Whether the air quality is clean mainly depends on the content of harmful substances

in the air. Clean air usually has the following characteristics:

Low pollutant content: The content of pollutants in clean air is very low, especially 

fine particulate matter (such as PM2.5 and PM10), sulfur dioxide, nitrogen oxides, 

carbon monoxide, ozone and volatile organic compounds that are harmful to health.

Good visibility: Visibility is higher when there are fewer suspended particles in the air, 

which usually means the air is cleaner.

Odor-free: Clean air usually has no pungent or unpleasant odors.

Appropriate humidity and temperature: Although these factors do not directly

 determine whether the air is clean, appropriate humidity and temperature can 

make the air more comfortable.

Low microbial content: Clean air contains lower levels of viruses, bacteria and 

other microorganisms.

To assess the air quality of an area, the Air Quality Index (AQI) is often used,

 which takes into account the concentrations of multiple pollutants mentioned 

above to provide a quantitative air quality score. The lower the AQI value, 

the better the air quality and the closer it is to a "clean" state. Generally, an 

AQI value between 0 and 50 is considered "excellent" or "good" air quality.