Yes. In my state there are no minimum requirements for the mask. You can wear a scarf, a bandana or whatever.
Asthma. Mostly it's "managed" through my current preventer Breo (which is far more effective than my previous preventer, Symbicort), but there can be times when it's what they classify as "unmanaged". It's when its unmanaged the mere physical effort to breathe is an undue exertion on the body, and cumulatively over time it can be exhausting. Add any kind of filter to that process, and that exertion increases in order to receive enough oxygen and airflow. That cumulative exertion manifests as chronic fatigue. My father, and his recently deceased mother have had similar experiences.
Understood, which is why I've been happy to answer your questions.