For COVID-19, the first steps in reducing the indoor concentrations of the virus are wearing face masks, physical distancing, and reducing occupancy levels. Improved ventilation is an additional prevention strategy. For ventilation systems, increasing outdoor air above the code minimum requirements, increasing total ventilation, and increasing filtration efficiencies are more effective at controlling infectious disease transmission than controlling indoor temperature and humidity. However, the use of temperature and/or humidity to reduce the risk of disease transmission should be considered on a case-by-case basis, taking into account the building enclosure, heating, ventilation, and air-conditioning (HVAC) system capabilities, level of control and/or building automation, local COVID-19 transmission rates, any unique clinical features of the occupants, and local climate. Both temperature and humidity can influence the transmission of infectious diseases, including COVID-19, but that influence has practical limitations. Research on the impact of temperature has shown that SARS-CoV-2, the virus that causes COVID-19, is sensitive to elevated temperatures, with over 99.99% inactivation in only a few minutes at 70°C (158°F). However, this temperature is far outside the limits of human comfort and could damage some building materials. While temperatures lower than 70°C (158°F) are also effective, the required exposure time for inactivation increases as the temperature decreases. So, elevated temperatures offer the potential for decontamination of SARS-CoV-2 virus in the air or on surfaces, but the use of increased temperature solely for decontamination is not generally recommended and is not realistic for occupied spaces. Another important consideration is that when the temperature in a space is elevated, the corresponding relative humidity level decreases. Current evidence is not persuasive that humidity significantly reduces transmission of SARS-CoV-2 beyond the level resulting from good ventilation and filtration. Some research studies have shown that the survival of viruses, including human coronaviruses, may be reduced when the relative humidity is in the 40–60% range. However, the reductions are modest and there are outliers to these findings. Consequently, neither ASHRAE nor CDC recommends introducing humidification for the sole purpose of limiting transmission of COVID-19. While not affecting transmission, there are peer-reviewed studies that suggest preventing excessive dryness in the air could help maintain the effectiveness of the human body’s immune system. Some HVAC systems can actively control both temperature and humidity. However, the majority of HVAC systems do not have dedicated humidification capabilities. Some dehumidification happens during warmer months as a byproduct of cooling humid warm air below its dew point and causing water to condense out of the air. Less common is the ability to limit low humidity by introducing water vapor into the dry supply air. Most existing residential and commercial buildings located in cold climates are not constructed to resist the corrosion and excessive moisture accumulation that can result from long-term, whole-building humidification. If additional winter humidification is used to maintain comfort and prevent excessive dryness of nasal and ocular membranes, first analyze the building enclosure to verify that condensation and moisture accumulation will not become a problem. ASHRAE Standard 160 (Criteria for Moisture-Control Design Analysis in Buildings) provides guidance for hygrothermal analysis of building enclosures. For commercial buildings that are properly constructed to allow for long-term humidification, and which have humidification capabilities already installed, there is no reason not to humidify the air to comfortable levels during the winter months. In residential settings, portable in-room humidifiers may be used for sensory comfort and to reduce excessively low relative humidity levels. In these instances, use a humidifier with a built-in humidistat and control the relative humidity level near 40%. Higher humidity levels are not necessarily better and may lead to localized mold growth, mildew, and other long-lasting indoor air quality issues. Maintenance and cleaning of portable humidification systems is very important. Change the water in the humidifier daily and maintain and clean the humidifier in accordance with manufacturer recommendations.