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 Nature Is Generally Quiet, with Noise Indicating Danger
The National Park Service (2017) noise maps (Figure 1) show that without human activity, environmental sound pressure levels are remarkably low, in the 20-30 dB(A) range. In nature, loud sounds are rare and may include thunderstorms, earth- quakes, volcanic eruptions, waterfalls, and certain animal sounds. In all animals, hearing is used to obtain information about the environment, to detect danger, for communication, and for entertainment.
In nature, loud noise often indicates danger and causes “fight or flight” responses. These involuntary physiological responses involve two primitive systems, the autonomic
nervous system and the neurohormonal or hypothalamic- pituitary axis. Noise causes almost instantaneous increases in blood pressure and pulse via the sympathetic nervous system (Babisch, 2014). It takes a little longer, but noise causes release of adrenocorticotropic hormone that, in turn, causes the release of steroid hormones from the adrenal gland and increases in serum epinephrine and norepinephrine levels. More recently, it has been shown that stress causes inflam- mation of the vascular lining (Tawakol et al., 2017) and that noise exposure specifically causes this inflammatory change (Radfar et al., 2018). These physiological effects and their clinical outcomes, best studied for transportation noise, are summarized in Figure 2.
Specific Noise Levels Affecting Human Health and Function
It has long been known that specific noise levels affect human health and function (Passchier-Vermeer and Vermeer, 2000).
These levels are based on expert systematic reviews of exten- sive published research. Despite their age, these specific noise levels remain valid. In some cases, such as studies of noise exposure causing hearing loss, older studies cannot be replicated because it is now unethical to endanger research subjects when the risk of harm is certain.
The National Institute for Occupational Safety and Health (NIOSH; 1998) recommended an 85 A-weighted decibel [dB(A)] equivalent continuous sound pressure level for 8 hours [LAeq(8); see Table 1 for a list of abbreviations] as the level to reduce the risk of hearing loss from occupational noise exposure in 1972 (NIOSH, 1998). The monograph by the Environmental Protection Agency (EPA; 1974), still authoritative and never rescinded, listed a 45 dB day-night- weighted sound pressure level (Ldn) as interfering with indoor
Figure 2. Proposed pathophysiological mechanisms of noise-induced cardiometabolic disease. Noise is stressful, causing chemical changes leading to vascular dysfunction, which, in turn, leads to disease. AngII, angiotensin II, a chemical causing vasoconstriction; HPA, hypothalamic-pituitary axis. The other factors affect the stress response and cause vascular dysfunction. For more details and abbreviations, see Münzel et al., 2018. From Münzel et al., 2018, with permission.
activities, a 55 dB Ldn as interfering with outdoor activities, a 70 dB(A) maximum time-weighted and A-weighted sound pressure level (LAmax) as an ambient-noise level interfering with speech comprehension in people with normal hearing, and a daily average of 70 dB A-weighted equivalent continu- ous sound pressure level for 24 hours [LAeq(24)] as the noise exposure level to prevent hearing loss. Moulder (1993) added 58 dB(A) LAmax as the ambient-noise level interfering with speech comprehension in people with hearing loss dining in restaurants. [For ease of use, this has been rounded up to 60 dB(A) LAmax in Table 1.] This is still much quieter than almost all restaurants in which noise levels were measured in Manhattan (Scott, 2018). In 1999, the World Health Organization (WHO) added 30 dB(A) LAeq(8) as the noise level required for uninterrupted sleep, noted that 45 dB LAmax will disrupt sleep, and recommended only 1 hour of exposure
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