Tobacco smoke contains more than 7,000 chemicals, more than 70 of which are known carcinogens. Smoking will kill half of all long-term users. E-cigarettes do not contain tar or carbon monoxide, 2 of the most harmful elements in tobacco smoke. They may contain some chemicals also found in tobacco smoke, but at much lower levels.
Research has found that vaping poses only a fraction of the risks of smoking, and switching completely from smoking to vaping can offer substantial health benefits. You may have heard about an outbreak of serious lung disease in the US from vaping; the substances that are believed to have caused this are not allowed in e-cigarettes in the UK.
There is a strong agreement among health bodies including the Royal College of Physicians, British Medical Association, Cancer Research UK, UK Centre for Tobacco and Alcohol Studies and others, that e-cigarettes support smokers to quit cigarettes.