Scientists have come to realize the coronavirus isn’t just spread through close contact. The virus can float through the air and be inhaled by someone, even if they’re diligently practicing social distancing.

And as more people head back to work, start shopping in malls and doing other things in public, indoor spaces, the airborne virus is a real concern — especially as it gets hotter and we rely more on air conditioning.

Ed Nardell is an infectious disease expert at the Harvard T.H. Chan School of Public Health. Nardell said that when we exhale, most of the particles that come out are wet.

"Now they're falling, falling, falling, and some will hit the ground and they won't evaporate anymore. They're settled,” Nardell said. “But others, before they hit the ground, start to evaporate. And pretty soon, they are airborne. And those can float on air currents indefinitely unless they're vented out or inhaled."

That could lead to infection. But Nardell said it's unlikely that an HVAC system would suck up a virus particle from one room and deposit it out another vent. Nardell said there’s no evidence of that happening.

But AC is a problem, he said. "Mainly because it makes people go indoors."

Outside, there's infinite space to dilute the air you're breathing.

"And then suddenly you're inside and you are more likely to be less distant and you're re-breathing air that other people have exhaled," Nardell said.

Indoor AC units can spread the virus, Nardell said. The units create air currents that can blow the virus around a room. He said that problem was clear in the case of one restaurant in Wuhan, China, where researchers studied why so many people eating there became sick.

"Where apparently someone infected not only people at their own table, but at the next table and the table after that, which happened to be that in the direction of the airflow from a wall unit air conditioner," he said.

Since not everyone has the option to stay outside, the American Society of Heating, Refrigerating and Air-Conditioning Engineers is figuring out ways to make indoor spaces safer.

Penn State professor William Bahnfleth chairs the society's epidemic task force, which has been busy writing up guidance for people in the HVAC industry and building managers.

"It's almost 400 pages of recommendations on how to protect different types of buildings from risk," Bahnfleth said.

The key is to lower the concentration of possibly infected particles in the air, he said.

"One of the best ways to do that is simply to bring in a lot more outside air into a building, because that outside air replaces indoor air that may be contaminated, and that lowers the concentration," Bahnfleth said.

That’s what they’re doing at the restaurant Coppersmith in South Boston, where they’re skipping the air conditioning and getting fresh air by leaving open big garage-style doors.

General Manager Sheila Senat said that customers seem happy with that choice. “I think they feel safer with the AC being off and the doors open,” she said.

But a lot of buildings don’t even have any windows that open. And most HVAC systems in those buildings keep recirculating cool air, because it's more energy efficient.

Bahnfleth said now’s the time to sacrifice some of that energy efficiency and dial up how much air the system is taking in from outside.

"Another thing you can do is to use filters to remove the particles from the air that may contain viruses," he said.

Filters are rated on what’s called the MERV scale, and the HVAC recommendations say most buildings should step it up to a level 13 filter to reduce particles carrying coronovirus.

So if you have to go back to work soon, Bahnfleth has some advice.

"You just have to ask, 'What have you done?'” he said. “I have the same questions for my employer at the university. 'What are you doing to make our classrooms safe?'"

Tamara Small of the commercial real estate development group NAIOP Massachusetts said she's hearing building managers are stepping up filtration to the recommended MERV-13.

“It's a balance to ensure that there is more ventilation, and definitely new air filters in every building are probably the most common responses we've seen," she said.

There is one other technology that Ed Nardell of Harvard said could help reduce risk: Upper room germicidal UV. Basically, that means shining ultraviolet light at the ceiling level to kill any virus floating up there.

"I first encountered it in the eighties when we tried to deal with an outbreak of tuberculosis in a homeless shelter in Boston," Nardell said.

It worked then, and Nardell said it could work now. The idea's been around for almost 100 years, he said. But it’s rarely used in the U.S. these days.

Nardell’s working on research now to show how effective it could be at killing off the coronavirus.

In the meantime, the best we can do is to try to get some fresh air.