Air pollution is caused by

Directions: Read the following passage and answer the question that follows.

Air pollution refers to the contamination of air, irrespective of indoors or outdoors.
Air pollution is the introduction of particulates, biological molecules, or other harmful materials into Earth’s atmosphere, causing diseases, death to humans, damage to other living organisms, food crops, or the natural or built environment.         
Air pollution can be further divided into two sections - Visible air pollution and invisible air pollution. The sustainment of all things living is due to a combination of gases that collectively form the atmosphere, the imbalance caused by the increase or decrease of the percentage of these gases can be harmful for survival.

Types of pollutants:
Direct pollutants/Primary pollutant: These are direct results of pollutant process. For example, Sulfur-dioxide emitted from factories.
Indirect pollutants/Secondary pollutants: These are caused by the intermingling and reactions of primary pollutants. Smog created by the interactions of several primary pollutants.