Better

Nov 16, 2023

Using a mannequin, researchers demonstrate how the respiratory cloud, and possibly coronaviruses with it, spread in different scenarios.

New research shows that 9.8 feet (3 meters) of social distancing are not enough to ensure protection from Covid-19. Even at that distance, it takes less than five minutes for an unvaccinated person standing in the breath of a person with Covid-19 to become infected with almost 100% certainty.

That's the bad news.

The good news is that if both people are wearing well-fitting medical masks, or, even better, N95 or FFP2 masks, the risk drops dramatically.

Researchers have investigated to what extent masks protect under which wearing conditions. In the process, the researchers determined the maximum risk of infection for numerous situations and considered several factors not included in similar studies. The comprehensive study published Dec. 7 in the Proceedings of the National Academy of Sciences (PNAS).

The team at the Max Planck Institute for Dynamics and Self-Organisation in Göttingen, Germany, which includes Eberhard Bodenschatz, adjunct professor of physics in the College of Arts and Sciences and aerospace and mechanical engineering in the College of Engineering, was surprised at the ease of coronavirus transmission.

"We would not have thought that at a distance of several meters it would take so little time for the infectious dose to be absorbed from the breath of a virus carrier," said Bodenschatz, director at the Max Planck Institute.

At that distance, the researchers found that air that has been exhaled has already spread in a cone shape in the air; the infectious particles are correspondingly diluted. In addition, the particularly large and thus virus-rich particles fall to the ground after only a short distance through the air.

"In our study we found that the risk of infection without wearing masks is enormously high after only a few minutes, even at a distance of three meters, if the infected persons have the high viral load of the delta variant of the Sars-CoV-2 virus," said Bodenschatz. And such encounters are unavoidable in schools, restaurants, clubs or even outdoors.

Nonetheless, medical or FFP2 masks protect effectively. The study confirms that FFP2 or N95 masks are particularly effective in filtering infectious particles from the air – especially if the masks are sealed at the face as tightly as possible. If both the infected and the non-infected person wear well-fitting FFP2 masks, the maximum risk of infection after 20 minutes is hardly more than one per 1,000, even at the shortest distance. If their masks fit poorly, the probability of infection increases to about 4%. If both wear well-fitting medical masks, the virus is likely to be transmitted within 20 minutes with a maximum probability of 10%. The study also confirms the intuitive assumption that for effective protection against infection, the infected person in particular should wear a mask that filters as well as possible and fits tightly to the face.

The infection probabilities determined by the Max Planck team indicate the upper limit of the risk in each case. In daily life, the actual probability of infection is certainly 10 to 100 times smaller. "This is because the air that flows out of the mask at the edges is diluted, so you don't get all the unfiltered breathing air," Bodenschatz said. "But we assumed this because we can't measure for all situations how much breathing air from one mask-wearer reaches another person, and because we wanted to calculate the risk as conservatively as possible.

"Under these conditions, if even the largest theoretical risk is small, then you’re on the very safe side under real conditions."

For the comparative value without the protection of a mask, however, the safety buffer turns out to be much smaller. "For such a situation, we can determine the viral dose inhaled by an unprotected person with fewer assumptions," said Gholamhossein Bagheri, a research group leader at the Max Planck Institute and lead author of the study.

In their calculations of the risk of infection, the researchers considered a number of factors that had not previously been included in comparable studies, including how a poorly fitting mask weakens the protection and how this can be prevented.

"The materials of FFP2 or KN95 masks, but also of some medical masks, filter extremely effectively," said Bagheri. "The risk of infection is then dominated by the air coming out and going in at the edges of the mask." This happens when the edge of the mask is far from the face.

In elaborate experiments, Bagheri, Bodenschatz and their team measured the size and amount of respiratory particles that flow past the edges of masks that fit differently.

"A mask can be excellently adapted to the shape of the face if you bend its metal strap into a rounded W before putting it on," said Bodenschatz. "Then the infectious aerosol particles no longer get past the mask – and glasses no longer fog up, either."

Get Cornell news delivered right to your inbox.