October 28, 2020

Airborne Coronavirus: What You Should Do Now

The coronavirus can stay aloft for hours in tiny droplets in stagnant air, infecting people as they inhale, mounting scientific evidence suggests.

This risk is highest in crowded indoor spaces with poor ventilation, and may help explain super-spreading events reported in meatpacking plants, churches and restaurants.

It’s unclear how often the virus is spread via these tiny droplets, or aerosols, compared with larger droplets that are expelled when a sick person coughs or sneezes, or transmitted through contact with contaminated surfaces, said Linsey Marr, an aerosol expert at Virginia Tech.

What is clear, they said, is that people should consider minimizing time indoors with people outside their families. Schools, nursing homes and businesses should consider adding powerful new air filters and ultraviolet lights that can kill airborne viruses.

Here are answers to a few questions raised by the latest research.

For a virus to be airborne means that it can be carried through the air in a viable form. For most pathogens, this is a yes-no scenario. H.I.V., too delicate to survive outside the body, is not airborne. Measles is airborne, and dangerously so: It can survive in the air for up to two hours.

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Updated 2020-07-12T10:24:24.672Z

For the coronavirus, the definition has been more complicated. Experts agree that the virus does not travel long distances or remain viable outdoors. But evidence suggests it can traverse the length of a room and, in one set of experimental conditions, remain viable for perhaps three hours.

The Daily Poster

Listen to ‘The Daily’: Four New Insights About the Coronavirus

A look at what scientists have learned about how the virus takes hold in the body and where the risk of infection is highest.

0:00/29:28

transcript

Listen to ‘The Daily’: Four New Insights About the Coronavirus

Hosted by Michael Barbaro, produced by by Alexandra Leigh Young and Austin Mitchell, and edited by Larissa Anderson

A look at what scientists have learned about how the virus takes hold in the body and where the risk of infection is highest.

michael barbaro

From The New York Times, I’m Michael Barbaro. This is “The Daily.”

[music]

Today: As infection rates break new records across the U.S. this July 4 weekend, four new insights into the virus from my colleague, science reporter Donald G. McNeil Jr. It’s Monday, July 6.

Let me start by, Donald, saying happy 4th of July.

donald g. mcneil jr.

Thank you.

michael barbaro

How did you spend it?

donald g. mcneil jr.

Saturday, I played softball in the morning — socially distant softball.

michael barbaro

Softball?

donald g. mcneil jr.

Yeah.

michael barbaro

In New York City?

donald g. mcneil jr.

In New York City. Yeah, Riverside Park. And then we went to dinner last night in a friend’s backyard on Long Island, where we all sat as couples together, but six feet apart from each other, and sort of took turns going up to the table to get to the food, and sat, and had a really nice time.

michael barbaro

I have to imagine that even a socially distanced meal with you is challenging, in that I think I would feel quite seen and judged, given your role.

donald g. mcneil jr.

Why?

[laughter]
donald g. mcneil jr.

I mean, I do tend to say six feet, six feet, six feet, a lot at people.

michael barbaro

See?

donald g. mcneil jr.

Because masks give people a false sense of security. I mean, this is a big thing on the softball field, is that people would both put on masks and they’d sit next to each other in the dugout, making up the batting order and stuff like that. And I’d go, no! Air leaks out of the side of your masks. And you’re not always wearing masks. Sometimes you’re wearing it as a sort of a Captain Ahab blue beard under your chin. So it’s better to stay six feet apart. That way if your mask —

michael barbaro

Do your remember a couple seconds ago how you asked me why it might be challenging to have a —

donald g. mcneil jr.

[LAUGHS]

michael barbaro

— a meal with you?

donald g. mcneil jr.

All right. [LAUGHTER]

michael barbaro

Ask and —

donald g. mcneil jr.

All right, touche, touche.

michael barbaro

So everything you just described, of course, is occurring in New York, where the infection rate has been generally declining or at least stabilizing. So let’s talk about the state of the pandemic in the rest of the country. I wonder if you can give us a quick status update on this end of July 4 weekend.

donald g. mcneil jr.

OK. I think it’s 39 states now have cases going up. And it’s hitting hardest in the South and in the West. Texas, Florida, Arizona, California, a number of other states. And it’s exactly what was predicted back in May when states were debating opening by Memorial Day. All public health experts said, if you open up when your case counts are rising, they’re going to continue rising and start rising even faster. And now we’re seeing that.

