Nicotine is the addictive substance present in tobacco plants. Benzoic acid is also naturally found in plants, 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 liner of the lungs. One small medical examine from 2018 that found that the compounds changed vapers’ sputum—the mucosal lining that covers the interior respiratory tract—to express totally different immune proteins. Some of those protein changes have been linked to bronchial asthma. Sign up for our Health Tip of the Day newsletter, and receive daily tips that can allow you to live your healthiest life.