Nicotine is the addictive substance found 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 clinical studies recommend that glycerin and propylene glycol irritate the liner of the lungs. One small scientific study from 2018 that found that the compounds changed vapers’ sputum—the mucosal lining that covers the interior respiratory tract—to express totally different immune proteins. Some of those protein adjustments have been linked to bronchial asthma. Sign up for our Health Tip of the Day publication, and obtain daily tips that may allow you to live your healthiest life.