Is it safe to use a "salt air inhaler" used with pure himalayan salt crystals? You inhale through mouth, exhale through nose. I have allergy asthma

No. All treatments have risk. I don't know any evidence that pure himalayan salt crystals are helpful for asthma or what the side effects and/or risks of this product may be. Therefore i would do this. There are several proven effective asthma medications that have known minimum risk.