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I’ve read that masks hardly work, and one mask caused health problems.

You did not read that in the medical literature. That is a story widely spread by professional antivaxxers who coordinated early on to undermine public health efforts to stop the pandemic.

Here are some selected medical/scientific journal articles on masks:

Experimental Evidence for the Optimal Design of a High-Performing Cloth Mask. Figure 7 in this paper shows efficiency of a single layer (top 6 panels), a double layer (middle 6 panels), and a triple layer (bottom 6 panels) of fabric for blocking sneeze droplets. The authors of this study also found that cotton and polyester cloth masks were more efficient after being washed once or twice.

Human coronavirus data from four clinical trials of masks and respirators. This is a review of several studies of transmission of coronavirus under three conditions: no mask, surgical mask, and N95 or P2 mask. All studies showed the highest transmission among the non-masked study participants. The coronaviruses in this study were those that cause the common cold, not Covid (but transmission characteristics are quite similar).

Impact of community masking on COVID-19: A cluster-randomized trial in Bangladesh. This study of 342,183 participants randomized into mask wearing and non-masked groups found that masking decreased transmission of Covid by 11.1% overall

I can keep posting links to studies showing efficiency of masks in decreasing the spread of Covid. There are a lot of such studies. The value of scientific studies is that they are based on observation and data collection and analysis. Recommendations to wear a mask are made because the data supports such recommendations.

As for the antivax claim that masks are dangerous because they block oxygen? Also not true. SARS-CoV-2 viruses are about 100 nm in diameter; an oxygen (O2) molecule is only 0.346 nm in diameter. That means that a SARS-CoV-2 virus particle is 289 times as large as an O2 molecule. Unless you wear a solid plastic mask, the mask cannot block oxygen.

SARS-CoV-2 (COVID-19) by the numbers. (Diameter of SARS-CoV-2 virus particle.)

Kinetic diameter. (Diameter of O2 molecule.)