Business aircraft crews and passengers are generally aware of the danger of prolonged exposure to noise in terms of hearing loss. Now there is a growing body of evidence that prolonged exposure to a combination of high-intensity and low-frequency noise may pose far more serious health threats. They include thickening of cardiovascular structures–carotid arteries, pericardium and cardiac valves; lung fibrosis; and neurological problems, including late-onset epilepsy and disabling balance disorders.
The noise in question is best described as that over the 110-decibel level (high intensity) and below 100 hertz (low frequency). Typical sources of such noise include dance clubs, powerful car audio systems, motorcycles and passenger cabins on twin-turboprop airplanes. In particular, it is the low-frequency noise (LFN) that researchers have singled out as primarily responsible for what they are calling “vibroacoustic disease” (VAD). It may even be of such low frequency as to be inaudible to the human ear.
Scientists have been examining the effects of LFN on the human body since the 1980s, and in more recent years, with the development of new and improved medical diagnostic tools, they are more convinced that it poses a serious neurophysiological health hazard far beyond hearing loss.
It was in 1980 that researchers in Portugal discovered that 10 percent of the aeronautical technicians employed at an aircraft manufacturing, maintenance and repair facility were diagnosed with late-onset epilepsy. The diagnosis was in sharp contrast to the 0.2-percent rate in the Portuguese general population. In later studies, a group of 60 LFN-exposed workers with an average age of 42 years exhibited magnetic resonance imaging (MRI) abnormalities normally seen in a much older population segment. In a group of 60 workers with an average age of 43 years, 30 exhibited cognitive deterioration. And finally, in a group of 140 LFN-exposed workers–average age 42 years–57 percent suffered vertigo or dizziness compared with the 2 percent that would be expected in the general population.
Dr. Nuno A. A. Castelo Branco of Portugal is one of the world’s foremost researchers into the extra-aural symptoms found in workers exposed to high-level LFN. He and his colleagues at the Occupational Medicine Research Center in Lisbon, Portugal–Doctors Augusto J. F. Martinho Pimenta, Jose M. Reis Ferreira and Mariana Alves-Pereira– have authored numerous studies that should be setting off alarms in the aviation community.
A paper delivered in late 2002 and authored by all four doctors– Monitoring Vibroacoustic Disease– noted the identification of certain symptoms of VAD among military and commercial aircrews. LFN exposure, the four scientists concluded, induces mood swings, rage, depression and aggressiveness; extracellular matrix changes (thickening of blood vessels); abnormal cardiovascular structures; the appearance of pulmonary fibrosis that has been replicated in LFN-exposed rodents under lab conditions; and lung tumors, also replicated in LFN-exposed rodents.
The appearance of symptoms depends on the amount of exposure to LFN, but they create a neuro-physiological picture as follows:
• Stage 1 (mild), one to four years of exposure–Slight mood swings, indigestion and heartburn, mouth/
throat infections and bronchitis.
• Stage II–(moderate), four to 10 years of exposure–Chest pain, definite mood swings, back pain, fatigue, fungal, viral and parasitic skin infections, inflammation of stomach lining, blood in urine, conjunctivitis and allergies.
• Stage III (severe), more than 10 years of exposure–Psychiatric disturbances, hemorrhages of the nasal, digestive and conjunctive mucosa (small nose bleeds), varicose veins, hemorrhoids, duodenal ulcers, spastic colitis, decrease in visual acuity, headaches, severe joint pain, intense muscular pain, neurological disturbances, epilepsy, balance disorders, suicide, stroke and heart attack.
An example of LFN comes in the form of special audio speakers, some of which might draw up to 5,000 watts of electricity, creating a bone-rattling wave of noise. But it is not the noise one hears so much as the subsonic vibration that one can, quite literally, feel. According to Dr. Robert Fifer, director of audiology and speech language pathology at the University of Miami’s Mailman Center, “When the human body is exposed to high-intensity sound, it releases stress hormones, starting with adrenaline. The blood pressure shoots up, respiration becomes more rapid, the heart rate increases and the body in general is working much harder than it usually would.”
It is part of the body’s “fight or flight” response and it can be triggered by sound you can’t even hear– large-pressure amplitude and low-frequency noise. He noted that long-term exposure to stress hormones produced by LFN exposure can increase an individual’s chances of a heart attack, ulcers, fatigue, back pain, joint pain and headaches. Low frequency sound can also affect the peripheral balance system in the inner ear, said Fifer, resulting in a difference in the signal being passed to the brain by the inner ear and the eyes. The result can be nausea and dizziness.
Shelby Carr, president of Quiet Flight, a Dallas-based company that produces active noise-cancellation systems for aircraft, agrees. He pointed out that people who complain of fatigue similar to jet lag after a flight of only two or three hours are actually feeling the cumulative effect of two or three hours of exposure to LFN.
Most employers offer no solution to LFN exposure. In compliance with OSHA and other government regulations, they will require hearing protection for workers exposed to high noise levels. But few are even aware of the long-term effects of exposure to LFN.
One of the few workplaces with a solution to LFN is the business aircraft cabin, and even there, protection from LFN is relatively rare. It comes in the form of the active noise cancellation (ANC) system.
Carr explains that thermal/acoustic sound absorbing or damping materials and systems do little or nothing to reduce the impact of LFN. An active noise-cancellation system, however, identifies and destroys the frequency and amplitude of unwanted noise in the lower range between about 80 and 360 hertz.
Microphones placed strategically throughout the cabin pick up noise produced in a designated range. This noise, in the form of an analog signal, is sent to a control box, where it is identified in terms of phase, amplitude and frequency. A computer then recreates the identical amplitude and frequency but shifts the phase 180 degrees, converts it back to an analog signal and sends it to a series of speakers. Because the noise from the speakers is 180 degrees out of phase with the original noise, the original noise is effectively destroyed.
A more recent variation is the so-called “shaker can” system, more accurately described as “active tuned vibration absorbers” (ATVA). The process is similar to the speaker-based system, but the out-of-phase signal is sent back to the ATVA units, which produces an identical but out-of-phase vibration to destroy the noise.
According to Mike Turner, director of marketing at Elliott Aviation in Moline, Ill., an ATVA system is particularly effective at eliminating prop-tip noise.
The only other solution is a program of constant medical monitoring, using psychometric and performance tests, neurophysiology examinations and other diagnostic technology.
In Vibroacoustic Disease: The Need for A New Attitude Towards Noise, Alves-Pereira and Castelo Branco pointed out that “among LF noise-exposed workers, VAD can be successfully prevented from reaching the severe stages if a yearly echocardiogram is administered to the noise-exposed workforce.” This allows the employer to remove the workers exhibiting VAD symptoms from the noise environment. They also recommend screening of applications for “noisy” jobs for pre-existing LFN exposure or the existence of VAD.
In their final commentary in Monitoring Vibroacoustic Disease, they noted that research is not conclusive and that the difficulty in finding an adequate control population that has not been exposed to LFN has resulted in some false controls. However, they concluded, “The lack of recognition of LFN as an agent of disease and the continued erroneous assumption that noise affects only the ear impedes objective and conclusive scientific results, a status-quo situation that is “tolerated by many and convenient for a few.”