Sept. 16, 2020 — If you live in an apartment building or condo complex, you already know itâs 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 buildingâs HVAC unit? Could it be spreading the virus from one apartment or condo to another through the air vents?
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, thatâs reassuring news, says Amesh Adalja, MD, an infectious disease specialist at the Johns Hopkins University Center for Health Security.
âCan an HVAC unit facilitate the spread of the virus in and of itself? We donât have that type of epidemiological evidence,â 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 theyâre already in the air. So they can be âincidentalâ 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 unitâs 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.
âWe conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation,â the authors wrote. âThe key factor for infection was the direction of the airflow.â
Adalja says this means the virus spread from one diner to others because they âwere sitting in a draft coming from a vent that was pushing the droplets forward.â
âThatâs different than [viral particles] being sucked into the HVAC and then being disseminated by it, which I donât think we have evidence for,â he says. âIf there was a table fan behind that person, you might have gotten the same effect.â
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 restaurantâs 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.
Thatâs particularly true if the building is not properly ventilated or has a faulty HVAC system that is merely recirculating the air indoors — whether itâs a residential facility, a retail store, office, restaurant, gym, hotel, or other building with a shared HVAC system.
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 itâs important to recognize that whenever indoor air is recirculating — whether itâs 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.
âIt’s not HVAC units per se â¦ itâs not that the virus is hanging out in the HVAC and is waiting to jump out at you,â says Marr, whose work centers on aerosols, the smallest airborne viral particles.
âItâs 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, itâs the fact that youâre not bringing outdoor fresh or clean air to replace the air thatâs in there, and everything is just building up.â
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 âshouting from the rooftopsâ 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 âshared or congregate housingâ — science-speak for large residential buildings — are not primary ways the virus is spread.
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 âfly 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,â 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 âaerosols,â and they are 10 times smaller than the period at the end of this sentence.
âThey donât fly like cannonballs; theyâre more like a mist that is going to float around in the air for a while,â Marr explains. âAnd just like cigarette smoke, theyâre going to be more concentrated closer to the person thatâs infected.â
So if youâre near an infected person, large respiratory droplets containing viral particles can land on you and cause an infection. But youâre 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 âhighly effectiveâ at capturing and filtering out those tiny particles in the air, she says.
Marr says thereâs 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 âmass gathering in the recreational areas.â Itâs also possible some passengers were infected by touching contaminated surfaces, they found. The vesselâs 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.
She adds that HVAC units can filter out contaminants, cutting the chance that viral particles will migrate from one room or apartment to another.
âMost air HVAC units are designed in a way such that they donât recirculate or mix the air between unitsâ in housing complexes, Marr notes. âIf one air handling unit serves, letâs say, 10 apartments, and one has a person thatâs sick, the [virus] is going to get filtered out or diluted across those 10 units.â
Adalja says itâs also reassuring when you compare COVID-19 to other infectious diseases that can be transmitted through indoor air. For instance, the âattack rateâ 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.
âIf one person in a household has measles, the entire household will get it,â he says. âWeâre 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 whoâs infected get infected. If it was the HVAC unit, wouldnât you expect it to be 100%?â
Adalja also points out that outbreaks have not occurred in other places youâd expect to find them if HVAC units were major spreaders of the virus.
âShopping malls in most states are open right now, and those are air-conditioned, and weâre not hearing about outbreaks at shopping malls,â he says.
Even so, itâs 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 itâs 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 buildingâs 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.
âIf you put in a good filter, you can remove 80% of pathogens that might be in the air,â 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. âThey remove aerosols from the air, and so they will drop down the levels in the air,â Marr says.â Thereâs less likelihood that someone is going to breathe in a lot of [respiratory droplets] from an infected person whoâs been in there.â
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 whatâs 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 roomâs area where it will be used, AHAM recommends.
So, for a room that is 120 square feet — 10 feet by 12 feet — youâd 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. Theyâre also equally effective in all indoor environments — heated, air-conditioned, or naturally ventilated.
Thatâs worth keeping in mind as we head toward colder months and the coming cold and flu season.
âThereâs always something you can do,â Allen says. âI never came across a sick building that couldnât be rehabilitated. There are practical steps that nearly every building can take.â
The Harvard Gazette: âIs air conditioning helping spread COVID in the South?â
CBS Austin: âAs COVID-19 spikes in the south, researchers look to see if A/C can transmit the virus.â
USA Today: âMost air conditioning systems donât protect against the coronavirus. In some cases, they can actually facilitate spread.â
The Times Picayune: âIs air conditioning helping spread coronavirus? In southern states, being indoors may be a factor.â
The Houston Chronicle: âTexans have a love affair with air conditioning. But what if turning it off could slow COVID-19?â
Annals of Internal Medicine: âProbable Evidence of Fecal Aerosol Transmission of SARS-CoV-2 in a High-Rise Building.â
The New York Times: âHow Coronavirus Infected Some, but Not All, in a Restaurant,â âThe Scientist, the Air and the Virus,â âAirborne Coronavirus: What You Should Do Now,â âAboard the Diamond Princess, a Case Study in Aerosol Transmission.â
The Journal of the American Medical Association: âAirborne Spread of SARS-CoV-2 and a Potential Role for Air Disinfection.â
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE): âASHRAE Issues Statements on Relationship Between COVID-19 and HVAC in Buildings,â âCoronavirus (COVID-19) Response Resources From ASHRAE and Others.â
Medical Hypotheses: âAirborne route and bad use of ventilation systems as non-negligible factors in SARS-CoV-2 transmission.â
Prevention: âIs the Novel Coronavirus Airborne? Hereâs Why Experts Donât Have One Answer.â
News – Coronavirus and Apartments: Whatâs the Risk?