The World Health Organization (WHO) must take swift action to establish global guidance on indoor air quality to reduce the spread of airborne bacteria and viruses in buildings, urges a new petition

A new petition has called on the WHO to take 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.

Supported by members of the global scientific and medical community, the petition is designed to not only increase global awareness on the role indoor environmental quality plays in physical health but also to call on the WHO to drive meaningful policy change.

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.

Keep indoor air at 40-60%

Professor Dr Akiko Iwasaki PhD, 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: “90% of our lives in the developed world are spent indoors in close proximity to each other.

“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 to 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:

  • 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.

Optimum humidity can improve indoor air quality

One of the leading forces in the charge for a globally recognised 40-60%RH guideline for public buildings, Dr Stephanie Taylor MD, infection control consultant at Harvard Medical School, ASHRAE distinguished lecturer & member of the ASHRAE Epidemic Task Group, 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 center 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, MD, general physician, former lecturer Inst. 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.”

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

If these recommendations were implemented, 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.

The group of scientists urges the public to read and sign the petition, which can be found here:


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