Irene - The guide I use to decide when to mask is air volume & movement.

Think of this from a science perspective. The size of most grocery stores, with their high ceilings and wide aisles, as well as the need for air movement to carry away the heat from the refrigeration and freezer units, means the air exchange is usually very good. That means any particles in the air are very diluted and quickly whisked off into the ventilation systems. Thus, low density of problem particulates. For me, that means no mask.

On the other hand, the neighborhood dollar store is often built in a strip mall, with low ceilings and narrow aisles, and whatever ventilation the builder installed. And people must always pass/stand close to one another. That means any particles linger longer and closer to where we breathe. So, if I walk in and there are more than a few people, I mask.

Does that make sense to you?
Sue