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Noise health effects, the collection of health consequences of elevated sound levels, constitute one of the most widespread public health threats in industrialized countries. Roadway noise is the main source of environmental noise exposure. Aerodynamic noise created at freeway speeds is particularly intense. Current conditions expose tens of millions of people to sound levels capable of causing hearing loss, but also are known to induce tinnitus, hypertension, vasoconstriction and other cardiovascular impacts. Vasoconstriction can also be contributory to erectile dysfunction. Beyond these effects, elevated noise levels create stress, increase workplace accident rates, and stimulate aggression and other anti-social behaviors. The most important sources of sound levels that create the above effects are motor vehicle and aircraft noise, with industrial worker noise exposure also being notable. Secondary exposures may arise from loud audio media especially if practiced as a lifestyle such as prolonged portable audio player use.
The pinna (visible portion of the human ear) combined with the middle ear amplifies sound levels by a factor of 20 when sound reaches the inner ear. Approximately ten percent of the population in industrialized societies have significant hearing loss, and millions more are steadily progressing to that outcome. The major source of hearing loss is exposure to elevated sound levels. Once it was thought that only extremely high sound levels create hearing loss; however, more careful investigations showed that cumulative exposure to relatively moderate levels, such as 70 dB(A), can lead to the irreversible loss of hearing. Another myth of noise effects is the overstated role of presbycusis, or loss of hearing associated with aging. It has been demonstrated that the most important factor of hearing degradation is not aging alone, but rather the cumulative long-term exposure to environmental and occupational noise that create the harm. In the Rosenhall study, age cohort populations were tracked, with the result that noise-exposed persons had much greater hearing loss than their age cohorts who were relatively unexposed to noise. In fact, it has been shown that people in non-industrialized countries do not experience the same progressive hearing loss. Due to loud music and a generally noisy environment, young people in the United States have a rate of impaired hearing 2.5 times greater than their parents and grandparents.
The mechanism of hearing loss arises from trauma to stereocilia of the cochlea, the principal fluid filled structure of the inner ear. The pinna (visible portion of the ear) combined with the middle ear amplifies sound pressure levels by a factor of twenty, so that extremely high sound pressure levels arrive in the cochlea, even from moderate atmospheric sound stimuli. The cilial damage is known to be cumulative and can be irreversible. The most recent research indicates that high noise levels create elevated levels of reactive oxygen species in the inner ear, which interfere with the regenerative process for cochlear cilia repair. This research shows why high noise levels have differing effects over a given population, and lead to a possible preventative strategy of adequate antioxidant intake.
In 1972 the U.S. EPA told Congress that at least 34 million people were exposed to sound levels on a daily basis that are likely to lead to significant hearing loss. The worldwide implication for industrialized countries would place this exposed population in the hundreds of millions.
Important cardiovascular consequences follow from elevated sound levels, principally because the elevated adrenaline levels trigger a narrowing of the blood vessels (vasoconstriction). Sound levels, again of fairly typical roadway noise exposure, are known to constrict arterial blood flow and lead to elevated blood pressure; in this case, it appears that a certain fraction of the population is more susceptible to vasoconstriction. (Independently, high noise levels are known to produce medical stress reactions, another risk associated with cardiovascular disease.) Noise-induced medical stress is significant for two reasons. First, it often results from prolonged exposure for 8 to 16 hours per day, leading to elevated blood pressure for much of the day. Second, unlike emotional stress, it has a very clear effect on blood pressure, whereas this is not always true of emotional stress. These effects may be compounded by other environmental vasoconstrictors such as over-illumination or light pollution.
