Air Conditioning Appears to Spread Coronavirus—But Opening Windows Could Stop It, Studies Suggest

Opening windows in buildings, including our homes, may prevent the spread of the coronavirus, scientists believe

Experts in health, the built environment and microbiology at the University of Oregon and the University of California, Davis, made the recommendations by reviewing existing studies on germs including SARS-CoV-2 (the virus which causes COVID-19 disease).

They also looked at data on other members of the large coronavirus family of bugs which trigger severe acute respiratory syndrome (SARS) and Middle East Respiratory Syndrome (MERS). They published their findings in the journal mSystems.

The team hope their recommendations will help those tasked with deciding when and how to ease social distancing measures as the pandemic continues. According to Johns Hopkins University, 1.9 million people worldwide have so far been diagnosed with COVID-19, more than 126,700 people have died, and almost 494,000 have recovered.

As the map below by Statista shows, every continent has been touched by COVID-19.

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A map showing COVID-19 cases worldwide as of April 15, 2020. Statista

According to the team, most people spend more than 90 percent of their daily lives in the built environment, giving viruses a range of ways to infect people: from close interactions with other individuals, to touching surfaces, and breathing in contaminated air.

How a building is ventilated and how paths of air travel in a space can affect which microbes are present, the authors explained.

"For most buildings, the easiest way to deliver outside air directly across the building envelope is to open a window," they wrote.

But the researchers warned the outdoor temperature should be taken into account to avoid exposing people to extremes, and to prevent viruses spreading between residences.

Past research also indicate light is useful for controlling the spread of some infectious diseases.

Microcosm studies, where a controlled environment is set up to imitate an ecosystem like the home or office, suggest daylight affects the communities of bacteria present in a space. Both visible and UV daylight can impact the viability of the germs, with one study on the flu virus, cited by the team, showing simulated sunlight cut its half-life from around 31 minutes in the dark to two minutes.

"Further research is needed to understand the impact of natural light on SARS-CoV-2 indoors; however, in the interim, daylight exists as a free, widely available resource to building occupants with little downside to its use and many documented positive human health benefits," the team argued.

People should try to open blinds and shades when possible to let in "abundant daylight and sunlight," they suggested.

The humidity of a building, the team said, may also affect the spread of infections like COVID-19. The limited data available on SARS-CoV-2 indicates it is active longest on plastic surfaces at a relative humidity of 40 percent, with an average half-life of 15.9 hours, and shortest in aerosol form with an average half-life of 2.74 hours with a relative humidity of 65 percent.

This is supported by evidence suggesting viruses, including coronaviruses, struggle to survive in typical indoor temperatures and at a relative humidity of above 40 percent. Targeted in-room humidification could therefore be considered as a means for preventing COVID-19, while remembering that humidity above 80 percent could create its own health problems, the team advised.

Considering the potential use of air filtration systems, the authors found the majority of viruses, including coronavirus, are too small to capture bugs like SARS-CoV-2 and "no filter system is perfect."

Those with air delivery systems should take care not to recirculate indoor air which "could potentially increase the transmission potential." Boosting airflow rates could meanwhile risk sending germs into the air from surfaces and "increase the potential for contamination throughout the building by distributing indoor air more quickly, at higher velocities and volumes, potentially resuspending more ultrafine particles."

However, systems that regulate air quality and temperature, known as heating, ventilation, and air conditioning or HVAC, could also help prevent the spread of disease, the authors said. Bringing in more outside air from outside and having higher rates of air exchanged in buildings "may help to dilute the indoor contaminants, including viral particles, from air that is breathed within the BE [built environment]," they wrote.

Keeping air filters properly installed and maintained could help to minimize the spread of SARS-CoV-2 according to the team, who stressed "it is important to understand that filters should not be assumed to eliminate airborne transmission risk."

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Health workers look out a window of the Honduran Social Security Institute in Tegucigalpa on April 13, 2020. ORLANDO SIERRA/AFP via Getty Images

The scientists also reviewed studies on how SARS-CoV-2 is spread, highlighting that at the time their study was submitted in mid-March, there had been no documented cases of a person developing COVID-19 from a surface, only through droplets.

Regardless, "steps should still be taken to clean and disinfect all potential sources of SARS-CoV-2 under the assumption that active virus may be transmitted by contact with these abiotic surfaces."

They added: "With an abundance of caution, it is important to consider the possibility that the virus is transmitted through aerosols and surfaces "

The team said: "We believe this information is useful to corporate and public administrators and individuals responsible for building operations and environmental services in their decision-making process about the degree and duration of social-distancing measures during viral epidemics and pandemics."

A separate group of scientists in China have also considered the potential for the built environment to aid the spread of SARS-CoV-2. They found air conditioning may have lead three families in Guangzhou, China to develop COVID-19.

Between January 26 and February 10 of this year, 10 members of three families who ate at the same air-conditioned, windowless, restaurant in the port city northwest of Hong Kong caught the coronavirus. One of the families had recently returned for the central Chinese city of Wuhan, the original epicenter of the pandemic.

"We conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation," the team said. However, the authors acknowledged their study was limited as they did not conduct an experiment simulating the potential spread in the restaurant, and did not estimate the risk of infection of asymptomatic family members and other diners.

The results were published as an early release article of a research letter due to be published in the journal Emerging Infectious Diseases.

This article has been updated with information on air delivery systems.

Centers for Disease Control and Prevention Advice on Using Face Coverings to Slow Spread of COVID-19

  • CDC recommends wearing a cloth face covering in public where social distancing measures are difficult to maintain.
  • A simple cloth face covering can help slow the spread of the virus by those infected and by those who do not exhibit symptoms.
  • Cloth face coverings can be fashioned from household items. Guides are offered by the CDC. (
  • Cloth face coverings should be washed regularly. A washing machine will suffice.
  • Practice safe removal of face coverings by not touching eyes, nose, and mouth, and wash hands immediately after removing the covering.

World Health Organization advice for avoiding spread of coronavirus disease (COVID-19)

Hygiene advice

  • Clean hands frequently with soap and water, or alcohol-based hand rub.
  • Wash hands after coughing or sneezing; when caring for the sick; before, during and after food preparation; before eating; after using the toilet; when hands are visibly dirty; and after handling animals or waste.
  • Maintain at least 1 meter (3 feet) distance from anyone who is coughing or sneezing.
  • Avoid touching your hands, nose and mouth. Do not spit in public.
  • Cover your mouth and nose with a tissue or bent elbow when coughing or sneezing. Discard the tissue immediately and clean your hands.

Medical advice

  • Avoid close contact with others if you have any symptoms.
  • Stay at home if you feel unwell, even with mild symptoms such as headache and runny nose, to avoid potential spread of the disease to medical facilities and other people.
  • If you develop serious symptoms (fever, cough, difficulty breathing) seek medical care early and contact local health authorities in advance.
  • Note any recent contact with others and travel details to provide to authorities who can trace and prevent spread of the disease.
  • Stay up to date on COVID-19 developments issued by health authorities and follow their guidance.

Mask and glove usage

  • Healthy individuals only need to wear a mask if taking care of a sick person.
  • Wear a mask if you are coughing or sneezing.
  • Masks are effective when used in combination with frequent hand cleaning.
  • Do not touch the mask while wearing it. Clean hands if you touch the mask.
  • Learn how to properly put on, remove and dispose of masks. Clean hands after disposing of the mask.
  • Do not reuse single-use masks.
  • Regularly washing bare hands is more effective against catching COVID-19 than wearing rubber gloves.
  • The COVID-19 virus can still be picked up on rubber gloves and transmitted by touching your face.

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