For example, in Houston, doctors who knew the situation in New York are saying that what’s happening there looks like what happened in New York in early April. Finding new beds, finding ventilators, lots of patients who were sick, patients who were on oxygen. Not as many dying yet, but with people on oxygen and on ventilators they may find themselves in the situation where they have to park refrigerated trucks behind hospitals to hold the bodies, as they did in New York.

michael barbaro

So Donald, correct me if I’m wrong. I think the U.S. is at about 50,000 infections a day right now. Dr. Anthony Fauci said we could get up to 100,000. And if that’s the case, what do we expect the attendant death count to start to look like in the U.S.? I assume that’s going to catch up with that.

donald g. mcneil jr.

Yeah, it’ll catch up with it. I mean, but there’s no reason to believe that 100,000 is the upper limit. It all depends on how much social distancing we practice. I mean, this is the dance. And do you close bars and restaurants? Do you open or close schools? Do you wear masks? How much attention do people pay to the directions you give them? How much do they practice good social distancing. That very much affects the rate of spread of the virus.

michael barbaro

Well, with that in mind, those big questions of kind of how we proceed through the rest of this pandemic, you have been doing a lot of reporting about the latest learnings and insights into the virus that will very much guide how we answer those questions. And we want to talk through those with you. So where do you think we should start?

[music]
donald g. mcneil jr.

Some of these insights are really more theories with some evidence to them. And some there’s quite a bit of confidence in. So we probably ought to start with something that there’s a pretty high degree of confidence in among doctors.

michael barbaro

And what is that?

donald g. mcneil jr.

When this all started, we thought of it as a lung disease, a respiratory disease, because the first cases we heard about, people got pneumonia. And that of course reminded us that the model for this disease was the 1918 influenza epidemic. But we’re learning that this coronavirus is very different from an influenza virus.

The influenza viruses attach to receptors in the lungs and the airway. This gets into the body through the airway, through the lungs. But it really attaches to the insides of the blood vessels. And so that makes it a vascular disease, a blood vessel disease.

michael barbaro

And what are the implications of a virus like this being a vascular disease, a blood vessel disease, and not just a respiratory disease?

donald g. mcneil jr.

It means it affects every organ in the body that has lots of fine blood vessels in it, and not even just organs.

I mean, so it affects the lungs, which are the filter where the air gets into the blood, and you have lots of little fine blood vessels surrounding the little sacs at the ends of your breathing tubes. It attacks the kidneys, because that’s the filter where the urine comes out of the blood. So you have very fine networks of blood vessels there. It attacks the gut, because you have a network of blood vessels in your gut where food gets into your body. It attacks the brain, because you have lots of fine blood vessels in the brain. It doesn’t attack the nerve cells in the brain, which most of the brain is made of. It doesn’t attack the muscle cells in the heart. But it attacks the blood vessels that go through all those other parts.

And so when they do autopsies they find thousands of tiny little blood clots all over the body. We have lots of people who have strokes. And as those blood clots clot up blood vessels to small areas of the brain, you may get dementia or disorientation. And then in kids, when you have ‘Covid toes’ in teenagers and young adults, this is the little capillaries in the hands and feet getting blocked, and getting this inflamed, painful, red or purple toe and finger syndrome. So it’s more complicated to deal with a disease that can travel to any organ in the body.

michael barbaro

So how does this new insight about the coronavirus, how does it change the way we are going to approach the pandemic?

donald g. mcneil jr.

Before, the main thing you’re looking for when you’re looking to see if somebody is having a problem is their blood oxygen level. Because you’re assuming that their problem is going to be pneumonia. But if you realize that the problem could be kidney damage, heart damage, you do a whole different battery of blood tests.

michael barbaro

So what you’re saying is that doctors who previously were diagnosing Covid-19 through a set of well-established symptoms now need to expand that set of symptoms pretty broadly. Because it turns out this is looking to be vascular, not respiratory.

donald g. mcneil jr.

Yeah, that’s right. It means that virtually anybody who comes into a doctor’s office feeling sick might have the coronavirus. If they come in with symptoms of a stroke, it might be Covid. If they come in with symptoms of a heart attack, it might be Covid. If they come in with what seems like arthritis in their feet, it might be Covid toe.

