Recommendations regarding indoor relative air humidity (RH) differ between countries, e.g. recommending at least 30% and a maximum of 65% RH to maintain a comfortable room climate. However, there is no clear consensus on an optimal RH value, which may differ according to the working environment and the symptoms addressed. Concurrently, recommendations relating to the room temperature should also be considered, since raising temperature leads to a decrease of RH. Therefore, we aim to compile the available evidence about health effects associated with air humidification amongst workers and in educational settings.