Well-fitting masks are more effective than an extra meter or two of distance for preventing COVID-19 spread, a new study concludes. Even standing 3 meters (10 feet) apart, it would take less than five minutes for an unmasked unvaccinated person talking to an unmasked person that has COVID-19 to become infected with almost 100 percent certainty. Masks dramatically reduce that risk, providing they are appropriately fitted. 

There’s no ethical way to conduct trials that really capture the risk of infection from spending time with a virus carrier. However, in Proceedings of the National Academy of Sciences, scientists have measured infection risk in various scenarios as best they can and can finally offer risk estimates to allow people to find the exposure danger with which they are comfortable.

Social distancing recommendations have been based on pre-pandemic science, some of which has turned out to be wrong, or at least not applicable to the SARS CoV-2 virus. Science takes time, and health authorities had to provide recommendations before the research could be updated, although arguably many have been too resistant to change as new evidence emerged.

Scientists at the Max Planck Institute for Dynamics and Self-Organisation (MPIDS) combined data on the distribution of respiratory particle sizes and leakage from different types of face masks with what we know about the airflow from breathing and speaking. They combined these with the way liquid particles shrink as they evaporate and what is suspected about absorption of SARS-CoV-2 viruses from susceptible airways to estimate exposure risk. The study took into account previously neglected factors, such as the way partially dried particles rehydrate on inhalation.

Even standing 3 meters apart there is a 90 percent chance an unmasked infectious person would transmit the disease to an unmasked counterpart within five minutes of conversation, the authors conclude. The work assumes still air around the two individuals, so the authors stress it represents an upper bound that can be lowered by good ventilation. Nevertheless, standard recommendations of 1.5-2 meters look deeply inadequate, regardless of how cute the memes to help measure those distances are.

"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 Professor Eberhard Bodenschatz, Director at MPIDS, in a statement. 

The study used estimates for Delta viral load, so it’s possible the recommendations were not so inadequate for people carrying earlier strains of the virus. However, the key difference is we now know COVID-19 can spread in small particles, which travel further, rather than only being carried in larger droplets.

The good news, however, is Bodenschatz and co-authors’ work backs up the growing evidence masks make an enormous difference. “In daily life, the actual probability of infection is certain 10 to 100 times smaller” when wearing masks, Bodenschatz said.  

Standing 1.5 meters (5 feet) apart the risk of transmission is at most 0.4 percent, even after an hour, if both parties are wearing well-fitted FFP2 masks. (Other studies have found FFP2 and N95 masks have similar protective values).

Not all masks are equal, but the study indicates even badly fitted masks count for a lot. "The materials of FFP2 or KN95 masks, but also of some medical masks, filter extremely effectively," said first author Dr Gholamhossein Bagheri. "The risk of infection is then dominated by the air coming out and going in at the edges of the mask." The study didn’t consider the usefulness of reusable cloth masks, although previous work suggests their value depends on their material. It also didn’t estimate the value of a mask worn without covering the nose, but you can probably guess that.

Overall the research makes a strong case for wider use of FFP2 masks, finding that even badly fitted protection of this sort is 2.5 times as effective as a well-adjusted surgical mask.