The vast majority of studies indicate that platelets contain only COX-1, as would be expected of an anucleated cell type in which the COX-2 gene could neither be induced nor COX-2 protein expressed. However, as platelets are derived from nucleated megakaryocytes, there is the possibility that COX-2 induced in these precursor cells could carry over to the mature platelet [34, 35], or even that residual mRNA within formed platelets could be transcribed into protein [36]. This phenomenon has been demonstrated following coronary artery bypass surgery, presumably as a consequence of megakaryocytes being exposed to the systemic rise in cytokines that follows such an invasive procedure [37].
Irreversible(2002) Dual Audio 72
Sound frequency is perceived as pitch (i.e., how high or low a tone is). The frequency range sensed by the ear varies considerably among individuals. A young person with normal hearing can hear frequencies between approximately 20 Hz and 20,000 Hz. As a person ages, the highest frequency that they can detect tends to decrease.
The threshold of hearing is the quietest sound that can typically be heard by a young person with undamaged hearing. This varies somewhat among individuals but is typically in the micropascal range. The reference sound pressure is the standardized threshold of hearing and is defined as 20 micropascals (0.0002 microbars) at 1,000 Hz.
Sound intensity is heard as loudness, which can be perceived differently depending on the individual and their distance from the source and the characteristics of the surrounding space. As the distance from the sound source increases, the sound intensity decreases. The sound power coming from the source remains constant, but the spherical surface over which the power is spread increases--so the power is less intense. In other words, the sound power level of a source is independent of the environment. However, the sound pressure level at some distance, r, from the source depends on that distance and the sound-absorbing characteristics of the environment (OTM/Driscoll).
These contours represent the sound pressure level necessary at each frequency to produce the same loudness response in the average listener. The nonlinearity of the ear's response is represented by the changing contour shapes as the sound pressure level is increased (a phenomenon that is particularly noticeable at low frequencies). The lower, dashed curve indicates the threshold of hearing and represents the sound-pressure level necessary to trigger the sensation of hearing in the average listener. Among healthy individuals, the actual threshold may vary by as much as 10 decibels in either direction.
Conductive hearing loss results from any condition in the outer or middle ear that interferes with sound passing to the inner ear. Excessive wax in the auditory canal, a ruptured eardrum, and other conditions of the outer or middle ear can produce conductive hearing loss. Although work-related conductive hearing loss is not common, it can occur when an accident results in a head injury or penetration of the eardrum by a sharp object, or by any event that ruptures the eardrum or breaks the ossicular chain formed by the small bones in the middle ear (e.g., impulsive noise caused by explosions or firearms). Conductive hearing loss may be reversible through medical interventions such as hearing amplification (e.g. hearing aids) or surgical treatment. It is characterized by relatively uniformly reduced hearing across all frequencies in audiometric tests of the ear, with no reduction using hearing tests that transmit sound through bone conduction.
Sensorineural hearing loss tends to be a permanent condition that is often associated with irreversible damage to the inner ear. The normal aging process and excessive noise exposure are both notable causes of sensorineural hearing loss. Studies show that exposure to noise damages the sensory cilia that line the cochlea. Even moderate noise can cause twisting and swelling of the cilia and biochemical changes that reduce cilia sensitivity to mechanical motion, resulting in auditory fatigue. As the severity of the noise exposure increases or if the noise exposure is chronic, the cilia and supporting cells disintegrate and the associated nerve fibers eventually disappear. Occupational noise exposure is a significant cause of sensorineural hearing loss, which appears on sequential audiograms as declining sensitivity to sound, typically first at high frequencies (4,000 Hz), and then lower frequencies as damage continues. Often the audiogram of a person with sensorineural hearing loss will show a "Notch" between 3,000 Hz and 6,000 Hz, and most commonly at 4,000 Hz. This is a dip in the person's hearing level at 4,000 Hz and is an early indicator of sensorineural hearing loss due to noise. Results are the same for audiometric hearing tests and bone conduction testing. Sensorineural hearing loss can also result from other causes, such as viruses (e.g., mumps), congenital defects, and some medications. Modern hearing aids, though expensive, are able to adjust background sounds, changing signal-to-noise ratios, and support hearing and speech discrimination despite the diffuse nature of sensorineural hearing loss. The role of cochlear implants remains unclear.
