Why the fight against COVID-19 must include indoor air humidity

As the fight to stem the impact of COVID-19 continues, without a viable vaccine in sight for the immediate future, social distancing and stringent personal hygiene remains the key advice from governments around the world in halting the spread of the virus. Whilst maintaining hand and surface hygiene is a sensible strategy for preventing spread through direct contact and short-range large droplet spray, to truly control disease outbreaks we must focus on the role of indoor air on disease transmission and occupant health.

For example, a largely overlooked aspect of indoor air that contributes to both disease transmission and poor occupant health is dry air. In climates with cold winters, outdoor air is drawn into a building and heated to comfort temperatures. This drastically lowers the indoor relative humidity, creating a dangerous condition for people indoors, especially in this time of COVID-19 disease. When the indoor relative humidity is less than 40 per cent, human occupants are more vulnerable to viral respiratory infections and the SARS-CoV-2 virus is more plentiful and infectious in the inhaled air.

Often when we talk about humidity, we think about outdoor conditions, but there is significant scientific evidence that suggests maintaining mid-range indoor humidity is an effective infection control mechanism for airborne germs and viruses. This is especially true when considering public sector buildings such as schools, hospitals and offices, where a great number of people are convened in one space, yet it is not a factor that has been placed highly on the priority list for planners and governments alike.

Real estate and viral transmission

When we enter a building, we are essentially entering a microclimate, and how hospitable this is to viral pathogens is dependent on the temperature, humidity and building materials in question. Humans shed approximately 37 million bacteria per hour, resulting in something known as a microbiome, a unique indoor community of microbes that adapt to the indoor temperature and lower relative humidity (RH) levels that we frequently find in seasonal regions.

Studies have shown air humidity at 40 to 60 per cent RH is the optimum level for human health. This works in three ways: firstly, germs, including the novel coronavirus that causes COVID-19 is transmitted through airborne droplet infection. When the RH in a building is lower than 40 per cent, the comparatively moisture-free air is optimal for long distance spread of tiny infectious aerosols.  Airborne viral particles can travel further, become inhaled by other occupants, and settle on surfaces where they can survive for days on surfaces such as stainless steel and plastic. Secondly, for reasons that we do not understand, low relative humidity levels actually increase the infectivity of many viruses, including SARS-CoV-2. This effect is in addition to higher numbers of infectious aerosols in the air.

Thirdly, dry air impairs our respiratory immunity.  When we inhale low RH air, the mucus in our nose, sinuses and throat dries and becomes more viscous, lessening the ability of cilia, the hair-like projections on cells lining airways, to expel viral particles. It also lessens the effectiveness of surface barriers such as mucus membranes, eyes and skin. Research indicates that low RH compromises the immune system’s ability to respond to pathogens effectively. In fact, evidence suggests that not only is 50 per cent RH the optimum level for our immune system to fight viral infection, but airborne viruses such as coronavirus and seasonal influenza are deactivated more rapidly in this bracket.

Seasonal influences

It is no coincidence that countries such as Canada, who have lower outdoor temperatures during the winter months, are most affected by seasonal illnesses. However, whilst it is a lack of indoor humidity, not outdoor warmth that is the driving force; cold, winter air carries very little water vapour and when this is drawn into buildings and heated without humidification, the RH drops even further, typically to around 20 per cent.

We know that the chances of transmission are increased by physical contact with an infected person, and that droplets can also be expelled through sneezing, coughing and breathing. But when conditions allow virus particles to survive in the air, they can move through a HVAC system and reach any number of a building’s inhabitants, making even those who have not come into contact with an infected individual at risk of infection. Combined with the high-density populations in public buildings, this creates a perfect storm for viral transmission. While social distancing decreases the risk of getting COVID-19 from someone else through large, short range contamination by large droplets, it does not prevent the transmission of tiny infectious aerosols in the air.

Policy for prevention

Microbiologists are on the frontlines of developing effective vaccines and anti-viral medications but building and facility managers have a vital role to play in reducing the spread of viral illnesses such as COVID-19 too.

Governments do set indoor air quality standards for temperature, fresh air introduction and pollutants, but there are presently no employment regulations or real estate planning policies in Canada that require a minimum RH for public buildings.

Canadian buildings are advised to maintain humidity levels between 35 and 55 per cent, but this is not enforced; international research has shown that less than five per cent of buildings currently comply with ASHRAE’s standard dictating the environmental conditions for human occupancy, which includes relative humidity as a factor. Right now, many indoor spaces do not meet the minimum humidity threshold to combat this.

As we look towards the future, setting a standard of 40 per cent relative humidity for minimum indoor humidity for public buildings will not only reduce the impact of COVID-19, but it will also reduce the impact of further viral outbreaks, both seasonal and novel. This will also reduce absenteeism, which costs Canadian businesses billions each year, increase productivity and reduce healthcare costs but most importantly, it will save lives.

We cannot predict outbreaks of viral infections, but our knowledge of how they spread does allow us to make vital changes to our built environments that will prevent a pandemic of the scale we are experiencing from happen again. For this to happen, we need the relationship between the medical community and policy makers, planners, architects and developers to strengthen, and the understanding of the powerful influence of indoor environments on occupant health to translate to tangible changes that will have long-lasting, positive impacts for us all.


Dr. Stephanie Taylor, M.D., M. Arch, CIC, of Harvard Medical School and CEO of Taylor Healthcare Consulting, is a former physician and specializes in infection control.

During her long career Stephanie has focused on improving the health care physical environment and clinical work processes to help patients recover quickly.

Follow her on Twitter @staylorvt

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