Both first-hand smoke and second-hand smoke are very harmful, but second-hand smoke is more harmful in comparison. The smoke emitted by a person while smoking can be categorized into mainstream smoke, exhaled smoke and sidestream smoke. Mainstream smoke, which is often referred to as first-hand smoke, refers to the smoke that smokers inhale directly into their bodies when tobacco products are burned, while second-hand smoke is a general term for exhaled smoke and sidestream smoke. The reasons are as follows: 1. When smokers inhale cigarettes, only 10% of the smoke and harmful substances produced by cigarettes are inhaled into the smoker’s lungs, and the remaining smoke is discharged into the air. In the smoke formed by secondhand smoke, the surrounding population inhales relatively more harmful substances, which is naturally more harmful. 2. In the case of insufficient combustion, the content of polycyclic aromatic hydrocarbons (PAHs) in secondhand smoke is more, and PAHs are carcinogens, so the harm of secondhand smoke is greater. 3. In addition, the duration of second-hand smoke is longer than that of first-hand smoke. While the smoke from first-hand smoke lasts for 5-10 minutes, the smoke from second-hand smoke in the air lasts for half an hour or even an hour. 4. Due to the hidden nature of second-hand smoke, second-hand smoke is inhaled by the human body more, which is more harmful to the human body. Long-term smoking and secondhand smoke can lead to lung cancer, coronary heart disease, stroke, breast cancer, sinus cancer and other diseases, for your health and the health of people around you, please actively quit smoking.