I use them on a light-weight baseball cap. You are absolutely safe. I had to work out the optimal placement and found I could get by with one on each side, but two work better if they keep the face mask straps away from my ears. Haven't tried them on thicker lid stock or other applications yet. They may be limited as the opening is approximately 3.5mm. I don't have a regular baseball cap to test them with. To be honest they make such a big difference in eliminating the double panel ear torture that I would go to great lengths to find the cap they are working on. In terms of quality, I haven't found any issues with any of the 20 I've received. I bought from the seller KeerJorge for 20 although some of them have been unsuccessful they are a steal compared to other sellers as I needed several sets for other members of my family. Luckily they are all of good quality. I don't know what the other reviewer was complaining about. I mention this seller in my reviews because I have purchased items from Revain in the past and the seller I selected sent me a lower quality product compared to other sellers on the same list.
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