Relative humidity (RH) represents an underestimated outdoor and indoor environmental parameter. Conditions below and above the optimal range could facilitate infectious transmission as well as the exacerbation of respiratory diseases. The aim of this review is to outline the consequences for health of suboptimal RH in the environment and how to limit this negative impact. RH primarily affects the rheological properties of the mucus, modifying its osmolarity and thus the mucociliary clearance. The integrity of the physical barrier, maintained by mucus and tight junctions, is critical for protection from pathogens or irritants. Moreover, the control of RH seems to be a strategy to prevent and control the spread of viruses and bacteria. However, the imbalance of RH in the outdoor and indoor environments is frequently associated with the presence of other irritants, allergens, and pathogens, and therefore the burden of a single risk factor is not clearly defined in different situations. Nonetheless, RHmay have a synergistic negative effect with these risk factors, and its normalization, if possible, may have a positive impact on a healthier environment.