Nicotine is the addictive substance present in tobacco crops. Benzoic acid is also naturally present 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 liner of the lungs. One small medical research from 2018 that found that the compounds modified vapers’ sputum—the mucosal lining that covers the interior 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 receive every day tips that will help you live your healthiest life.