How it affects your body

How could exposure to noise have such devastating effects on human health as causing cardiovascular disease?

Key to solving this puzzle is recognizing that noise can create a form of chronic stress that keeps our bodies in a state of constant alert. Research published in 2006 by Wolfgang Babisch of Germany's Federal Environmental Agency in Berlin shows that even when you are asleep, your ears, brain and body continue to react to sounds, raising levels of stress hormones such as cortisol, adrenalin and noradrenalin.

This makes evolutionary sense, as all animals need to be alert to threats even when they are asleep, so they can wake up and flee if necessary, researchers say.

However, if these stress hormones are in constant circulation, they can cause long-term physiological changes that could be life-threatening. The end result can be anything from heart failure and strokes to high blood pressure and immune problems.

"All this is happening imperceptibly, and this is the key," says Deepak Prasher of University College London, who collaborated on the WHO study. "Even when you think you're used to noise, these physiological changes are still happening."

What's more, there are a wide range of sources of noise stress. Some are big and obvious, such as constant heavy traffic or aircraft taking off, while others are much more subtle and difficult to define as "pollution," yet can still cause intense anxiety and irritation. In the case of noisy neighbors, for example, stress might be triggered simply by knowing a neighbor is in, even if they are not being noisy at that point.

Noise can aggravate stress still further if it disturbs sleep, which can result in constant fatigue and outbursts of aggressiveness and irritability. People exposed to noise during their sleep have been shown to wake up more often and fidget more in their sleep -- both indicators of sleep disruption.

There's also mounting evidence that excessive noise disrupts learning and education. As far back as 1975, studies in New York showed that the reading skills of children in classrooms next to noisy railways lagged three to four months behind those of their peers in quieter classrooms.