And because we often don’t have enough tests, or it takes a long time to get test results, the patient’s at a real disadvantage. Because if you don’t know your patient has coronavirus, whatever symptom they’ve got now might become greater, might spread to other organs. So the problem in your toes might literally spread to your kidneys or your brain. And you want to know that that patient has a disease that can spread throughout the body.

michael barbaro

In other words, more testing, fast testing becomes more imperative once we have learned that so many symptoms may actually be a sign of Covid-19.

donald g. mcneil jr.

Absolutely.

michael barbaro

OK. So what is the next big new insight we have into the coronavirus?

donald g. mcneil jr.

Well, people are always asking, is the virus mutating? Is it becoming different? And the answer is yes. This virus always mutates. It makes one mutation about every two weeks.

michael barbaro

Wow.

donald g. mcneil jr.

The question is are any of those mutations important. And most of them aren’t. Most of them don’t change the function of the virus at all. But there has been one mutation that has become the object of a great deal of interest. We know for sure that there are sort of two general clades of the virus, the Wuhan strain and the other one called the Italian strain or sometimes a European strain. Now, the Wuhan strain is obviously the original one. That’s where the virus started. But it went around Asia. Then it went to Iran. Then it went to Italy. And in Italy sometime in February, presumably, this mutation took place. Now, it has definitely not made the virus more dangerous, more lethal, more likely to kill you. But it appears to have made it more transmissible.

michael barbaro

How so?

donald g. mcneil jr.

Well, it appears that it transmits between people five to 10 times more easily. Now, this is in dispute. But there’s been work done in cells in the laboratory where they infected them with the two different strains. And the mutation in the Italian strain seems to make the spikes on the outside of the virus — the spikes of the corona — more stable. Better able to infect. And so that they appear to be five to 10 times more capable at infecting cells as the old Wuhan version.

michael barbaro

So the strain of this virus that has a better spike — the Italian strain — and is therefore more transmissible, is crowding out the previous strain, because it’s just doing a better, more effective job of infecting people.

donald g. mcneil jr.

Yeah, that’s right. It’s the natural progression for a virus. It’s the way they tend to go.

michael barbaro

What do you mean?

donald g. mcneil jr.

Well, viruses, over the course of infecting lots of hosts, tend to become less lethal to those hosts and more transmissible. Like, for example, if I have the virus and it mutates inside me, and it turns into a more deadly strain, I’ve now got two strains. And I pass on that virus to two people, the person who gets the more deadly strain is more likely to go home, go to bed and die. Whereas the person who gets the less lethal, more transmissible strain is going to go out to a disco and infect 40 people.

And if you do that enough times in the course of the virus, the virus always sort of naturally moves in the direction of the more transmissible, less lethal one, because that’s the one that spreads whenever it’s given that kind of fork in the road.

And so this is what happened in 1918. The virus started off extremely deadly. It blew through an enormous chunk of the population, probably 60 to 70 percent of all the people in the world. And then it disappeared for a while. Then it turned up in pigs, and it was a pig virus for a while. And then when enough humans who’d never had the virus were born, it reappeared in people. But it reappeared as the H1N1 seasonal flu, the one that we know about as one of the seasonal flus every year. But that became less lethal and more transmissible. And basically all viruses do that. And we might be beginning to see the very first hints of that happening with this virus.

michael barbaro

So if I’m in Texas or Arizona right now and I’m testing positive for Covid-19, it sounds quite likely that I’ve gotten the Italian mutation of this virus, right? And that means I’m quite likely to spread it to somebody else and not have the most horrible symptoms. So does that partly help explain why infection rates are rising so rapidly in the U.S.?

donald g. mcneil jr.

Well, infection rates are rising rapidly in the U.S. more because of human behavior than because of any changes in the virus. I think it’s wishful thinking to think that this virus is not dangerous. It’s really dangerous, and it’s highly transmissible.

michael barbaro

But because the Italian version of the virus spreads more effectively, that does suggest that the virus is becoming better at doing the thing it was designed to do, which is to infect lots and lots of people.

donald g. mcneil jr.

Yes. But I mean, the Italian version versus the Wuhan version isn’t the dead end. There are going to be many more mutations. It mutates every two weeks. There may be other mutations turning up in the virus that turn out to be important. And we may call those the Texas strain or the California strain, or whatever.

But we don’t know them yet. There’s a lot of disagreement about this among scientists as to whether or not it really is more transmissible. And there’s zero agreement that it — not even really any thought that it’s less dangerous. That completely remains to be seen.

