Nighttime Noise and Blood Pressure

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By Sora Song

The din of airplanes landing, motorcycles roaring or a bedmate snoring can make for patchy sleep and strained nerves. But even when you manage to slumber through a rackety night, your body still registers the noise by raising blood pressure, according to a small new study.

Volunteers living near four major European airports with night flights — in Athens, Milan, Stockholm and London — took part in the study published this week in the European Heart Journal. Study participants were outfitted with ambulatory blood pressure monitors, which were programmed to take readings at 15-min. intervals throughout the night. The volunteers' bedrooms were also equipped with an MP3 recorder and a noise-meter, which recorded all ambient noise, its timing and its volume. Researchers considered a "noise event" to have occurred if any sound, from road traffic, aircraft or a partner's snoring, exceeded 35 decibels (dB) — not a very high threshold, considering that a quiet whisper from 3 ft. away measures about 30 dB and the hum of a refrigerator logs about 40 dB. Noise levels recorded in volunteers' bedrooms fluctuated between about 30 dB or 40 dB to about 80 dB or 90 dB, roughly the volume of a food blender.

Researchers found that people's blood pressure rose reliably in response to a noise event, even when it wasn't loud enough to wake them. The response was consistent across all sources of sound, whether from the runway or the other side of the bed. Airplane noise, for example, caused an average 6.2 mmHg increase in systolic blood pressure (the pressure of blood in the artery when the heart contracts — i.e., the larger, top number) and a 7.4 mmHg increase in diastolic pressure (when the heart relaxes between beats). A snoring partner and road traffic had similar impact. And the effect was dose dependent: The louder the noise, the higher the jump in blood pressure. For every additional 5 dB in volume of aircraft noise, systolic and diastolic blood pressure rose another 0.65 mmHg each. "It's a small increase in the blood pressure, obviously, but it is significant," says co-author Dr. Lars Jarup, who specializes in environmental and occupational medicine at Imperial College London.

The new report was a corollary of a much larger study conducted by the same research group, examining the relationship between hypertension and nighttime exposure to noise near airports or daily exposure to road traffic noise. That study, which appeared online in the journal Environmental Health Perspectives last December, involved 4,861 participants, aged 45 to 70, who had lived at least five years near a major European airport. Researchers found that nighttime airport noise was linked to a significant increase in risk for hypertension; every 10 dB increase in exposure led to a corresponding 14% rise in high blood pressure risk. In addition, the study found, daily exposure to road traffic noise (at average levels above 65 dB) led to a more than 50% increased risk of hypertension — but, curiously, only among men, not women.

The new study, which included 140 middle-aged volunteers with normal blood pressure, was designed to take a closer look at the link between noise and hypertension risk — a relationship that researchers still don't fully understand. "It seems plausible that if you have a lot of these transient [blood pressure] changes during the night — if you live around the airport for many years, for example — that in the end you might get some long-term effects on your blood pressure," says Jarup, "but we don't really know." Why the body responds to nighttime noise is also somewhat mysterious. While the research in humans is new, previous lab experiments in animals have shown that they register blood pressure blips in response to noise, even during sleep or sedation. "That was the same here," says Jarup of the current study, suggesting that the human body's response may be similarly automatic. "It's not that you're annoyed and that's why your blood pressure goes up. It's something that's in the brain, and we really don't know what the mechanism is.... It could well be some kind of stress reaction, which is recorded subconsciously."

Hypertension — defined as having systolic blood pressure of 140 mmHg or diastolic pressure 90 mmHg, or higher — is known to increase risk of stroke and heart attack. Risk factors like nighttime noise are perhaps less decisive than other changeable variables like weight, exercise and alcohol intake. But, in general, says Jarup, "I would say that the main point is to reduce your risk factors — the fewer the better."

http://www.time.com/time/printout/0,8816,1713178,00.html

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