A e-cigarette is a device that simulates tobacco smoking by using a power source to heat liquid, usually nicotine and flavourings, which are inhaled through the mouthpiece to create a vapor. In 2019, e-cigarette use was reported by over 5 million middle and high school students. A wide variety of appealing flavours are used to lure current smokers and attract new users.
The varying concentrations of nicotine in e-liquids and the method used to deliver nicotine may affect how harmful vaping can be. The nicotine level in a single e-liquid can range from 0 mg/mL to 54 mg/mL and the way it is delivered from the cigarette to the user affects how much nicotine is absorbed. 
Whether or not an e-cigarette is used with nicotine, the vapour produced by these devices contains numerous other toxic chemicals. The inhalation of these chemicals has been shown to cause respiratory tract diseases such as bronchiolitis obliterans, also known as popcorn lung. This scarring in the lungs is caused by the chemical diacetyl and can lead to heart disease, stroke and other life-threatening conditions.
There is growing evidence that e-cigarettes and vaping are associated with negative cardiovascular consequences including thrombotic events, platelet activation, vascular endothelial dysfunction, and increased inflammation and cytokine release. However, the exact mechanisms remain unknown.
E-cigarette flavouring chemicals have been shown to cause a range of short-term effects in the lungs such as platelet activation, increased levels of soluble CD40 ligand and adhesion molecules, and alterations in blood pressure regulation and vascular tone. Similarly, exposure of e-cigarette aerosols to human bronchial epithelial cells and murine macrophages induced cell-specific toxicity ().
Nicotine is a highly addictive substance that can cause serious health problems. It has been found to damage the development of brain circuits that control attention and learning, and can increase the risk for addiction to other substances such as marijuana and cocaine. The long-term use of e-cigarettes can damage the lungs and increase your risk for lung infections such as COVID-19, which can lead to severe pneumonia. Talk to your doctor or nurse about safe, proven ways to quit smoking or vaping. Ask about free resources like online, texting and phone quit support services and apps. Get support from family and friends to help you stay smoke- or vape-free. And make sure your home and car are smoke- and vape-free. 電子煙