London - We are surrounded by the sounds of the machinery that make our lives comfortable and convenient. The constant thrum of traffic, the thunder of jet engines overhead.
But when we have to listen to these noises for too long or at the wrong time, they can inflict silent and stealthy damage. Increasing evidence shows this damage isn't just to our ears, but to our blood vessels and hearts.
Nor is this just a problem for people who live near busy roads or under flight paths. New research suggests noise pollution also causes harm in places such as hospitals.
Recently the world's experts gathered in Japan to discuss the latest findings about noise and health. Perhaps most eye-catching was the study that linked noise pollution to your waist size.
In a four-year project published last year, researchers from Karolinska University in Sweden found that the louder the traffic noise to which people in different parts of Stockholm were exposed, the greater the increase in their waist size - there was nearly a centimetre increase for every ten-decibel rise in the noise levels. This is like the difference between a whispered conversation and the noise level in an average house or personal office.
Last month, scientists from Karolinska University found an even more dramatic effect from plane noise. After tracking more than 5 000 people for ten years, they reported that the waistlines of those most exposed to plane noise increased on average by 6cm.
A similar study last year by the School of Public Health at Imperial College London found that being exposed to higher levels of aircraft noise around Heathrow raised the risk of admission to hospital for heart disease by 20 percent.
The effects of noise pollution are even felt by babies in the womb.
In another of the studies presented at the conference, researchers from Utrecht University in the Netherlands examined data from more than 68 000 births and found that for every six-decibel increase in traffic noise there was a drop of 15g to 23g in birth weight.
Low birth weight is linked to a range of long-term health problems, including high blood pressure, diabetes and heart disease.
Of course, the most common response to noise exposure is annoyance. But while this may be limited to making you feel angry or exhausted, a major review of research published in the Lancet last October showed it can also disturb sleep and increase the risk of hypertension and cardiovascular disease.
It also affects schoolchildren's academic performance.
“We are gathering more and more evidence that noise in the environment can have a direct effect on health,” says Professor Adrian Davis, one of the authors of the Lancet review and director of population health science for Public Health England.
Sound affects you even in sleep
What is unsettling is that noise pollution can affect you without you even consciously hearing it.
At night, heavy traffic is a major cause of insomnia, with all the knock-on effects of missing out on the restorative phase of sleep, such as depression, weight gain, raised blood sugar levels as well as daytime sleepiness.
The result can be an increased risk of type 2 diabetes and heart disease.
But the noise doesn't even have to keep you awake to have an impact. When the Imperial College researchers monitored sleepers' blood pressure, they found it went up in response to a quite low sound, such as a passing car - even though this might register only 50 decibels, and even though the sleeper didn't wake.
“It seems as if the subconscious never stops monitoring what is going on around you,” says Professor Paul Elliott, an epidemiologist (expert in the incidence of disease) at Imperial College.
Noise pollution makes you fat
So, what's going on here? How can the sound of traffic make you put on weight or raise you risk of heart disease? “The connection comes from stress,” says Professor Elliott.
It dates back to our Stone Age ancestors, he says, explaining that back then, being alert to new sounds around you could mean the difference between life and death.
“In the wild, sounds are often a sign that something dangerous might be about to happen,” he says.
“When they catch our attention because they are new or annoying, our bodies produce stress hormones, such as adrenalin and cortisol, to make us ready to fight or flee.”
These hormones raise energy and make blood more ready to clot.
That's good for a short-term emergency when you may be injured, but long term it's one of the factors pushing up heart attack risk - which might explain the link between noise pollution and heart disease.
One of the chemicals from this cocktail, cortisol, is a hormone that makes it more likely you will pack on the pounds around the middle. This is the so-called visceral fat that also pushes up heart attack risk.
Noise can also increase blood pressure, another risk factor. When we're stressed, the endothelium - the fine lining of the arteries - contracts, raising blood pressure.
The effect of noise stress isn't limited to making us physically ill, it can also make it harder to concentrate, especially for children.
The background noise in the classroom shouldn't be more than 35 decibels, but that can be doubled by cars passing by or planes overhead.
Research shows heavy traffic or being under a flight path is linked with learning more slowly. The children pay less attention or become more annoyed.
Hospital din can harm patients
Around 20 percent of people in the EU have to put up with traffic noise greater than 65 decibels in the day, while nearly a third have noise measurements greater than 55 decibels outside their houses at night - the level that can trigger problems, says the World Health Organisation.