[music]
michael barbaro

We’ll be right back.

So Donald, what is the next big new understanding we have into the virus at this point?

donald g. mcneil jr.

Well, there’s more and more confirmation that you are much safer outdoors than you are indoors. There’s a study in China that looked at 318 clusters of transmission. And only one case involved outdoor transmission. And that was between two neighbors who had a long conversation with each other. And there’s recently been another study from Japan that suggests that your chances of getting the virus indoors are 20 times as high as it would be outdoors.

michael barbaro

And what are these studies finding about why exactly that is? I think we all have some understanding that when you’re outside the virus is just going to disperse and become more diffuse. Is that as complicated as it is?

donald g. mcneil jr.

Well, there’s always a little bit of wind outside. Humidity also makes droplets fall out of the air. But mostly it’s the wind. And when people talk within a few feet of each other, especially when they talk loud, or when they laugh, or when they sing or shout or do anything like that, you put out this kind of invisible mist of little tiny droplets that spews out of your mouth and sort of hangs around your head. But it also drifts towards the other person. And so you’re sitting inside each other’s droplet cloud. And those little tiny droplets, even if you’re not feeling the other person in effect spitting on your face, that droplet cloud can hold enough virus to transmit the disease from one person to the other.

And indoors when there’s no windows open, it can sort of drift through the room, more or less at head level, and go past one person after another at a cocktail party or inside a bar like that. And each person inhaling a little bit of that droplet cloud, until the disease has spread to 20, 30, 40 people. Whereas outdoors, the breeze just blows that away. So standing six feet away outdoors, even without masks, is considered safe.

michael barbaro

This is the idea that the virus becomes aerosolized. And you’re saying that indoors, that poses a very significant danger. Outdoors, because of wind, nowhere near as much.

donald g. mcneil jr.

Yeah, that’s right.

michael barbaro

So if being outdoors is less risky, and it’s now been clinically shown, I wonder if that explains something you mentioned the last time that we spoke, which is that you did not have a tremendous amount of fear that these protests that have occurred all over the United States over race and policing, that they would be a major source of infection. And is that because they occurred outdoors? And is it so far the case that they haven’t led to a meaningful spike in infections?

donald g. mcneil jr.

We have not seen any big spike in infections in the cities where most of the protests took place. So it looks like they didn’t lead to a lot of transmission. That doesn’t imply that everything is safe just because it’s outdoors. The important thing is how far apart people are when they’re outdoors. So sitting right next to somebody else in front of a stage at Mount Rushmore, for example, where the chairs are zip tied together, is not safe. Masks or no masks, you still really want to try to keep six feet distance.

michael barbaro

Donald, a couple of moments ago you mentioned the danger of being indoors because of this aerosolized virus mist that is not as great a danger outdoors. But I want to linger on this question of the indoors for a moment. Because the more we think about it, that aerosolized mist would seem to make any indoor activity inherently dangerous. I wonder if that’s an accurate assessment?

donald g. mcneil jr.

Yes. I mean, we’ve seen transmission of virus to large numbers of people in funerals, in choir practices, at birthday parties, inside bars, in business meetings. Virtually any kind of indoor environment you can imagine, there have been super spreader events. There may be ways to eventually make indoor spaces safer. There’s going to be no way to make them completely safe.

And all this talk about what’s safe to do indoors brings us to really the most important question, which is the most important indoor space we want to get functioning again, which is schools. Can kids go to school safely? And again, the science isn’t firm yet. But there are more and more hints that it may be safe, or pretty safe, to open the schools in the fall, especially for very young kids.

There’s growing evidence that kids are not big transmitters of the virus to adults. Denmark opened its schools in April. Did not see a big spike up in cases. Finland opened its schools in May. Did not see a big spike up in cases. Even from the beginning in China, the Chinese said, every time they looked at clusters in families, almost never did they see a case where the child, particularly the youngest child, was the one who introduced the virus into the family. Usually it was parent infecting the kids, not the other way around.

We know that kids are big transmitters of flu viruses. And they do it because they cough and sneeze like crazy. But if the biggest symptom that they’re getting is inflammation, rather than coughing and sneezing, — and that’s the case; kids tend to get more sort of cranky, inflammatory, unpleasant manifestations of the disease, rather than something that looks like a cold. Then it would make sense that that might be a reason why they’re not big transmitters.

michael barbaro

And what is this new insight about kids being less likely to transmit mean for the teachers who are going to stand or sit in front of them all day? Does it mean that an adult teacher in a school is pretty safe teaching? Or does it not mean that at all?

donald g. mcneil jr.

