Nicotine is the addictive substance present in tobacco crops. Benzoic acid can be naturally present in vegetation, and is used commercially in dyes, perfumes, insect repellants, and food preservatives. Existing cell, animal, and medical studies counsel that glycerin and propylene glycol irritate the lining of the lungs. One small clinical research from 2018 that found that the compounds changed vapers’ sputum—the mucosal lining that covers the internal respiratory tract—to specific different immune proteins. Some of these protein adjustments have been linked to bronchial asthma. Sign up for our Health Tip of the Day publication, and receive daily tips that can assist you to live your healthiest life.