In autumn and winter in recent years, the central and eastern regions of China frequently encounter "smog" weather. We call particulate matter with a diameter less than or equal to 2.5 microns in the

In autumn and winter in recent years, the central and eastern regions of China frequently encounter "smog" weather. We call particulate matter with a diameter less than or equal to 2.5 microns in the air PM2. PM2.5 mainly harms the human respiratory system. When air enters the lungs, it passes through the nasal cavity, pharynx, larynx, trachea and bronchus. However, PM2.5 particles are tiny and are not easily blocked by organs such as the nose. It can directly enter bronchi and alveoli, affecting gas exchange in the lungs and causing asthma and other diseases. After entering the lungs, it can be quickly absorbed by capillaries outside alveoli and distributed throughout the body through blood circulation. Particles with a diameter less than or equal to 2.5 microns in the air can induce respiratory diseases.

Therefore, choose: B.