Nicotine is the addictive substance present 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 scientific research recommend that glycerin and propylene glycol irritate the lining of the lungs. One small medical examine from 2018 that found that the compounds modified 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 publication, and obtain every day tips that may help you live your healthiest life.