If you want to speed up the flow of outdoor air into a room, you could also take a box fan, place it in a window and blast it outward, Dr. Jimenez said. When any amount of air leaves, that same amount of air returns — it’s a fixed volume. Therefore, the fan should help pull in the same amount of outdoor air.

If you have air-conditioning in your home, no one is saying that you need to give up on it entirely. When it’s sweltering out, air-conditioning can be essential not only to help you function but also to avoid heatstroke.

But if you are going to spend time in a cooled space with other people, it may be worth understanding a bit more about the cool air you are breathing. Basically, all air-conditioning falls into one of three categories.

  • The unit cools both indoor and outdoor air.

  • The unit cools and recirculates only indoor air.

  • The unit relies entirely on pulling in outdoor air. (These are uncommon outside hospitals and labs.)

Centralized-air systems, such as those common in office buildings, dorms and some large apartment buildings, often fall in category one. Dr. Jimenez and other building scientists involved in coronavirus prevention are currently advising owners of businesses and buildings with category one systems to adjust the ratio to pull in more outdoor air, an enterprise that can be costly. Take a casino in Las Vegas, which is kept cool enough to keep people gambling inside while it’s 120 degrees Fahrenheit outside. Cooling that hot outdoor air will be more expensive than recirculating the already cool inside air. But given that keeping customers healthy is also a priority, more are willing to revisit their approach, Dr. Jimenez said.

Few of us have the ability to adjust our air-conditioning in this way. Most window units sitting with their rears facing the outdoors, for example, fall into category two. Instead of pulling in outdoor air, they are dumping heat from the room outdoors, said William Bahnfleth, a professor of architectural engineering at Penn State’s Institutes of Energy and the Environment.

If you live alone, or with people you’re sure aren’t infectious, those units are fine. But if you give in to throwing that birthday dinner for your parents, or if your teenager has been less than strict about staying home, it’s worth remembering that “any virus that’s present will be mixed in” to the recirculating indoor air, Dr. Jimenez said.

And so, if you have to have people over, it may be preferable to revert to rule one: When in doubt, open the windows. Or better yet, go outside.

So what do you do if you’re stuck with a unit that primarily recirculates indoor air and it’s unrealistic to open the window? This is where filters come in. The right filter is just as effective as pulling in outside air, said Dr. Edward A. Nardell, a professor at Harvard Medical School who has written about the role that air-conditioning plays in spreading airborne diseases.