There is agreement that N95 respirators, when properly used by trained professionals, can protect the wearer from exposure to COVID-19. Sick people who wear surgical masks are thought to be less likely to infect others, but there is disagreement on whether surgical masks are protective for the wearer.
Unlike respirators, surgical masks are loose pieces of fabric that are not airtight, and so in theory they should not prevent the inhalation of airborne viruses. However, this does not mean that they cannot provide some protection – the question is how much.
There is some evidence from the early 2000s SARS epidemic that surgical masks can protect the wearer. In this particular study of five hospitals, there were no infections among surgical mask-wearers, compared with 7% infection among those who wore makeshift paper masks. There is also evidence for the effectiveness of masks amongst non-healthcare workers. One 2009 study looked at transmission of swine flu on a commercial flight, and found that none of the mask-wearers became ill, compared to 35% of the non-wearers. In more controlled studies in which some flu patients’ families were told to wear masks and compared to control families, masks appeared to be protective, but only when those who did not comply fully with the mask-wearing were excluded.
While this might sound like solid evidence, there are some very important problems to consider. Those who are more diligent in their use of masks probably follow other hygiene precautions more strictly, and so the data from these studies could have nothing to do with masks at all. The studies that occur following an epidemic rely on self reporting of mask use, and those who are inconsistent with their mask use are more likely to report that they didn’t use a mask if they became ill.
So if the evidence for surgical mask benefits is unclear, what of N95 respirators? These are more expensive masks designed to create an airtight seal, which should theoretically offer much better protection. However, this relies on them being used correctly, and according to one study in California, 65% of people wear their respirators incorrectly even among health workers, let alone the general population.
Overall, there is some evidence that wearing a surgical mask could protect against exposure to coronavirus. However, staying at home remains the most effective protection, and with many hospitals currently suffering from a mask shortage, healthcare workers who are exposed to coronavirus every day should be prioritised. That means not buying up masks until supply is plentiful. As for respirators, while some people might be able to wear them properly, evidence suggests that most wearers will not gain a significant benefit over surgical masks.
FACE MASKS: MUCH MORE THAN YOU WANTED TO KNOW: https://slatestarcodex.com/2020/03/23/face-masks-much-more-than-you-wanted-to-know/
Protection by Face Masks against Influenza A(H1N1)pdm09 Virus on Trans-Pacific Passenger Aircraft, 2009: http://dx.doi.org/10.3201/eid1909.121765
Tuberculosis isolation: comparison of written procedures and actual practices in three California hospitals.: DOI: 10.1086/501693
Effectiveness of precautions against droplets and contact in prevention of nosocomial transmission of severe acute respiratory syndrome (SARS): https://doi.org/10.1016/S0140-6736(03)13168-6