I wear a mask when around other people. In my car, no. I have asthma and even though it's relatively light fabric, it does impede my breathing somewhat. Not enough to be uncomfortable and not anywhere near an asthma attack or leaving me on the verge of passing out, but noticeably more restrictive. I wouldn't use one if I was engaging in any kind of vigorous physical activity.

Even the CDC guidelines say not to wear a mask if it makes it hard for you to breathe.