I don’t think we know the answer to that yet. I mean, schools — you’re bringing together a lot of kids. But schools also bring together a lot of adults. Teachers, staff, parents picking up the kids, things like that. So schools are not going to be completely safe under any circumstances.

But opening schools is so important to society, much more important than opening restaurants, much more important than opening movie theaters. It probably needs to be done really carefully. Not just all back into the classroom, 30 kids to a classroom, at all. But it looks like it could be done. And that’s really important. Because it’s important for the kids, for their development, for their feeding, for their socialization. And it’s also important for the parents. Parents can’t go back to work if they’re stuck at home with their kids. So it’s a crucial part of getting both the economy going and just the health of kids and health of parents.

michael barbaro

So of all the insights that you have shared today, this one seems like the silver lining. That reopening schools may be a somewhat safe undertaking.

donald g. mcneil jr.

Yeah. And that would be very good news for us.

michael barbaro

Because if I’m being candid, everything else you have said sounds pretty bad, right? I mean, it seems to be vascular, not respiratory. So it’s going to be easy to miss symptoms. It seems it’s becoming more transmissible through mutations. And the indoors presents very significant threats for non-kids because of this aerosolized mist. And once the temperature drops, which it will do in a few months, and tens of millions of us are suddenly stuck indoors, then we’re in for a lot of trouble.

donald g. mcneil jr.

Yeah, and the number of cases per day could rise well over 100,000 if we’re not careful. So yeah, I guess, it’s mostly bad news.

[laughter]

Sorry. I’m hoping that the fact that the virus is becoming more transmissible also means that it will become less lethal, which would be good news. But it hasn’t done that yet. So more transmission of a virus that’s already bad is not a good thing. No question about it.

michael barbaro

And all these things that we have just talked about would also seem to reinforce the need, not just for social distancing, but for these government-mandated lockdowns. I mean, specific requirements that say, don’t go to a bar. Don’t go to a restaurant. And those will become even more urgent as the warm weather yields to cold weather.

donald g. mcneil jr.

Yeah. We have to realize we are just in the opening phases of this pandemic. I mean, this is the second inning. And there’s still — there’s more than 120,000 people dead. So we are doing the dance in, dance out of various forms of lockdown. But we need to get to the point where we’re all basically dancing to the same music. Where all governors accept the notion that when they have a problem that’s getting out of control in their state, they react quickly.

And if they do that, they will save lives of their own citizens. And I think we’re beginning to see that.

In places like Texas, places like Arizona, places like Florida the governors have made major about-faces in the last couple of weeks. And they’re getting the science that the thing you do today doesn’t produce good effects until a month from today, because the people who got infected yesterday are the ones who are going to be in your hospital three weeks from now. So they’re beginning to catch on.

But we need to arrive at sort of a common understanding that we don’t all have to move in lockstep as a nation, but at the crucial moments we need to take similar steps to save lives.

[music]
michael barbaro

Thank you, Donald. We appreciate it.

donald g. mcneil jr.

Thank you. I was glad to be here.

michael barbaro

We’ll be right back.

Here’s what else you need to know today.

archived recording (donald trump)

In our schools, our newsrooms, even our corporate boardrooms, there is a new far-left fascism that demands absolute allegiance.

michael barbaro

In a pair of back-to-back speeches over the weekend, President Trump delivered harsh attacks against what he called the radical far-left forces who are protesting police brutality and tearing down monuments to America’s racist past, describing them as a threat to American values and heritage.

archived recording (donald trump)

If you do not speak its language, perform its rituals, recite its mantras and follow its commandments, then you will be censored, banished, blacklisted, persecuted and punished. It’s not going to happen to us.

michael barbaro

The Times reports that the speeches, delivered in front of Mount Rushmore and the White House, signaled that Trump would seek, once again, to exploit racial and cultural divisions in an effort to win re-election.

archived recording (donald trump)

I am here as your president to proclaim before the country and before the world this monument will never be desecrated. These heroes will never be defaced. Their legacy will never, ever be destroyed. Their achievements will never be forgotten. And Mount Rushmore will stand forever as an eternal tribute to our forefathers and to our freedom.

michael barbaro

Neither event enforced social distancing rules. And both were held despite pleas from public health officials that they be canceled to avoid spreading the coronavirus.

