The self-cleaning capacity of the atmosphere is controlled by hydroxyl (OH) radicals in the troposphere. There are primary and secondary OH sources, the former through the photodissociation of ozone, the latter through OH recycling. We used a global model, showing that secondary sources are larger than assumed previously, which buffers OH. Complementary OH formation mechanisms in pristine and polluted environments, connected through transport of ozone, can maintain stable global OH levels.