With temperatures beginning to tumble in many parts of the country and winter right around the corner, Americans have begun reaching for their thermostats. And with millions of employees working from home, more time than ever has been spent indoors this year as the public fights to curb the spread of COVID-19.
But could those thermostats and heating units that we rely on every year to stay comfortable amid the season's harshest conditions enhance the pandemic threat in our indoor spaces? Could the indoor climates we create in our living rooms or office buildings be just as comfortable for the coronavirus as it is for us?
According to air quality expert Dr. William Bahnfleth, an architectural engineering professor at Pennsylvania State University, the increased chance for potential transmission may be more likely to come from secondary behaviors that come with turning on heating systems.
"Behaviorally, in homes, people are less likely to open their windows in the middle of the winter," Bahnfleth said. "Unless they're intentionally opening their windows now and then to air the house out, then the air inside of it is going to be older and staler, which is not good for a lot of reasons." Bahnfleth cautioned that if somebody living in the house is infected, then that person "would have probably a higher chance of infecting other people because you'd get higher concentrations of aerosol inside."
More and more is being understood about what weather conditions allow respiratory droplets containing SARS-CoV-2, the virus that causes COVID-19, to spread. Temperature, humidity, sunlight and wind speed have all been factors analyzed by experts.
Recent research has found that higher levels of relative humidity may not deter transmission as much as previously believed, which may explain why many warmer, more humid areas around the world had higher infection rates over the summer. Bahnfleth said that humidity will need to be a considered factor for indoor spaces as they kick on the heat. "We must humidify," he told AccuWeather.
"There's strong evidence that it really cuts down infection rates." Bahnfleth said before noting that other studies have suggested the opposite.
But more needs to be understood about the pathogen's behavior, he said. "Coronavirus isn't an influenza and different viruses and bacteria that may be in the air don't necessarily respond to temperature and humidity the same way. But that might be a concern, and I like some humidification in buildings just in general, because it's good for comfort," he said. And "it's probably better for asthmatics to not have really dry air."
Similar to air conditioning units, heating systems in both residential and commercial buildings could both help and hurt transmission prevention efforts depending on the efficiency of the units and the amount of outdoor air that is getting into a space.
In residential buildings, a typical furnace HVAC system will circulate air through the house and "mix it up into one big zone," according Bahnfleth. But in homes, that shared air may not be as large of a concern because people are likely in the same bubble and not worried about social distancing from one another.
However, in different sized buildings, that risk changes. For larger companies in commercial buildings, air filters could make a huge difference.
"It's been demonstrated that the virus can get transmitted through HVAC systems, but it seems like there's not enough of it for the concentration to get high enough anywhere else for infections," Bahnfleth said. "It's one thing to have an air conditioning system that's serving 10,000 square feet of office space," he continued. "If someone is sick in one room, their air goes into the return and then gets spread out over 10,000 feet, so the concentration is low. But if you're in a 1,500-square-foot house and you're there more than half the day and walking around, the concentration could get fairly high everywhere over time."
When discussing the impact of air conditioners in the spring, Bahnfleth told AccuWeather that air filter changes should be treated like oil changes for a car and that multiple changes per year are necessary.
With heating systems entering their busy season, Bahnfleth said building owners should be proactive in changing those filters and ensuring that their heating units are performing up to par by ensuring that the outgoing flow rate is where its supposed to be.
"If it's an office building or a restaurant you probably ought to be changing the filters maybe as frequently as every 90 days, although maybe three to four times a year maybe depending on how they run," he said. "Checking the outdoor air supply and making sure that you have good filters that are properly installed are the first two things to do and if a professional is looking at a building they can assess whether there's likely to be a need to go beyond."
While many people are looking for a clear green light or red light as to whether it's safe to return to normal activities like regular indoor dining, Bahnfleth said there's no way to say when that risk is nonexistent because of the many different transmission factors.
"There's very little we can do from a practical point of view to reduce risk to zero, except staying away from those situations entirely," he said.
Dr. Blythe Adamson, an infectious disease epidemiologist and former member of the White House coronavirus task force, told Good Morning America that an individual is 20 times more likely get COVID-19 from an infected person sitting indoors rather than in an outdoor setting.
Even as a clearer understanding of the virus' behavior and aerosol transmission continues to grow, there isn't yet a clear set of criteria for how much ventilation or air circulation is necessary to prevent virus exposure and widespread infection.
However, one clear factor that should be focused on is reducing air irritants in an indoor environment. But, going too far with humidification can lead to a surplus of condensation, which could spur the growth of mold. According to Bahnfleth, other problems such as dust mites are also occasionally made worse by humidity.
In peoples' homes, Bahnfleth added, there are plenty of steps that people can do to produce air contaminants, such as smoking, cooking, lighting candles, or having pets.
In an office setting, those contaminants are more likely to come from pieces of machinery or other work processes. As those contaminants gather in the air, the chance for respiratory infections increases. In the age of COVID-19, that comes with far more risk than in past winters.
So while Bahnfleth reiterated that the spread of hot air by heating systems themselves may not directly lead to a spike in cases, the changes in behavior could certainly affect the way people are exposed this coming winter. Thus, the efforts to wear a mask and practice proper social distancing may be more important than ever.
As Bahnfleth said, many of the modifications that need to be made have already been suggested for months and there isn't a high-priced piece of innovation that can save lives more than social distancing.
Even with high-priced HVAC tweaks, it may not mean much if social distancing, hand washing and mask-wearing are ignored.
"Often when something like this comes around, everyone thinks we need some new technological miracle to save us," Bahnfleth said. "But really, the most useful tools we have are still ventilation and good filtration."