'Citation: Anfinrud, P., Stadnytskyi, V., Bax, C. E., & Bax, A. (2020). Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering. New England Journal of Medicine, NEJMc2007800. https://doi.org/10.1056/NEJMc2007800;
Extract from the paper: "Aerosols and droplets generated during speech have been implicated in the person-to-person transmission of viruses,1,2 and there is current interest in understanding the mechanisms responsible for the spread of Covid-19 by these means... We found that the number of flashes increased with the loudness of speech; this finding was consistent with previous observations by other investigators..." '
Don't know about the oxygen-reduction effects, which I guess could lead to problems from long term use. Aren't there exemptions for those with pre-existing respiratory problems? The way I see it they're not perfect either for preventing the wearer from catching or spreading the virus, but if worn by a high percentage of the population they can significantly slow down the spread of infection.