Presbycusis is a gradual sensorineural hearing loss associated with aging. The onset and the degree of hearing loss can vary considerably and is related to genetics, other impacts such as an accumulation of diseases, medications, and the cumulative effect of noise in the modern environment. Presbycusis and noise induced hearing loss appear to be additive and both can contribute to hearing loss in older people. Both types of hearing loss affect the upper range of an audiogram. A sloping audiogram with tapering to the lowest levels at 8,000 Hz often indicates that the hearing loss is aged-related, but after years of exposure, noise-induced hearing loss can have the same pattern. As humans begin losing their hearing, they often first lose the ability to detect quiet sounds in the high frequency range. This progresses to difficulty understanding conversations in noisy environments, even when amplified by hearing aids.
The primary effects of workplace noise exposure include noise-induced temporary threshold shift, noise-induced permanent threshold shift, acoustic trauma, and tinnitus. A noise-induced temporary threshold shift is a short-term decrease in hearing sensitivity that displays as a downward shift in the audiogram output. It returns to the pre-exposed level in a matter of hours or days, assuming there is not continued exposure to excessive noise.
If noise exposure continues, the shift can become a noise-induced permanent threshold shift, which is a decrease in hearing sensitivity that is not expected to improve over time. A standard threshold shift (STS), as defined by OSHA, is a change in hearing thresholds of an average of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear when compared to a baseline audiogram. Employers can conduct a follow-up audiogram within 30 days to confirm whether the STS is permanent. Under 29 CFR 1910.95(g)(8), if workers experience an STS, employers are required to fit or refit the workers with hearing protectors, train them in the use of the hearing protectors, and require the workers to use them. Recording criteria for cases involving occupational hearing loss can be found in 29 CFR 1904.10; also see information and examples in Appendix I.
Tinnitus, or "ringing in the ears," is a common byproduct of overexposure to noise and can occur after long-term exposure to high sound levels, or sometimes from short-term exposure to very high sound levels, such as gunshots. Other physical and physiological conditions are also known to cause tinnitus. Regardless of the cause, this condition is actually a disturbance produced by the inner ear and interpreted by the brain as sound. Individuals with tinnitus describe it as a hum, buzz, roar, ring, or whistle, which can be short term or permanent. Noise-exposed workers may not associate tinnitus with noise exposure or be aware that tinnitus may be an early indicator of overexposure to noise. Hearing conservation training is often focused on noise-induced hearing loss (NIHL) and may not address tinnitus awareness and prevention adequately.
In some individuals, excessive noise exposure can contribute to other physical effects. These can include muscle tension and increased blood pressure (hypertension). Noise exposure can also cause a stress reaction, interfere with sleep, and cause fatigue.
The upper frequency of audibility of the human ear is approximately 15 to 20 kilohertz (kHz). This is not a set limit: some individuals may have higher or lower (usually lower) limits. The frequency limit normally declines with age.
Ototoxicity is the property of being toxic to the ear (oto-), specifically the cochlea or auditory nerve. Ototoxic agents (ototoxins) have adverse effects on organs or nerves involved in hearing or balance and may be physical (e.g., noise), biological, or chemical. Substances including pesticides, solvents, pharmaceuticals, asphyxiants, nitriles, and metal compounds that contain ototoxicants may expose workers via inhalation, ingestion, or skin absorption. Severity of health effects caused by ototoxicants vary depending on compound characteristics and properties; exposure route; exposure concentration, frequency, and duration; exposures to other hazards, and individual characteristics such as age.
In 1998 NIOSH published Criteria for a Recommended Standard: Occupational Noise Exposure (DHHS 98-126). That publication recommends a 3 dB rather than a 5 dB exchange rate. Although OSHA enforces its own standard, the Council for Accreditation in Occupational Hearing Conservation (CAOHC), which trains and certifies clinicians in managing audiometric programs, expects clinicians to understand how the different exchange rates influence estimates of noise dose and therefore affect attribution to hearing loss. 2ff7e9595c
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