There are many tiny particles suspended in the atmosphere, the larger ones we can see or even perceive with the naked eye, such as sand, dust and black smoke, but the very small ones can only be observed under a microscope. PM is the abbreviation of particulate matter, which means tiny particles. PM10 refers to particles with a diameter between 2.5 and 10 microns, also known as respirable coarse particles, while PM2.5 is particles with a diameter of 2.5 microns or smaller. The value behind PM2.5 indicates the weight of fine particles per cubic meter in the air, generally expressed in micrograms, and the higher the value indicates the higher the concentration of fine particles, which means the heavier the pollution. PM2.5 in developed countries mainly comes from industrial emissions after burning coal (oil), automobile exhaust, and in developing countries a small portion of PM2.5 also comes from burning coal or firewood when cooking and heating at home. These toxic particles are directly inhaled into the alveoli and come into contact with the alveolar epithelium, inducing oxidative reactions or DNA damage mediated by these metals. Therefore, the lung is the organ most seriously affected by PM2.5. PM2.5 enters the alveoli and clears very slowly, and high concentrations of PM2.5 not only lead to inflammation in the lung itself but also trigger inflammatory reactions throughout the body. PM2.5 levels are positively correlated with lung cancer incidence and mortality, and studies show that for every 10μg/m3 increase in PM2.5, lung cancer mortality rises by 8-37%, and the data varies widely due to different research methods and regions. According to statistics, in South Africa, a developing country, 5.1% of respiratory tract tumors (including lung cancer) are caused by air pollution. Compared to PM2.5, the harm of smoking is far more serious, and 80% of lung cancers in men in developed countries in Europe and America are smoking-related. Last winter and this spring, the nationwide hazy weather has caused major public concern about environmental pollution, and in recent decades, China’s rapid economic development, but the damage to the environment is also quite serious. Therefore, the current environmental pressure and tumor prevention and control situation we are facing is very serious. To reduce the harm of PM2.5, we should first reduce industrial emissions and automobile exhaust, strengthen greening and reduce dust from urban construction sites, and the public should avoid outdoor activities in polluted weather with high PM2.5. Only with joint efforts can we return our blue sky and beautiful environment.