore, of the utmost importance for maintaining the chemical composition of the atmosphere. Due to a variety of human activities, leading especially to increasing emissions of CH4, CO and NOx, it is conceivable that the concentrations of ozone are increasing in polluted and those of hydroxyl decreasing in clean tropospheric environments. The result is most likely an overall decrease in the oxidation efficiency of the atmosphere and consequently a buildup of several longlived trace gases that are primarily removed by reaction with OH. Here we discuss the most important processes that determine the oxidation efficiency of the atmosphere and give some examples of changes in O3, CO, and OH concentration distributions that may have occurred as a result of human activities.