That’s it for “The Daily.” I’m Michael Barbaro. See you tomorrow.

Aerosols are droplets, droplets are aerosols — they do not differ except in size. Scientists sometimes refer to droplets less than five microns in diameter as aerosols. (By comparison, a red blood cell is about five microns in diameter; a human hair is about 50 microns wide.)

From the start of the pandemic, the W.H.O. and other public health organizations have focused on the virus’s ability to spread through large droplets that are expelled when a symptomatic person coughs or sneezes.

These droplets are heavy, relatively speaking, and fall quickly to the floor or onto a surface that others might touch. This is why public health agencies have recommended maintaining a distance of at least six feet from others, and frequent hand washing.

But some experts have said for months that infected people also are releasing aerosols when they cough and sneeze. More important, they expel aerosols even when they breathe, talk or sing, especially with some exertion.

Scientists know now that people can spread the virus even in the absence of symptoms — without coughing or sneezing — and aerosols might explain that phenomenon.

Because aerosols are smaller, they contain much less virus than droplets do. But because they are lighter, they can linger in the air for hours, especially in the absence of fresh air. In a crowded indoor space, a single infected person can release enough aerosolized virus over time to infect many people, perhaps seeding a superspreader event.

For droplets to be responsible for that kind of spread, a single person would have to be within a few feet of all the other people, or to have contaminated an object that everyone else touched. All that seems unlikely to many experts: “I have to do too many mental gymnastics to explain those other routes of transmission compared to aerosol transmission, which is much simpler,” Dr. Marr said.

ImageStephanie Stevens gets her nails done at Million Nails as New York City enters Phase 3 of its reopening on July 6th.
Credit…Gabriela Bhaskar for The New York Times

Physical distancing is still very important. The closer you are to an infected person, the more aerosols and droplets you may be exposed to. Washing your hands often is still a good idea.

What’s new is that those two things may not be enough. “We should be placing as much emphasis on masks and ventilation as we do with hand washing,” Dr. Marr said. “As far as we can tell, this is equally important, if not more important.”

Health care workers may all need to wear N95 masks, which filter out most aerosols. At the moment, they are advised to do so only when engaged in certain medical procedures that are thought to produce aerosols.

For the rest of us, cloth face masks will still greatly reduce risk, as long as most people wear them. At home, when you’re with your own family or with roommates you know to be careful, masks are still not necessary. But it is a good idea to wear them in other indoor spaces, experts said.

As for how long is safe, that is frustratingly tough to answer. A lot depends on whether the room is too crowded to allow for a safe distance from others and whether there is fresh air circulating through the room.

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This is a matter of intense debate. Many schools are poorly ventilated and are too poorly funded to invest in new filtration systems. “There is a huge vulnerability to infection transmission via aerosols in schools,” said Don Milton, an aerosol expert at the University of Maryland.

Most children younger than 12 seem to have only mild symptoms, if any, so elementary schools may get by. “So far, we don’t have evidence that elementary schools will be a problem, but the upper grades, I think, would be more likely to be a problem,” Dr. Milton said.

College dorms and classrooms are also cause for concern.

Dr. Milton said the government should think of long-term solutions for these problems. Having public schools closed “clogs up the whole economy, and it’s a major vulnerability,” he said.

“Until we understand how this is part of our national defense, and fund it appropriately, we’re going to remain extremely vulnerable to these kinds of biological threats.”

Do as much as you can outdoors. Despite the many photos of people at beaches, even a somewhat crowded beach, especially on a breezy day, is likely to be safer than a pub or an indoor restaurant with recycled air.

But even outdoors, wear a mask if you are likely to be close to others for an extended period.

When indoors, one simple thing people can do is to “open their windows and doors whenever possible,” Dr. Marr said. You can also upgrade the filters in your home air-conditioning systems, or adjust the settings to use more outdoor air rather than recirculated air.

Public buildings and businesses may want to invest in air purifiers and ultraviolet lights that can kill the virus. Despite their reputation, elevators may not be a big risk, Dr. Milton said, compared with public bathrooms or offices with stagnant air where you may spend a long time.

If none of those things are possible, try to minimize the time you spend in an indoor space, especially without a mask. The longer you spend inside, the greater the dose of virus you might inhale.