Nicotine is the addictive substance found in tobacco crops. Benzoic acid can also be naturally found in vegetation, and is used commercially in dyes, perfumes, insect repellants, and food preservatives. Existing cell, animal, and clinical studies counsel that glycerin and propylene glycol irritate the lining 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 express totally different immune proteins. Some of those protein modifications have been linked to asthma. Sign up for our Health Tip of the Day e-newsletter, and receive daily tips that will allow you to live your healthiest life.