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 scientific research suggest that glycerin and propylene glycol irritate the liner of the lungs. One small medical study from 2018 that found that the compounds changed vapers’ sputum—the mucosal lining that covers the interior respiratory tract—to precise different immune proteins. Some of these protein modifications have been linked to asthma. Sign up for our Health Tip of the Day e-newsletter, and receive day by day ideas that can allow you to reside your healthiest life.