Nicotine is the addictive substance present in tobacco plants. Benzoic acid can be naturally present in plants, and is used commercially in dyes, perfumes, insect repellants, and meals preservatives. Existing cell, animal, and clinical research suggest 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 internal respiratory tract—to express different immune proteins. Some of these protein changes have been linked to bronchial asthma. Sign up for our Health Tip of the Day newsletter, and obtain daily tips that will allow you to reside your healthiest life.