Nicotine is the addictive substance found in tobacco crops. 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 research suggest 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 precise different immune proteins. Some of these protein adjustments have been linked to asthma. Sign up for our Health Tip of the Day newsletter, and receive daily ideas that will allow you to reside your healthiest life.