Nicotine is the addictive substance found in tobacco vegetation. 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 liner of the lungs. One small clinical research from 2018 that found that the compounds modified vapers’ sputum—the mucosal lining that covers the inner respiratory tract—to express 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 suggestions that will assist you to live your healthiest life.