Though some public health experts expected coronavirus transmission to wane in the summer as temperatures rise and the air becomes more humid, cases have actually skyrocketed in some of the hottest and stickiest parts of the country.
Engineers and ventilation experts said this may be in part because residents escape the heat by retreating indoors where heating, ventilation and air conditioning (HVAC) systems could exacerbate airborne transmission with unplanned air currents.
“The main way (air conditioning) can contribute to spreading coronavirus is by creating strong air currents that can move the droplets … and contribute to increase risk,” said William Bahnfleth, chair of the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ Epidemic Task Force (ASHRAE) and professor at Penn State University.
Even in bars and restaurants where social distancing is observed, air ventilation can carry respiratory droplets or aerosols that contain virus, said Len Horovitz, pulmonary specialist at Lenox Hill Hospital in New York City.
The Centers for Disease Control and Prevention published initial findings of an outbreak linked to the airflow in a Guangzhou, China, restaurant. Over the course of 12 days, nine people who dined at the restaurant Jan. 24 fell ill as a result of another patron with a COVID-19 infection, the authors determined.
Within five days, three people sitting at the infected patron’s table were infected along with another below the air conditioner. Of the 91 people in the restaurant during that hour, only those at tables in the way of the air conditioner’s airflow contracted the virus.
The World Health Organization only recently recognized that aerosolized droplets can lead to infection after more than 200 medical experts wrote an open letter urging the agency to react to mounting evidence and go a step further with its recommendations.
“Ventilation is the key control point for an airborne virus,” said Dr. Julian Tang, one of the authors of the paper. “Based on multiple studies done by the authors, we believe that optimized ventilation is the way to move forward, removing the virus from the air before people inhale it. We think that’s one of the main ways it’s transmitted.”
The best ventilation will always be outside. In hot Southern states where people want to stay indoors and enjoy the air conditioning, ventilation is dependent on HVAC systems.
Though Bahnfleth said it’s possible to increase outside air through these systems, experts are not sure how much outside air is enough to diffuse virus particles as the dose of infection is unclear.
“Even if you try to increase the ventilation rate, HVAC systems have not been designed to prevent transmission of these infectious airborne diseases,” said Dilip Goswami, president, co-founder and CTO of Molekule, an indoor air purification company.
After ventilation, the next line of defense in an HVAC system is filtration. Most commercial and residential HVAC systems have a MERV 6 or 8 filter, which takes care of the basic pollen, dust, dust mites, mold and bacteria. It’s unable to capture small particles that contain virus, which can be about 1 micron.
More air-quality-conscious establishments boast a MERV 13 filter that can partially capture virus carriers, but some HVAC systems aren’t equipped to handle it. Though ventilation from the outside is able to diffuse virus particles in the air, filters can capture the virus only when it comes into the system.
“Most air conditioning systems won’t filter (virus) out, and if it did filter it out, it probably went right by and right into your face first,” said Wendell Porter, senior lecturer at the University of Florida.
Which brings us back to air currents. Goswami said it’s possible to manipulate air currents to make it safer for people indoors, but most establishments don’t think that far ahead.
ASHRAE’s task force was established to respond to the COVID-19 pandemic and provide guidance to ensure buildings are prepared for epidemics. Its recommendations include ventilation control, filtration and maintenance.
ASHRAE is a professional society, not a formal legal body. It’s up to states, localities and building codes to adopt recommendations to ensure a safe return to work, school and leisure.
Goswami said it’s important for people to take these recommendations seriously, especially in bars and restaurants where patrons need to remove their masks to eat and drink. Many establishments enforce social distancing and mask wearing, but few take a second look at their HVAC systems to see whether they go above the minimum standards.
“We knew something like this could have happened and that airborne transmission is a major problem to contain,” he said. “We need to be aware of it and make sure we do everything possible instead of the minimum of what’s possible.”