Immunologist Erin Bromage spoke to the BBC about whether we should be more concerned about coughing or sneezing by others and whether we are more at risk in the park, in the office … or in our own home.

What danger does that man who sneezes in the bus line pose to me? Should I really go to a restaurant? Or use public transportation?

As part of the world is gradually emerging from confinement and returning to social settings and activities, the risk of contracting and spreading the coronavirus increases, which causes fear of a second wave of infections.

We spoke with immunologist and associate professor of biology Erin Bromage about how we might reduce the risk of contracting covid-19.

She teaches an infectious disease epidemiology course at the University of Massachusetts at Dartmouth, USA, and has followed this pandemic closely.

More than an expert in this particular disease, he sees his role as that of a communicator of scientific information and wrote a text on the risks of the coronavirus on a blog that has been read almost 16 million times.

Here are her tips for staying safe when we get back to some kind of normalcy.

Where do people get sick?

Bromage says that most people get infected in their own home, because a member of the household that transmits it to others through continuous contact.

. Saliva droplets from an infected person can infect us.

But what about away from home?

Are we in danger on our daily walk in the park? Is that inconsiderate person running without a mask going to infect me?

Probably not, says the teacher.

“Outdoor, you have unlimited dilution ”, he told the BBC. “So you exhale and this dissipates very, very quickly.”

That means that you are unlikely to be exposed to the virus long enough to contract it.

“To become infected, you need to expose yourself to an infectious dose of the virus; according to infectious dose studies with MERS and SARS, some Estimated that as few as 1,000 viral SARS-CoV2 particles are needed for an infection to take rootHe wrote on his blog.

This number is the subject of much debate and must be determined by experiment, but it provides a useful reference to demonstrate how infection can occur.

The key point here is that you can reach that number, whatever it is, in several different ways.

“Through 1,000 viral particles you receive in one breath… or 100 inhaled viral particles with each breath for 10 breaths or 10 viral particles with 100 breaths. Each of these situations can cause an infection, “he explained.

And that means that in those instances in which you spend a short time with infected people (such as those who run without respecting social distancing) it is unlikely to pass on an infectious dose.

So which situations should concern us the most?

People with symptoms

Coughing and sneezing actually transmit disease, but at very different rates.

A single cough releases about 3,000 drops at about 80 km / h, according to Bromage.

Most of the drops are quite large and gravity makes them fall to the ground quickly, but others remain in the air and can cross a room.

But if the person you’re in an elevator sneezes with instead of coughing, your problems are multiplied by 10.

A single sneeze releases around 30,000 drops, much smaller and traveling much larger distances, easily through a room.

These can reach speeds of up to 320 km / hBromage says.

“If a person is infected, the drops in a single cough or sneeze can contain up to 200 million virus particles,” he wrote.

. Should we worry if someone runs past us and coughs?

“So if you’re talking face-to-face with a person and they sneeze or cough directly at you, it is quite possible that you will end up inhaling 1,000 virus particles and become infected.”

Even if you were not present when the cough or sneeze occurs, you may not be safe.

Some infected drops are small enough to stay in the air for a few minutes, and if you enter that room within that time, you could breathe enough to catch it.

The asymptomatic

We know that people can be infectious for about five days before they start showing symptoms, and some may never have them.

Further, even respiration releases copies of the virus into the environment.

But how many?

“A single breath frees 50 to 5,000 drops. Most of them go at low speed and fall to the ground quickly, ”according to Bromage.

When we breathe through the nose, even fewer drops are released.

“There it is even more filtered and it points directly down. So you find that very few pathogens, viral particles, are released by breathing, “he told the BBC.

“Importantly, due to lack of exhalation force when breathing, viral particles from the lower respiratory areas are not expelled.”

.Dentists and dental hygienists are at risk of contagion.

This is significant because the tissues found in these lower areas are where the coronavirus is in the highest concentration.

We don’t know exactly how many viral particles of SARS-CoV2 (this new coronavirus) are released through respiration, but Bromage cites a study saying that a person infected with influenza releases 3 to 20 copies of viral RNA per minute of respiration.

If this figure is valid for the coronavirus, an infected person releases 20 copies per minute into the environment.

