Why does Xi have smog every year?

The composition of smog is very complicated. Sulfur dioxide, nitrogen oxides and inhalable particles are the main components of smog. The first two are gaseous pollutants, and the last particulate matter is the chief culprit that aggravates smog weather pollution. They combine with fog and instantly turn the sky gray. In addition, there are more than 20 kinds of fine particles and toxic substances harmful to human body, including acid, alkali, salt, amine and phenol, as well as dust, pollen, mites, influenza virus, tuberculosis and pneumococcus. Its content is dozens of times that of ordinary atmospheric water droplets.

Haze has the greatest impact on the human respiratory system, and the diseases caused are mainly concentrated in respiratory diseases, cerebrovascular diseases, nasal inflammation and other diseases. However, because the composition of smog is so complex and diverse, the content is different in different regions and at different times, so it is difficult to say that smog itself is carcinogenic, but some of the substances are carcinogens. For example, the Comprehensive Scientific Assessment Report of Airborne Particulate Matter issued by the US Environmental Protection Agency in 2009 pointed out that there are enough scientific research results to prove that fine particulate matter in the air can adsorb a large number of carcinogens and genotoxic mutagenic substances, which bring negative effects that cannot be ignored to human health, including increased mortality, aggravation of chronic diseases, deterioration of respiratory and heart diseases, changes in lung function and structure, influences on reproductive capacity, and changes in human immune structure.