Sept. 16, 2020 -- If you live in an apartment building or condo complex, you already know it’s wise to mask up and keep your distance from others in common areas -- elevators, lobbies and hallways -- to avoid the coronavirus.

But what about your building’s HVAC unit? Could it be spreading the virus from one apartment or condo to another through the air vents?

Not to worry, experts say.

HVAC (heating, ventilation, and air conditioning) systems in large residential buildings are not spreading COVID-19 by moving the virus from one dwelling to others, research shows.

In fact, multiple studies have shown that large air-handling systems can filter out viral particles from the air and knock down the spread of the virus.

Those conclusions are based on a handful of COVID-19 outbreaks in big residential and commercial buildings, including a high-rise tower in Hong Kong, the Diamond Princess cruise ship, and a five-story Chinese restaurant.

With colder weather soon to send us indoors for longer periods of time, that’s reassuring news, says Amesh Adalja, MD, an infectious disease specialist at the Johns Hopkins University Center for Health Security.

“Can an HVAC unit facilitate the spread of the virus in and of itself? We don’t have that type of epidemiological evidence,” he says.

But he points out that the venting from HVAC systems can, in fact, recirculate indoor air and push viral particles around a room, if they’re already in the air. So they can be “incidental” spreaders of the coronavirus, he says.

For example, a small outbreak of the virus at a restaurant in Guangzhou, China, last January was linked to venting from an air conditioning unit that allowed the viral particles in the air to spread from one infected diner to nine other people.

Chinese researchers who investigated the incident found that the AC unit’s air vents had acted like a fan, propelling those airborne respiratory droplets from the infected diner to others nearby.

Their findings -- detailed in a paper in Emerging Infectious Diseases, a journal published by the CDC -- did not suggest the AC unit had somehow absorbed viral particles from the air and then spread them widely.

“We conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation,” the authors wrote. “The key factor for infection was the direction of the airflow.”

Adalja says this means the virus spread from one diner to others because they “were sitting in a draft coming from a vent that was pushing the droplets forward.”

“That’s different than [viral particles] being sucked into the HVAC and then being disseminated by it, which I don’t think we have evidence for,” he says. “If there was a table fan behind that person, you might have gotten the same effect.”

Worth noting: 73 other diners ate that day on the same floor of the five-story restaurant, and none became sick. You would expect to have seen more cases if the restaurant’s ventilation unit itself was a factor, Adalja says.

But the case does makes clear that simply being indoors, close to others, can increase your risk.

That’s particularly true if the building is not properly ventilated or has a faulty HVAC system that is merely recirculating the air indoors -- whether it’s a residential facility, a retail store, office, restaurant, gym, hotel, or other building with a shared HVAC system.

Airborne Transmission: Risks and Realities

Linsey Marr, PhD, a Virginia Tech scientist who specializes in the airborne transmission of viruses, agrees that HVAC units are not major spreaders of COVID-19 in multi-unit residential buildings.

But it’s important to recognize that whenever indoor air is recirculating -- whether it’s being pushed by air coming from an air conditioner, heating vent, or a fan -- it can help spread COVID if viral particles are already present.

“It's not HVAC units per se … it’s not that the virus is hanging out in the HVAC and is waiting to jump out at you,” says Marr, whose work centers on aerosols, the smallest airborne viral particles.

“It’s the fact that when you turn on the HVAC, you have a lot of recirculation of air, so that gives more chance for a virus to build up in the air. So, it’s the fact that you’re not bringing outdoor fresh or clean air to replace the air that’s in there, and everything is just building up.”

Scientific consensus has been shifting since January on how well the virus can spread through indoor air.

Early in the pandemic, the World Health Organization suggested COVID-19 was not a virus that could be easily transmitted through the air, like measles and tuberculosis. But in a commentary published in July in the journal Clinical Infectious Diseases, 239 scientists pressed the WHO to change its position and recognize this potential for airborne spread.

