Airborne allergens can trigger environmental allergies and asthma attacks. Indoors, you can keep your home free of allergens with environmental control products like air purifiers and HEPA vacum cleaners. Outdoors, however, it's impossible to clean up all of the allergens, pollutants, and asthma triggers. You can reduce exposure to these irritating outdoor particles with a well-fitting mask.  Masks also help allergy sufferers avoid indoor chemical fumes, particles, and dust (especially when cleaning)