A level of 45 decibels and below is considered ideal.
The problem isn't limited to towns and cities. Aircraft fly over the countryside and agricultural machinery can be intrusive.
And as Anna Hansell, a doctor at Public Health England, explains: “If aircraft noise has an effect on the heart, then so might other sorts of noise that people find stressful - such as building works or constant loud music from neighbours.”
One of the big shocks of the Lancet report was the harm caused by noise in hospitals.
The review found that noise levels in hospitals have increased and are typically 15 to 20 decibels higher than the recommended 40 decibels - that's the difference between the hum of a fridge and someone practising the piano.
Intensive care units, which look after some of the most vulnerable patients whose systems are already stressed, can be full of “irregular” sounds such as alarms sounding, phone ringtones and pagers going off, staff chatting and doors banging open and shut.
These are just the sort of sounds that can trigger the damaging stress “startle” response to noise.
The result is patients take longer to heal, need higher doses of painkillers and are likely to be readmitted to hospital, according to the Lancet review.
The review also found that staff, too, are affected by this constant noise, making them tired and suffer headaches. As Professor Davis points out: “Many of these noise sources can be dealt with simply with sound- absorbing ceilings and cutting the volume of ring tones - that would lower the rate of cardiovascular disease.”
Crying babies aren't a risk
The decibel levels linked to health problems such as cardiovascular disease don't seem too high - just 50 to 60 decibels for traffic noise.
That's not very different from having a conversation (60 decibels), let alone a crying baby (100 decibels) or a rock concert (120 and up). So, why is traffic noise harmful and a screaming baby not? “There are several reasons why you can't compare them directly,” says Dr Hansell.
“The rating for aircraft noise, for instance, is usually the average over 24 hours. So, you would have quiet moments and times when the level might reach 90 decibels. This sort of intermittent noise is much more annoying than a steady background noise.”
The sound of a screaming baby may be intermittent and stressful, but even the most distressed baby doesn't keep going 24/7 for four or more years - the length of time of the studies that found health damage.
The key difference is that plane noise counts as “nuisance noise” - you don't seek it out or enjoy it.
“Nuisance noise is harder to ignore and its effect is in addition to background noise,” says Dr Hansell.
And decibels aren't the only feature that decides how a sound is going to affect us. Some types of noise are more stressful than others, possibly because they are less regular or are high or low-pitched - this varies with the individual.
There is a significant subjective element to the way people respond to sound, says Geoff Leventhall, an independent noise consultant, author and expert on the noise from wind turbines. “The actual level of sound as objectively measured in decibels makes up only about 30 percent of how it is perceived. People's attitude can play a big part.
“Something that might be stressful viewed in one way, such as a thumping beat from a nearby club at 2am when you've got an early start, can be enjoyable if you are out for the weekend.”
Researchers such as Professor Elliott are well aware it's hard to be sure it's noise alone causing a problem.
“We found a connection between noise and heart disease in 3.6 million people living around Heathrow,” he says.
'But that population includes a large number of people from South-East Asia who have a higher risk of heart disease anyway. That could have contributed to the result.'
However, with the growing body of evidence that noise is harmful, Professor Davis insists action is needed to tackle it.
“We need programmes to cut levels of stressful noise and enforce standards,” he says.
“The results could include improved learning in schools, better sleep for millions, a drop in heart disease, and hospitals that were far more pleasant.”
How to protect yourself
There are things we can all do to reduce the effects of noise, says Dr Hansell
Earplugs: “I use them in the cinema, at rock concerts and for sleeping, especially in noisy hotels,” says Dr Hansell.
Complain: “If we don't all let people know when we're being affected, nothing will change. So if a cinema film is too loud, ask the staff to turn it down. I've even written in about the announcements on the Northern line Tube being too loud.”
Relax: “If you can't change it, try not to become annoyed or stressed by the noise, as that will increase your blood pressure!”
In decibels, how much noise are you exposed to?
10: Falling leaves
20: A whisper in a quiet library
30: Quiet conversation
40: The noise level in an average house or the hum of a fridge
50: Normal office noise or rainfall
60-70: Piano practice or the noise of a normal conversation at 3ft apart
70: Level at which most people play their radio
80: Noisy office or an alarm clock, loud singing, average sound of traffic
100: Bass drum, passing truck or a car horn
110: Screaming child
120: Rock concert
180: Plane taking off. - Daily Mail