Nicotine is the addictive substance present in tobacco crops. Benzoic acid can be naturally found in crops, and is used commercially in dyes, perfumes, insect repellants, and meals preservatives. Existing cell, animal, and medical studies counsel 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 precise 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 every day suggestions that may help you live your healthiest life.