1/ Here is one way to do a quick & dirty test of leakage around mask. Place several small mirrors or reading glasses in the fridge for 2 to 3 min. Avoid putting them in the freezer, as they will get too cold and you will get condensation on lenses when you remove it.
2/ Take first one out and place it directly in from of mask. As you breathe, the high water vapor content of your warm breath will condense on the lens, a good thing as it shows air going through mask. 
3/ Now take out a second pair of glasses and put at edge of mask near chin, nose crease, or cheeks. You want to see as little, and preferably no, condensation as possible. In example below I purposely tweaked seal around nose to yield a small amount of condensation.
4/ Repeated with good seal and no visible condensation. This took about two minutes. Lack of visible condensation does not mean zero leakage, but does give a good indication of relative fit. Aim for zero visible condensation around edges and a lot adjacent to front of mask.
5/ Best to do front of mask first & edges after, as glass lenses will become cooler as they sit in fridge and yields a safety factor (makes it easier to get ample condensation even with small leakage as per 2nd set of images above).
6/ This is not a perfect fit test, but it does make use of the high absolute humidity on breath and is a bit more scientific than other methods I've seen people reporting.
7/ Wearing a mask has a dual benefit. It lowers your inhaled dose of virus-laden aerosol particles if you are in a space w/ an infected person (whether in near or far field in indoor space) & also lowers your emissions if you are infected, thus lowering inhaled dose of others.
8/ Simple example. The infected person wears a mediocre mask that captures 40% of emissions & you use a mask that removes 40% of virus-laden aerosol particles. The combination yields a 64% reduction in your inhaled dose of aerosol particles (whether close contact or far field).
9/ Now, let's say that a better mask is used or a much better fit of the first mask is obtained, yielding 80% reduction in emissions and removal during inhalation by receptor. This yields an overall reduction of 96% in inhaled dose of aerosol particles. HUGE reduction.
10/ The delta variant is highly transmissible & destroying and taking lives. Wear a higher quality mask if possible. Try to get as good of a fit as possible. Protect yourself. Protect others. Just Do It!
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