Nicotine is the addictive substance present in tobacco crops. Benzoic acid is also naturally found 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 liner of the lungs. One small medical study from 2018 that discovered that the compounds changed vapers’ sputum—the mucosal lining that covers the inner respiratory tract—to specific different immune proteins. Some of these protein modifications have been linked to asthma. Sign up for our Health Tip of the Day e-newsletter, and receive day by day tips that can allow you to stay your healthiest life.