This document discusses air pollution, including its definition, composition of air, structure of the atmosphere, classification of air pollutants, sources of common air pollutants, and effects of common air pollutants. It defines air pollution and explains that the atmosphere consists of five main layers - troposphere, stratosphere, mesosphere, thermosphere, and exosphere - each with varying temperature and density properties. It also classifies air pollutants according to their origin as primary or secondary pollutants and state of matter as gaseous or particulate, and lists some common sources and health effects of major air pollutants.