Joseph Allen, an assistant professor of exposure assessment science and director of the Healthy Buildings program at Harvard University, says infectious disease experts were “shouting from the rooftops” that the airborne transmission route was more significant than the WHO or the CDC had concluded.

As a result of the letter and other evidence, the WHO and CDC now say COVID-19 is mostly spread through the air -- via close personal contact with infected people -- or by touching a contaminated surface.

Both health agencies have also said HVAC units in “shared or congregate housing” -- science-speak for large residential buildings -- are not primary ways the virus is spread.

Size Matters: Airborne Transmission

Marr, the Virginia Tech scientist who helped write the WHO letter, explains that SARS-CoV-2, the virus that causes COVID-19, spreads through respiratory droplets from one infected person to another.

When someone coughs, sneezes, or exhales, large respiratory droplets “fly through the air ballistically, like a cannonball almost, and you have to be close to the person [within 3 to 6 feet] for them to fly through the air and land in your eye, your nose, or your mouth,” she says.

But at the same time someone releases a few large respiratory droplets, hundreds to thousands of much smaller microscopic droplets are also expelled. Scientists call these droplets “aerosols,” and they are 10 times smaller than the period at the end of this sentence.

“They don’t fly like cannonballs; they’re more like a mist that is going to float around in the air for a while,” Marr explains. “And just like cigarette smoke, they’re going to be more concentrated closer to the person that’s infected.”

So if you’re near an infected person, large respiratory droplets containing viral particles can land on you and cause an infection. But you’re much more likely to inhale the virus in those smaller microscopic particles because they float around in the air longer and can travel more than 3 to 6 feet.

This is where HVAC units come in. Large ventilation systems are “highly effective” at capturing and filtering out those tiny particles in the air, she says.

Marr says there’s no evidence that HVAC systems in large facilities can suck in, store, and spread the coronavirus from one apartment or condo to another. But if viral particles are already in the air inside a room, the venting from HVAC units can spread it around, as in the case of the Chinese restaurant in Guangzhou.

Researchers who investigated two other widely reported outbreaks earlier this year -- on the Diamond Princess cruise ship and the Hong Kong high-rise tower -- also identified things other than the HVAC units in those facilities as primary spreaders of COVID-19.

Scientists who examined the February outbreak onboard the Diamond Princess found the virus that sickened nearly 700 passengers was mostly spread through “mass gathering in the recreational areas.” It’s also possible some passengers were infected by touching contaminated surfaces, they found. The vessel’s massive HVAC system was not identified as a factor in the case.

And in the widely reported outbreak in the high-rise residential tower in Hong Kong this year, the culprit was found to be a faulty plumbing network in the building.

All of this is good news for people living in large residential buildings, Marr says.

She adds that HVAC units can filter out contaminants, cutting the chance that viral particles will migrate from one room or apartment to another.

“Most air HVAC units are designed in a way such that they don’t recirculate or mix the air between units” in housing complexes, Marr notes. “If one air handling unit serves, let’s say, 10 apartments, and one has a person that’s sick, the [virus] is going to get filtered out or diluted across those 10 units.”

Adalja says it’s also reassuring when you compare COVID-19 to other infectious diseases that can be transmitted through indoor air. For instance, the “attack rate” for COVID-19 -- which gauges the percentage of people who are infected when exposed to a virus indoors -- is much lower for the coronavirus than measles, tuberculosis, and other diseases.

“If one person in a household has measles, the entire household will get it,” he says. “We’re not seeing that type of attack rate with this virus. The household attack rate is about 20% for COVID-19, meaning only 20% of people living in a household with someone who’s infected get infected. If it was the HVAC unit, wouldn’t you expect it to be 100%?”

Adalja also points out that outbreaks have not occurred in other places you’d expect to find them if HVAC units were major spreaders of the virus.

