From a health and safety point of view, keep in mind that persons using a respirator in the workplace are required to have an initial medical exam and periodic medical surveillance, as it's possible a respirator can do more harm than good in some persons with underlying medical conditions/constraints, and use of a respirator may induce respiratory distress. This is for masks that actually seal to your face and pass the air through a particle or vapor filter. If the masks do not seal, like the common dust mask sold in the big box stores, then no real worries but probably not filtering out a lot either.

I wouldn't use a filtering facepiece mask/respirator while cycling or exercising, unless it was the powered, supplied air, variety. expensive and heavy. Powered Air Purifying Respirators (PAPR) - Respiratory - Grainger Industrial Supply