These terms refer to all polluting emissions with the potential to cause significant health and environmental problems with prolonged or concentrated exposure. In practice, the term can be used to refer to substances in the air, whether emitted by human activity or naturally-occurring, which can affect either human health or the environment.

These terms don't necessarily refer to specific substances. Instead they may refer to substances that are hazardous in high concentrations or specific forms. For example, carbon dioxide is a necessary component of the Earth's atmosphere. It only poses a hazard if too much or too little is present. Ozone is another normal component of the atmosphere, but it is only considered hazardous when it occurs in high concentrations close to the ground.

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