Nicotine is the addictive substance present in tobacco vegetation. Benzoic acid can also be naturally found in plants, 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 liner 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 totally different immune proteins. Some of these protein modifications have been linked to bronchial asthma. Sign up for our Health Tip of the Day publication, and receive every day suggestions that will allow you to reside your healthiest life.