Scientists call for review of link between humidity and respiratory health in wake of COVID-19

Global petition calls on World Health Organization to set clear guidelines on minimum air humidity levels in public buildings

Dr Stephanie Taylor says, following the COVID-19 outbreak, it is more important than ever to listen to the evidence on the positive impact of air quality and respiratory health

A new global petition is calling on the World Health Organization (WHO) to produce clear guidelines on the minimum lower limit of air humidity in public buildings, given the wealth of evidence which shows mid-range humidity levels can prevent respiratory infections.

Scientists and physicians specialising in immunobiology and infection control have joined forces to support the petition, calling on the WHO to protect public health and save lives in the wake of the COVID-19 outbreak.

Every winter, buildings all over the world drop below the 40-60% mid-range humidity level threshold, contributing significantly to seasonal respiratory illness.

In light of the COVID-19 crisis, it is now more important than ever to listen to the evidence that shows optimum humidity can improve our indoor air quality and respiratory health

And the petition calls on the WHO to take swift and decisive action to establish global guidance on indoor air quality, with a clear recommendation on the minimum lower limit of air humidity in public buildings.

This critical move would reduce the spread of airborne bacteria and viruses in buildings, including hospitals, in order to protect public health.

Supported by leading members of the global scientific and medical community, it is designed to not only increase global awareness among the public on the crucial role indoor environmental quality plays in physical health; but also to call emphatically on the WHO to drive meaningful policy change; a critical necessity during, and after, the COVID-19 crisis.

As COVID-19 continues to put pressure on health systems and the economy globally, the group calls on the WHO to review the extensive research that shows an indoor humidity level of between the 40-60% relative humidity (RH), is the optimum threshold for inhibiting the spread of respiratory viruses such as influenza. This is a threshold that many public buildings drop below every winter.

Professor Dr Akiko Iwasaki, The Waldemar Von Zedtwitz Professor of immunobiology and professor of molecular, cellular and developmental biology at Yale, and an investigator for the Howard Hughes Medical Institute, said: “Ninety percent of our lives in the developed world are spent indoors in close proximity to each other.

It is time for regulators to place management of the built environment at the very centre of disease control

“When cold outdoor air with little moisture is heated indoors, the air’s relative humidity drops to about 20%.

“This dry air provides a clear pathway for airborne viruses, such as COVID-19.

“That’s why I recommend humidifiers during the winter, and why I feel the world would be a healthier place if all our public buildings kept their indoor air at 40-60% RH.”

Evidence shows the important role indoor humidity levels play in preventing virus transmission and improving immune system response.

There are three key notable findings suggesting why indoor air should be maintained at 40-60%RH in public buildings such as hospitals, care homes, schools and offices throughout the year:

  • Breathing dry air impairs our respiratory immune system’s ability to efficiently capture, remove, and fight airborne viruses and germs, rendering us more vulnerable to respiratory infections
  • When the RH is lower than 40%, airborne droplets containing viruses, such as SARS-CoV-2, shrink through evaporation making them lighter. This enables the particles to float for longer in the air, increasing the likelihood of infection
  • The vast majority of respiratory viruses suspended in dry atmospheres survive and remain infectious for much longer than those floating in air with an optimum humidity of 40-60%RH

One of the leading forces in the charge for a globally-recognised 40-60%RH guideline for public buildings, Dr Stephanie Taylor, an infection control consultant at Harvard Medical School, said: “In light of the COVID-19 crisis, it is now more important than ever to listen to the evidence that shows optimum humidity can improve our indoor air quality and respiratory health.

“It is time for regulators to place management of the built environment at the very centre of disease control.

“Introducing WHO guidelines on minimum lower limits of relative humidity for public buildings has the potential to set a new standard for indoor air and improve the lives and health of millions of people.”

Dr Walter Hugentobler, a GP and former lecturer at the Institute of Primary Care at the University of Zürich, added: “Raising air humidity by humidification reduces the risk of virus spread in hospitals and other buildings at low cost and without causing negative effects.

“It can also be easily implemented in public buildings, both in private and workplace environments with relative ease.

“Humidification gives people a simple means of actively combatting seasonal respiratory infections.”

The petition specifically and directly targets the WHO, due to the pivotal role the organisation plays in setting global guidelines for indoor air quality.

Today, there are no recommendations from WHO on the minimum lower limit of humidity in public buildings.

Raising air humidity by humidification reduces the risk of virus spread in hospitals and other buildings at low cost and without causing negative effects

But, if it publishes much-needed guidance on minimum lower limits of humidity, building standards regulators around the world would be encouraged to act urgently.

And, if these steps were implemented, there is optimism that the following effects could benefit global health systems and the world economy:

  • Respiratory infections from seasonal respiratory viruses, such as flu, being significantly reduced
  • Thousands of lives saved every year from the reduction in seasonal illness
  • Global healthcare services being less burdened every winter
  • The world’s economies massively benefiting from less absenteeism through illness
  • A healthier indoor environment and improved health for millions of people