Indoor Humidity and COVID-19

Scientists and engineers are still learning about how the novel coronavirus is transmitted from person to person, especially in indoor environments. But one thing has been known for decades: humidity levels are a major factor in the transmission of airborne and respiratory illnesses.

In 1986, E. M. Sterling published this summary chart showing that humidity levels between 40% and 60% were much safer than both higher and lower humidity levels.

Recently, researchers in Sydney, Australia, reported that for every 10% drop in relative humidity, the risk of COVID-19 transmission doubled.

When humidity is very low, humans’ mucous membranes dry out and are less effective at protecting from infection. At the same time, many viruses, including the new coronavirus, survive much longer at very low humidity levels. Even worse, droplets that are sneezed or exhaled decrease in size as they lose moisture, causing them to be suspended in the air rather than falling quickly to the floor.

High humidity encourages the growth of many harmful organisms including bacteria and molds. The novel coronavirus, interestingly, does not survive well as the humidity increases, but the other health risks of very high humidity argue for an optimum zone of 40% to 60% relative humidity.

What can building managers do about this? Most HVAC systems do not control directly for humidity. Instead, temperature, ventilation and infiltration are the primary determinants of indoor humidity.

Every thermostat Embue ships is able to measure humidity, so our customers can measure it in every space in their buildings. We have observed extremely low humidity -- drier than the Sahara desert -- when Embue is first installed in older buildings. These buildings are also chronically overheated. Customers have used the Embue platform to reduce overheating and raise humidity levels by instituting limits on how high residents can set their thermostat’s target temperature and by improving controls on the central heating equipment. This is both healthier and saves energy.

Another driver of low indoor humidity in winter and high humidity in summer is infiltration, which simply means that air from outside is entering the building through leaky windows and poor insulation. Correcting that is expensive, but can pay for itself through energy savings and improved occupant health.

If a building is well insulated and has very low infiltration, ventilation becomes critical. A “tight building” needs active mechanical ventilation to remove pollutants, maintain healthy CO2 levels, and balance the humidity levels. That’s the subject of a future blog post.

In the meantime, ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, has an FAQ page on operating your HVAC systems safely in these times.