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