Nicotine is the addictive substance present in tobacco crops. Benzoic acid can be naturally found in vegetation, and is used commercially in dyes, perfumes, insect repellants, and food preservatives. Existing cell, animal, and clinical research suggest that glycerin and propylene glycol irritate the lining of the lungs. One small scientific research from 2018 that found that the compounds changed vapers’ sputum—the mucosal lining that covers the internal respiratory tract—to specific different immune proteins. Some of those protein adjustments have been linked to asthma. Sign up for our Health Tip of the Day newsletter, and receive day by day ideas that can help you live your healthiest life.