Nicotine is the addictive substance found in tobacco vegetation. 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 studies recommend that glycerin and propylene glycol irritate the liner 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 express totally different immune proteins. Some of those protein modifications have been linked to asthma. Sign up for our Health Tip of the Day newsletter, and receive daily tips that can help you live your healthiest life.