Are pollen masks usually effective for preventing allergies leading to asthma?

It varies...: ...At least some relief of symptoms is likely, complete resolution is not.

Sometimes: It really depends on which allergens you are trying to avoid. All allergens have different size particles and pollens are generally light and distributed in the air and therefore can get through most filters. Dust mites are larger as are pet danders and thus do not spend much time in the air and can be avoided with some masks. Particularly niosh masks.