Nicotine is the addictive substance found in tobacco plants. Benzoic acid can also be naturally present 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 clinical research from 2018 that found that the compounds changed vapers’ sputum—the mucosal lining that covers the inner respiratory tract—to express completely different immune proteins. Some of those protein modifications have been linked to asthma. Sign up for our Health Tip of the Day e-newsletter, and receive day by day suggestions that can help you stay your healthiest life.