“Shopping malls in most states are open right now, and those are air-conditioned, and we’re not hearing about outbreaks at shopping malls,” he says.

What You Can Do

Even so, it’s a good idea to take extra steps to cut your risks if you live in a condo complex or apartment building.

First of all, indoor air experts say it’s important for large HVAC systems to be maintained well and adjusted to bring in as much fresh outdoor air as possible. Some systems limit fresh air inflow to save energy and money, but now is not the time for such trade-offs, Allen says.

He advises asking your landlord or manager how much fresh outdoor air your building’s HVAC system brings in and how much air it recirculates.

He also recommends asking if the HVAC has a high-efficiency filter, such as a HEPA (high-efficiency particulate air) filter, which can remove airborne contaminants.

HVAC filters are rated for efficiency using a MERV (minimum efficiency reporting value) scale. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends large residential and commercial buildings use HVAC filters with a rating of MERV 13 or higher -- a recommendation Allen endorses, too.

“If you put in a good filter, you can remove 80% of pathogens that might be in the air,” Marr says.

Deborah Thrope, deputy director of the National Housing Law Project, says state and local building, housing, and health codes require landlords to maintain safe and healthy conditions throughout their properties.

Thrope, a healthy-buildings specialist with the nonprofit tenant rights advocacy group, explains those codes vary, state to state, but most generally require property owners to provide safe, working HVAC units and plumbing systems.

If you think your landlord may be skirting these requirements, a phone call to city planners or health officials is often enough to bring a response, she says.

"Filing any type of complaint is highly likely to get the attention of the landlord," Thrope says. "If you have a valid complaint, you should absolutely pursue it, especially now."

She also notes some leases require landlords to provide a working HVAC unit. Finally, free legal aid services in most regions of the country are available in cases where tenants and landlords can't come to terms on their own.

"If someone is living in a unit they feel is a risk to their health and safety because of [COVID-19], they should seek legal advice," Thrope says. "They may qualify for free legal aid services in their community."

Allen adds that simply opening your windows is a low-tech way to allow more fresh air into the building. Just cracking a window can bring in enough fresh air to replace the air in a room up to 10 times every hour, he says.

Another option: Use portable air purifiers. “They remove aerosols from the air, and so they will drop down the levels in the air,” Marr says.” There’s less likelihood that someone is going to breathe in a lot of [respiratory droplets] from an infected person who’s been in there.”

For more information, the website of the Association of Home Appliance Manufacturers provides a helpful primer on air purifiers. AHAM says such devices are rated by what’s called a CADR -- short for clean air delivery rate -- which indicates the volume of filtered air a device delivers.

In general, the higher the CADR number, the faster the unit filters the air. As a rule of thumb, the CADR of your air cleaner should be equal to at least two-thirds of the room’s area where it will be used, AHAM recommends.

So, for a room that is 120 square feet -- 10 feet by 12 feet -- you’d want to use an air purifier with a smoke CADR of at least 80.

Another strategy: Use special wall or ceiling-mounted boxes that emit short-range UV radiation, which kills airborne viruses.

But the best advice for apartment and condo dwellers is what you already know: Wear a mask, wash your hands often, and keep your distance from others in highly trafficked common areas in your building.

And all of these strategies help fight other viruses and pathogens, in addition to COVID-19. They’re also equally effective in all indoor environments -- heated, air-conditioned, or naturally ventilated.

That’s worth keeping in mind as we head toward colder months and the coming cold and flu season.

“There’s always something you can do,” Allen says. “I never came across a sick building that couldn’t be rehabilitated. There are practical steps that nearly every building can take.”

Show Sources

Deborah Thrope, deputy director, National Housing Law Project.

Amesh Adalja, MD, infectious disease specialist, Johns Hopkins University Center for Health Security.

Linsey Marr, PhD, professor of civil and environmental engineering, Virginia Tech.

Joseph Allen, DSc, assistant professor, director of Healthy Buildings program, Harvard University. 

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