**Core Concept**
Air pollution is a significant environmental health hazard that affects millions of people worldwide. It is primarily caused by the emission of pollutants from industrial activities, transportation, and other human activities. Air pollution can be monitored using various indicators, which help in assessing its impact on human health and the environment.

**Why the Correct Answer is Right**
Air quality is typically monitored using several indicators, including particulate matter (PM), nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), and carbon monoxide (CO). These pollutants are emitted from various sources, such as power plants, vehicles, and industrial processes. The levels of these pollutants are measured using air quality monitors, which provide valuable information on the quality of the air in a particular region.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that air pollution is a major risk factor for respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and asthma. Exposure to poor air quality can also exacerbate cardiovascular disease and increase the risk of lung cancer.

**Correct Answer: D.**