Nicotine is the addictive substance found in tobacco vegetation. Benzoic acid is also naturally found in plants, and is used commercially in dyes, perfumes, insect repellants, and food preservatives. Existing cell, animal, and medical research counsel that glycerin and propylene glycol irritate the lining of the lungs. One small scientific study from 2018 that found that the compounds modified vapers’ sputum—the mucosal lining that covers the inner respiratory tract—to specific different immune proteins. Some of those protein adjustments have been linked to bronchial asthma. Sign up for our Health Tip of the Day newsletter, and receive every day ideas that will allow you to stay your healthiest life.