Indoor air is not coronavirus-ready, Condair reveals

By Oliver Zimmermann, CEO, Condair Group
Wednesday, 18 March, 2020

Indoor air is not coronavirus-ready, Condair reveals

A lack of humidity control regulations for public buildings will have a subsequent impact on possible coronavirus transmission and health.

Public building occupants are being exposed to increased risk from viruses, such as the coronavirus, because the regulations on indoor air quality (IAQ) are falling short of current scientific knowledge. Maintaining IAQ at above 40% relative humidity (RH) has been scientifically shown to reduce viral cross infection, including coronavirus and influenza. Yet out-of-date regulations on the topic result in buildings such as hospitals, offices and schools experiencing dangerously low humidity levels every winter.

Studies such as Casanova et al 20101 have specifically examined humidity’s role in coronavirus transmission. This study showed that coronavirus was deactivated fastest when exposed to a mid-range humidity (50%RH), rather than dry (20%RH) or damp (80%RH) air. There are many other studies, dating back from the 40s to now, that all indicate that an indoor humidity of 40–60%RH has a positive impact on cross infection and people’s susceptibility to viruses.

The building services sector accepts this indoor humidity level as being best practice, with many professional organisations endorsing a mid-range humidity for health in their recommendations. However, there are no official regulations that set an acceptable range of indoor humidity for public places. Therefore, building designers, who are driven to reduce energy consumption and costs, do not commonly include humidity control in their plans.

Oliver Zimmermann, Condair Group CEO.

The seasonality of viruses, such as coronaviruses and influenza, are further evidence of humidity’s role in their transmission. Indoor air is much drier in the winter and this corresponds to the rise of infections. Strategies to contain the spread of the virus frequently cite the assumption that infections will probably drop as warmer weather returns, and indoor humidity levels naturally return to a midrange of 40–60%RH. It doesn’t need to be this way! A healthy indoor humidity can be maintained during winter if buildings incorporated humidification as part of the ventilation system. This would significantly reduce seasonal ’flu transmission and save thousands of lives globally every year.

Yale University research scientist Professor Akiko Iwasaki led a recently published study on humidity’s impact on influenza infection.

“Our research found that mice housed in low relative humidity (20%) suffered from more severe disease and succumbed to death when infected with influenza virus compared to those that were housed in 50% relative humidity,” Professor Iwasaki said.

“We found that 20% humidity blocks the mucociliary clearance of inhaled particles and impairs innate immune response against viruses. The implication of our study is that humidified air (40–50%) is critical for the functioning of the respiratory immune system. This is particularly important for patients and elderly, whose immune system is already compromised.

“If everyone’s lung immune system is working well at 50% humidity, then if an infected person is introduced into that population, the virus will be cleared and transmission will be halted.

“Decades of research has shown that virus particles are least stable at humidity ranging 40–60%. Combined, there is ample evidence showing that maintaining humidity at this range is ideal to reduce the spread of viruses.

“At low humidity, viruses and bacteria can survive in the environment to increase transmission. In addition, low humidity will allow these pathogens to enter the lung and grow better due to reduced mucociliary clearance and innate resistance. At the other extreme, very high humidity conditions with poor ventilation in the building will allow moulds to grow on surfaces. Thus, keeping the humidity at optimal levels (40–60%) is ideal to prevent infections,” Professor Iwasaki said.

Advice on mitigating the risk from coronavirus largely focuses on hand hygiene and avoiding unwell people. However, viral cross infection occurs via the air as well as from physical contact. As the general public are largely helpless to manage this important aspect of infection control, government advice ignores this topic.

The responsibility to manage IAQ ultimately falls on building owners and operators to safeguard occupant health. This is particularly true for healthcare facilities, where people are most vulnerable and at risk to airborne infections such as coronavirus and influenza. The general public are being failed in this respect, with no health authority in the world specifying a minimum humidity level in waiting rooms or wards.

Given the overwhelming scientific evidence for indoor humidity of 40–60%RH being an effective infection control mechanism, and the pending viral pandemic on our doorstep, regulatory bodies must listen to the science and set acceptable indoor humidity levels for health.


1. Effects of Air Temperature and Relative Humidity on Coronavirus Survival on Surfaces. Lisa M. Casanova, Soyoung Jeon, William A. Rutala, David J. Weber, Mark D. Sobsey. Applied and Environmental Microbiology Apr 2010, 76 (9) 2712-2717.

Image caption: ©

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