Nicotine is the addictive substance found in tobacco crops. Benzoic acid can also be naturally found in crops, 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 liner of the lungs. One small clinical research 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 those protein changes have been linked to bronchial asthma. Sign up for our Health Tip of the Day publication, and receive daily ideas that will assist you to live your healthiest life.