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