Noise pollution is not easily defined. Part of the difficulty lies in the fact that in some ways it is different from other forms of pollution.

Noise is transient; once the pollution stops, the environment is free of it. This is not the case for chemicals, sewage, and other pollutants introduced into the air, soil, or water.

Other forms of pollution can be measured, and scientists can estimate how much material can be introduced into the environment before harm is done. Though we can measure individual sounds that may actually damage human hearing, it is difficult to monitor cumulative exposure to noise or to determine just how much is too much.

The definition of noise itself is highly subjective. To some people the roar of an engine is satisfying or thrilling; to others it is an annoyance. Loud music may be enjoyable or a torment, depending on the listener and the circumstances.

Broadly speaking, any form of unwelcome sound is noise pollution, whether it is the roar of a jet plane overhead or the sound of a barking dog a block away.

One measure of pollution is the danger it poses to health. Noise causes stress, and stress is a leading cause of illness and suicide. Therefore any form of noise can be considered pollution if it causes annoyance, sleeplessness, fright, or any other stress reaction.

The actual loudness of a sound is only one component of the effect it has on human beings. Other factors that have to be considered are the time and place, the duration, the source of the sound, and whether the listener has any control over it. Most people would not be bothered by the sound of a 21-gun salute on a special occasion. On the other hand, the thump-thump of a neighbour's music at 2 a.m., even if barely audible, could be a major source of stress.