1. The main ingredient in e-cigarettes is nicotine, which is less harmful than paper cigarettes and can be inhaled into the lungs, but long-term inhalation can lead to addiction.

2. In addition, there is a substance in e-cigarettes, propylene glycol, which mainly acts as an antifreeze, and can become an allergen after topical use and inhalation, leading to eye and respiratory discomfort, and may also lead to headaches, dizziness and drowsiness.

3. In addition, there is a diacetyl flavoring agent, a substance that gives a butter-like flavor, which can cause a life-threatening respiratory disease such as popcorn lung, so e-cigarettes are harmful and are not recommended for smoking.