The air cleaners are labelled either A or B, indicating if they are real or fake. Until closing of the trial, only the manufacturer will know the code. The patients will be randomised either to A or B, with no stratification. The air cleaners are sealed. Hence, it is not possible to open them to see what kind of filters they have, without breaking the seal. Even the placebo filters will seem real, for people not accustomed to looking inside air cleaners.