Nighttime Noise & Blood Pressure

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." [TM]