Figuring the threshold

Demonstrating that chronic exposure to noise can produce illness or death is not simple. For example, traffic generates air pollution at the same time as noise, so how do you separate out the effects of the two?

The principle the WHO uses is the same one used to assess the effects of other environmental pollutants such as cigarette smoke or ozone. Find households with abnormally high exposure to noise, and compare death and disease rates in these households with those in quiet neighborhoods.

The WHO's researchers also studied groups of people with particular diseases, such as coronary heart disease, and tried to work out whether abnormally high exposure to noise increased their risk of dying.

Finally, they combined this information with data from "sound maps" that highlight which parts of European cities are noisiest. Knowing the proportion of the population exposed, it was then possible to work out roughly how many people will die or suffer disease as a result of noise exposure, and to estimate the number of years of health that exposure to noise will wipe out.

So how loud is loud noise? Sound exposure is generally measured in decibels (dB), which reflect the pressure on the eardrum. While the WHO has yet to finalize what levels of chronic exposure cause ailments such as heart disease and tinnitus, its "Night Noise Guidelines for Europe" can be used as a rough guide. These include threshold exposure levels that, if routinely breached at night, would threaten health.

The threshold for cardiovascular problems, for example, is chronic night-time exposure of 50 dB or above. For sleep disturbance, the threshold is lower, at 42 dB, and lower still for general annoyance, at just 35 dB. The threshold of noise judged to have a negative impact on children's learning is 55 dB during night or day.

That 55 dB is roughly equivalent to the din you would expect in a busy restaurant, while 75 dB is roughly equivalent to the noise at a busy intersection in a big city.