You would have to inhale each particle that breathed for 50 minutes to reach the 1,000-copy figure discussed above in order to catch it (remember, this figure is only used as a reference and the exact value has yet to be determined).

So, we can see that it would be much less likely to contract the disease simply by being in the same room as someone who is infected but do not cough or sneeze.

Talking multiplies the release of respiratory droplets by about 10, that is, at about 200 copies of the virus per minute, according to Bromage.

Sing and shout

“You get to scream or sing and the drops really do project at that stage and they can come from deep in your lungs while you’re forcing that sound,” he told the BBC.

These drops also come from areas where the tissue is more likely to be contaminated.

. When singing or shouting the number of droplets ejected multiplies.

“Therefore, anything that causes vigorous emissions simply puts more of those respiratory droplets into the air from tissues that have a higher viral load,” he said.

Although it is much more difficult to contract the infection in this way, there are studies that estimate that many infections, and perhaps the majority of the transmissions outside the home, take place in people without any symptoms.

What environments are particularly risky?

Obviously, those professions that deal directly with infected people are at the highest risk.

We also know that certain environments have caused large-scale infections.

While cruises may be the first thing that comes to mind for the public, Bromage highlights events that occurred in open-plan offices and at sporting and social events like birthday parties, funerals, and a choir recital.

In these cases, people had a much higher risk of viral exposure due to time spent indoors in the presence of someone who was infected.

.There is more risk of contagion in closed spaces.

“Even if they were 15 meters away, such as in the choir or in the call center, when they were hit by air that contained a dose even if the virus was low for a long period, it was enough to cause a contagion,” he said. .

As we return to work, certain professions will also be of particular concern.

The Open-plan offices with poor ventilation are especially problematic.

Bromage cites an example where 94 of 216 office workers in a building in South Korea were infected, the vast majority of whom were on one side of a floor, where they shared a large open space.

Dentists do not make up much of the population, but will be particularly exposed to risk.

.Dentists and dental hygienists are at risk of contagion.

“It’s an occupation that creates a lot of aerosols, just because of the procedures they do, and they really have to think about their workplace to protect their employees primarily,” he says.

“Because it will not be these who can make the patient sick, but the patient who can make employees, dental hygienists, the dentist, get sick because of the drilling, the suction, all the fluids that go everywhere ”.

Teaching staff also face increased risk, he said.

“There is a population of older professors and professors with younger people in a high-density classroom. There will be a lot of thought going into how to make those workplaces safe. ”

In and out

Bromage says they have been reported very few episodes of outbreaks in outdoor settings.

Wind and space dilute viral load, and sunlight, heat and humidity can also have an effect on viral survival.

By maintaining social distance and limiting the duration of interactions, we can further reduce risk.

. Outdoors the risk of contagion is lower.

But some indoor interactions can be very risky.

Events filled with people talking, singing, or screaming will, of course, be high risk, and social distancing measures become less effective indoors over time.

Spaces with limited air exchange or recycled air are particularly problematic.

But buying, at least for the customer, is much less risky, as long as you spend relatively little time in one environment.

Assess the risk

As coronavirus restrictions are removed, according to Bromage, we must critically evaluate our activities in terms of risk.

.The time you spend in a place influences your risk of contracting the virus.

If you go indoors, consider the volume of the space, the amount of people that will be inside at any given time and how much time will you spend there.

“If you are sitting in a well-ventilated space with few people, the risk is low,” he said.

“If you’re in an open-plan office, you really need to critically assess risk (volume, people, and airflow). If you’re in a job that requires talking face-to-face or, worse, yelling, you need to assess the risk. “

. Large crowds are not a good idea.

For a visit to a shopping center, for example, “the low density, the high volume of air from the store together with the limited time you will spend in the store, means that the opportunity [para un cliente] receiving an infectious dose is low, “said the expert.

“But for the store worker, the extended time spent there provides a greater opportunity to receive the infectious dose,” he said.

Outside, the risk of infection is much lower because the infected droplets will dissipate more quickly, but remember that it takes “dose and time” to catch it.

“And while I’ve focused on respiratory exposure here, don’t forget the surfaces. Those infected respiratory droplets land somewhere. Wash your hands often and stop touching your face! ”He wrote.

You should probably also stop blowing out the candles on your birthday cake.

BBC