Brief summary

In scientific literature, standards and guidelines, different recommendations are given for lower and upper limits of relative humidity for indoor spaces in order to reduce adverse effects on people and their health. However, these limits are not uniform and in some sources only a lower or upper limit is given. Often, scientific publications state an optimal range of relative humidity for humans between 40% and 60%. In this paper, based on the evaluation criteria of comfort, health, and influence on pathogens and pollutants, the results of the considered sources are analyzed and the recommendations are put into an overall context. The results show that the influences of relative humidity should be considered individually for each criterion and that the requirement for a specific target range always represents a compromise. For example, low relative humidities can lead to a higher acceptance of perceived air quality and to a reduction in the proliferation of dust mites and the corresponding adverse health effects. Maintaining moderate relative humidities can significantly reduce eye, skin, and respiratory health impairments and associated absenteeism. With respect to pathogens, depending on the type of virus, relative humidity can affect their inactivation. For example, polioviruses are inactivated to a maximum degree at low relative humidities, influenza viruses at medium relative humidities, and coronaviruses at medium to high relative humidities. High relative humidities reduce the suspension time of particles and aerosols, but should be avoided because of the risk of microbial growth and mold. Overall, therefore, the compromise of a medium range of relative humidity is shown to have the least adverse effects on humans and their health.