#清新十周年# 世界卫生组织指出，空气污染是世界范围内导致过早死亡的第四大风险因素。

在各种大小的颗粒中，PM2.5尤为有害，这是因为其颗粒小到足以到达较深的肺部气道。非常小的颗粒（比如小于1微米）可以进入肺泡—气道的末端以及与血液进行气体交换的部位。最小的颗粒（比如小于0.1微米）可以穿透肺壁并进入血流。

臭氧、二氧化氮和二氧化硫也会对健康造成负面影响，尤其会影响呼吸系统。

AIR POLLUTION IS THE FOURTH LEADING RISK FACTOR IN PREMATURE DEATH WORLDWIDE ACCORDING TO THE WORLD HEALTH ORGANIZATION.  

Among various sizes of particles, PM2.5 is especially harmful because the particles are small enough to reach the deeper airways of the lung.  Very small particles (e.g., < 1 micrometers) can enter the alveoli of the lung — the ends of the airways and site of gas exchange with the blood. The smallest particles (e.g., < 0.1 micrometers) may penetrate the walls of the lungs and enter the bloodstream.

Ozone, nitrogen dioxide, and sulfur dioxide also have negative health effects, particularly affecting the respiratory system.