Stand on a busy street corner in almost any city and listen carefully. Beneath the conversations, you will hear the roar of engines, the screech of brakes, the beeping of reversing lorries and perhaps the distant drilling of a building site. Most city dwellers have learned to ignore this background noise so completely that they barely notice it. Their bodies, however, have not learned the same trick.
Noise is usually measured in decibels, a scale that works in an unusual way. Because it is logarithmic, a small increase in the number represents a large increase in sound energy. An increase of about ten decibels is generally experienced as a doubling of loudness. This is why a street that seems only slightly louder than another may in fact be exposing its residents to far more sound.
The most obvious harm is to sleep. Even when traffic noise does not wake us, it can cause the heart to beat faster and the body to release stress hormones. Over months and years, disturbed sleep and repeated stress responses may contribute to serious health problems, including high blood pressure. For this reason, the World Health Organization considers environmental noise one of the most important environmental threats to health, alongside air pollution.
Noise also affects how we think. Research carried out in schools located near busy airports and roads has linked constant noise to difficulties with reading and concentration among children. It is not hard to understand why: a pupil who must strain to hear the teacher, or whose attention is broken every few minutes by a passing plane, has less mental energy left for learning.
Humans are not the only ones affected. Many birds rely on song to attract mates and defend territories, and in noisy cities their messages can be drowned out. Some species have responded by singing louder, at a higher pitch, or at quieter times of day, including the middle of the night. These adaptations show remarkable flexibility, but they may also come at a cost, since singing in this way can require extra energy.
The good news is that, unlike some forms of pollution, noise disappears the moment its source is removed. Cities around the world are experimenting with solutions. Special road surfaces can reduce the sound of tyres, lower speed limits make traffic quieter, and carefully designed buildings can shield homes from busy roads. Electric vehicles help too, especially at low speeds, when engine noise is the main source of sound. Parks and quiet courtyards offer residents places to escape. Silence, it seems, is becoming something that cities must actively plan for.