Researcher and co-author of “Humidity as a non-pharmaceutical intervention for influenza” Dr Stephanie Taylor talks to CCN about the role of humidity in the prevention of COVID-19.

CCN: How much of an impact can humidity have on virus and disease control?

Dr Taylor: Studies have shown that dry indoor air promotes the spread of airborne bacteria and viruses through the air including SARS-COV-2, the virus which causes COVID-19. My research into the role of indoor air in infection control has shown that maintaining a consistent relative humidity of between 40-60% is the optimum range to mitigate the risk of the spread of the virus and helps your body’s immune system to fight the virus.

Put simply, when you cough, sneeze or talk, millions of droplets are expelled from your respiratory tract, including any infectious viruses. Part of the water in these droplets evaporate – the rate of which is impacted by temperature and humidity – leaving behind minute droplets containing pathogens small enough to remain in the air for a long-time. Ensuring a relative humidity of 40-60% has been proven to be more effective in deactivating these viral particles, both in the air and on surfaces. Beyond this, the 40-60% humidity range has shown to improve our body’s response to the virus – improving the ability of cilia cells in our airway to remove viral particles, and the ability of our cells to repair damage caused by the virus on the lungs.

CCN: What buildings are the most crucial spaces for virus control in this regard?

Dr Taylor: Public buildings like schools, workplaces and hospitals have undoubtedly been hotspots for the spread of the COVID-19 outbreak, not just because of viral cross infection via physical contact, but in large part due to the airborne transmission of infectious droplets, aggravated by low indoor humidity levels.

In these buildings, many people congregate, so regularly disinfecting high touch surfaces will be vital.

CCN: Could home humidifiers and other products achieve similar rates at home to ensure people working from home have the same level of immune strength?

Dr Taylor: The most comprehensive solution to improve the level of humidity in your air, and to mitigate the risk of spread of the virus in your home, is through purchasing a steam humidifier. It may seem like a simple solution but is the most effective action you can take within your home.

CCN: What are the risks when humidity is below the preferred range, and what are the risks when it is above it?

Dr Taylor:  When we have dry indoor air – where relative humidity is less than 40% – it promotes the spread of germs in the air, including the coronavirus. But this is something you can actively manage. By increasing the amount of water vapour, known as humidity, in your home you can effectively reduce the risk of spread and help your body’s immune system fight the virus.

When there is too much humidity, above the range of 40-60%, this can create several issues on the indoor space and have health consequences too. A too humid room can be shown in the condensation on windows, or stains on the walls, mould, musty odour etc. In the long term, too much humidity can rot some structural parts of a building, drawing pests and bugs.

High humidity can also create allergic reactions on humans and pose a threat to sufferers of allergy and asthma, which again, can develop into more severe respiratory issues if that person gets infected with COVID-19.

CCN: Beyond humidity, what other environmental factors could help Australians maintain their best possible health in the face of a pandemic virus threat? (Does some time in the sun, exposed to UV light wavelengths, have benefits in killing bacteria/viruses, for example?)

Dr Taylor: One misconception when it comes to viruses is that people believe that low temperatures are the driver of wintertime illnesses, or that a sunny weather will kill viruses. What is important to differentiate here is the importance of relative humidity levels over temperature.

Recent research found that there may be some hope for pandemic respite as we move into spring, but this depends on how indoor environments can adapt. If indoor relative humidity stays above 40%, which occurs more often in the spring and summer when outdoor air does not require as much heating, the risk of indoor transmission of viruses such as the one that causes COVID-19 is less. However, the virus can still be transmitted through contact and close proximity through all seasons.

The cold, dry air of winter clearly facilitates the spread of SARS-CoV2 — the virus that causes COVID-19 —among people, but as indoor humidity naturally increases during spring and summer, the risk of transmission of the virus through airborne particles decreases in buildings.

CCN: In conclusion what advice do you have for building managers and other professionals?

Dr Taylor: Building managers have a vital role to play in reducing the spread of viral illnesses such as COVID-19. We now know that viruses and bacteria can survive in tiny infectious aerosols that are transmitted in dry air. This brings into focus the importance of properly humidifying and ventilating buildings to reduce transmission of airborne infections. While we know that viruses can be spread through physical contact with an infected person, they can also travel in tiny droplets released during a sneeze, cough or even when an infected person breathes, and can travel through air into an HVAC system when the air quality in a building is dry. This puts building occupants at increased risk of infection even if they have not come into close contact with an infected person.

This concept of airborne transmission may sound scary but managing and maintaining buildings can absolutely control it by keeping relative indoor humidity at what I call the “magic zone” of 40-60%. Data and research show us that if people in offices, hospitals, and schools manage their indoor relative humidity at 40-60%, human immune mechanisms for physiological defence are improved. Not only does humidifying the indoor environment decrease infectious pathogens in the air we breathe, it also bolsters our own immune systems, which is critical, now more than ever.

There is currently little regulation on minimum indoor humidity levels globally. Building codes have focused on reducing energy consumption which has resulted in lowering the permitted minimum indoor humidity level at the expense of occupant health. Setting minimum indoor humidity levels for public buildings will reduce the burden of COVID-19 and other upcoming seasonal viral illness on society, reduce absenteeism and save lives. In fact, a recent study in a nursery school showed that there were fewer infectious droplets in the air, and that fewer children missed school, when the classroom humidity was maintained above RH 40% (Rieman J, “Humidity as a non-pharmaceutical intervention for influenza A”, 2018 ASHRAE abstract).

Governments currently set indoor air quality standards for temperature, fresh air introduction and pollutants. Setting a minimum indoor humidity level in public buildings is easily achievable and will ultimately save lives.




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