Other proven effects of high noise levels are increased frequency of headaches, fatigue, stomach ulcers and headrush. The same U.S. EPA study establishes links between high noise levels and fetal development. This body of research suggests a correlation between low-birth-weight babies (using the World Health Organization definition of 5.5 pounds) and high sound levels, and also correlations in abnormally high rates of birth defects, where expectant mothers are exposed to elevated sound levels, such as typical airport environs. Specific birth abnormalities included harelip, cleft palate, and defects in the spine. According to Lester W. Sontag of The Fels Research Institute (as presented in the same EPA study): “There is ample evidence that environment has a role in shaping the physique, behavior and function of animals, including man, from conception and not merely from birth. The fetus is capable of perceiving sounds and responding to them by motor activity and cardiac rate change." Noise exposure is deemed to be particularly pernicious when it occurs between 15 and 60 days after conception, when major internal organs and the central nervous system are formed. Later developmental effects occur as vasoconstriction in the mother reduces blood flow and hence oxygen and nutrition to the fetus. Low birth weights and noise were also associated with lower levels of certain hormones in the mother, these hormones being thought to affect fetal growth and to be a good indicator of protein production. The difference between the hormone levels of pregnant mothers in noisy versus quiet areas increased as birth approached.
Earlier researchers often grouped the non-physiological impacts of noise as “annoyance”. As research unfolded, it became clear that there are a host of psychological and behavioral effects result from elevated sound levels, including: sleep disturbance, reading development in children, stress, mental health (including disengagement and increases in aggressive behavior). These effects are statistical but measurable changes in a population of individuals compared to a control group of persons in a quiet environment. Obviously, other negative environmental factors are likely to be present in high noise areas such as higher air pollution levels and possibly poverty-induced nutrition deficits; however, the overwhelming weight of dozens of independent studies identify noise pollution to be responsible for significant increases in the psychological effects studied above.
Measurements of noise annoyance typically rely on weighting filters, which consider sound frequencies annoying only to the degree that they are audible, on average, to a human ear at a particular decibel volume. Common methods include the older dBA weighting filter used widely in the U.S., which underestimates the impact of frequencies around 6000 Hz and at very low frequencies, and the newer ITU-R 468 noise weighting filter, which is used more widely. It is important to note that these filters do not necessarily reflect the occurrence of adverse health effects from noise, which may not depend on its audibility to the ear, nor do they take into account the propensity of low-frequency noises to penetrate into buildings or to carry over long distances.
Annoyance effects of noise vary greatly by demographics and by the perception of how useful the entity is that originates the noise. For example, aircraft mechanics who live near an airport are less likely to be complainants, since their livelihood is based upon airport operations. Annoyance is also influenced by whether the noise source is visible, whether it has pure tones or hammer effects and whether the recipient believes the noise can be controlled. In any case, the onset of noise complaints can be as low as 40 dB(A). Whether the noise occurs at night is another critical variable for annoyance phenomena. Most commonly, concerted actions of the public appear at approximately 65dBA regarding roadway, aircraft or industrial noise in the environment. Closely associated with annoyance are sleep disturbance and speech interference phenomena. The threshold for sleep interference is 45 dB(A) or lower. The onset of speech interference is about 63dBA, or roughly the sound level of speech in a normal tone between two people separated by one meter.
When young children are exposed to speech interference levels of noise on a regular basis, there is a likelihood of developing speech or reading difficulties, because the auditory processing functions are compromised. In particular the writing learning impairment known as dysgraphia is commonly associated with environmental stressors in the classroom.
Effects of environmental noise upon aggression, mental health, anxiety, withdrawal and other psychological factors have been studied by numerous researchers. For example J.M. Field examines a variety of these outcomes and finds significant influence of moderate-level environmental noise upon human behavior and mood. There are also strong associative impacts when other stressors are present such as over-illumination and presence of certain drugs.
Environmental noise regulations usually specify a maximum outdoor level of 60 to 65 dB(A), while occupational safety organisations recommend that the maximum exposure to noise is 40 hours per week at 85 to 90 dB(A). For every additional 3 dB(A), the maximum exposure time is reduced by a factor 2, e.g. 20 hours per week at 88 dB(A). Sometimes, a factor of two per additional 5 dB(A) is used. However, these occupational regulations are acknowledged by the health literature as inadequate to protect against hearing loss and other health effects discussed above.