Nicotine is the addictive substance found in tobacco plants. Benzoic acid can also be naturally found in vegetation, and is used commercially in dyes, perfumes, insect repellants, and meals preservatives. Existing cell, animal, and clinical studies recommend that glycerin and propylene glycol irritate the liner of the lungs. One small scientific study from 2018 that discovered that the compounds modified vapers’ sputum—the mucosal lining that covers the internal respiratory tract—to express completely different immune proteins. Some of these protein changes have been linked to bronchial asthma. Sign up for our Health Tip of the Day e-newsletter, and obtain daily tips that may help you live your healthiest life.