Noise exposure levels in stock car auto racing.
Noise-induced hearing loss associated with the workplace has been well described. Far less is known, however, about the risks to hearing from recreational sources of noise. We investigated the popular sport of stock car racing as a potentially significant source of noise exposure, and we conducted a sound-level survey at a National Association for Stock Car Auto Racing (NASCAR) event. Noise levels measured during the race ranged from 96.5 to 104 dB(A) at 46 meters (~150 feet) from the track and 99 to 109 dB(A) at 6 meters (~20 feet) from the track. The peak sound pressure level at 6 meters was 109 dB (A). Although significantly less than that associated with an immediate permanent threshold shift, such an exposure could cause a temporary threshold shift. Alhough hearing protection is recommended, particularly for track employees with longer periods of exposure, racing fans with only occasional exposure to such noise levels are unlikely to develop a permanent noise-induced hearing loss.
Noise is a well-recognized and significant cause of hearing loss in our society. Alhough not reversible, noise-induced hearing loss (NIHL) is preventable through hearing-conservation programs and a better awareness of the various occupational and environmental sources of potentially damaging noise.
NIHL can be temporary or permanent. A temporary threshold shift (TTS) refers to a sensorineural hearing loss following exposure to a loud noise that recovers completely within 24 hours. Repeated exposures that result in a TTS may eventually lead to a permanent threshold shift (PTS). Permanent hearing loss can also result from acoustic trauma, or a single exposure with enough peak intensity to cause immediate physical injury to the inner ear, such as separation of the organ of Corti from the basilar membrane. This type of immediate and permanent hearing loss is generally associated with sound pressure levels greater than 140 dB, as might be generated by an explosive or a high-powered rifle discharging close to the ear.
Hearing loss caused by noise in the 85- to 140-dB range is thought to result from a metabolic rather than a mechanical change in the cochlea. During a TTS, the stereocilia of the outer hair cells become less stiff and disarrayed, responding poorly to stimulation. With time, these stereocilia will recover and function normally again. A PTS, however, is characterized by a loss of stereocilia, leading to loss of the outer hair cells themselves and replacement by scar. With continued exposure, loss of inner hair cells and supporting cells, as well as secondary neural degeneration, can occur.
The magnitude of a noise-induced threshold shift depends on certain acoustic parameters characteristic of the noise, including intensity, duration of exposure, and the frequency content. Clearly, the greater the intensity, the larger the threshold shift. Intensity is commonly expressed in decibels (dB), a logarithmic expression of intensity relative to the standard threshold of hearing. In addition, the intensity of a sound is inversely related to the square of the distance from that sound. In cases of NIHL, a longer duration of exposure results in a greater TTS and increased risk of a PTS. Interrupted exposures are associated with less risk of threshold shift than continuous exposure with the same total duration.
Noise generally consists of a broad spectrum of frequencies, although high-frequency sounds tend to be more damaging that lower-frequency sounds. For this reason, standard contour filters such as the "A-weighted scale" have been developed to filter out lower frequencies and give greater weight to those frequencies most hazardous to human hearing. The A-weighted scale, expressed in dB(A), is an accepted standard in the measurement of environmental noise. In addition, the average level for noise of fluctuating intensity is often reported as a Leq (level equivalent) level, referring to the level of a steady-state sound integrated over a period of time.
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The Occupational Safety and Health Administration (OSHA) has established a time-weighted average of maximum acceptable exposure for workers in noisy environments. The standard is a maximum exposure of 90 dB(A) for an 8-hour period with a 5-dB trading ratio. This means that the allowable time of exposure is cut in half for each 5-dB increase in noise level.
While federal regulations do not address NIHL outside the workplace, a number of recreational sources of noise have been previously described, including amplified music, (1) hunting and target shooting, (2) snowmobiles, (3) and the noise levels associated with aerobics classes in the health club setting. (4) Reported peak sound levels from rifles and shotguns range from 132 to 172 dB(A) and have been associated with PTSs. (2) A meta-analysis of previously published sound level data from rock concerts revealed a mean sound pressure level of 103.4 dB(A) and, while the risk of permanent hearing loss is uncertain, TTSs have been documented. (5) Lee et al demonstrated TTSs of up to 30 dB in volunteers who listened to portable headphone radios at levels near the maximum output. (6)
The purpose of our study was to characterize the level of noise experienced by fans attending professional stock car races, one of the world's fastest growing spectator sports. The best known stock car racing series in the United States, the National Association for Stock Car Auto Racing (NASCAR) Nextel Cup Series, attracted millions of spectators to its 36 races during the 2003-2004 season.
Materials and methods
Our data were collected at a major NASCAR event at the Lowe's Motor Speedway in Concord, North Carolina, on May 30, 1999 (figure 1). NASCAR officials were notified in writing prior to the study, and a written acknowledgment was received.
The race lasted 3 hours and 58 minutes. During this time, sound pressure levels were measured at 6 meters (~20 feet; approximate distance to the front row of seating) and 46 meters (~150 feet) from the track, at 30-minute intervals, using a portable precision sound-level meter (Model 2203; Bruel & Kjaer; Naerum, Denmark) with an A-weighted scale. In addition, an external filter was used to measure sound pressure levels at four specific frequencies (125, 500, 2,000, and 8,000 Hz) at each 30-minute interval, at 46 meters from the track.
At 46 meters, sound pressure levels ranged from 96.5 to 104 dB(A); the average measured level was 100.7 Leq dB(A) (figure 2). At 6 meters, the sound pressure levels ranged from 99 to 109 dB(A); the average measured level was 106.2 Leq dB(A) (figure 2). These interval sound level measurements and the associated averages are a representation of noise levels at the above distances, although actual noise levels at other points in time might have varied, including levels less than 90 db(A).
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The sound levels measured at four specific frequencies at each 30-minute interval at 46 meters (figure 3) were as follows: At 125 Hz, sound pressure levels ranged from 85 to 101 dB, with an average of 96.0 Leq dB. At 500 Hz, sound pressure levels ranged from 95 to 105 dB, with an average of 100.6 Leq dB. At 2,000 Hz, sound pressure levels ranged from 75 to 94 dB, with an average of 89.0 Leq dB. At 8,000 Hz, sound pressure levels ranged from 66 to 82.5 dB, with an average of 75.9 Leq dB.
The goal of this project was to characterize the intensity, frequency content, and duration of exposure to noise at a typical stock car race. Our results indicate an average noise level in the first row of 106.2 Leq dB(A) with a peak intensity of 109 dB(A). This peak sound pressure level is significantly less than the 140 dB level generally associated with the potential for an immediate PTS. However, those exposed to such noise levels should be considered at risk for a TTS. By OSHA standards, this level of exposure over a 4-hour period (the approximate length of the race) exceeds the 90 dB(A) time-weighted average by fourfold.
The risk of permanent hearing loss from recreational noise clearly depends on each individual's pattern of exposure. While the risk for most fans attending a single stock car race is low, the overall cumulative exposure for drivers, crews, and track employees may be far greater. Lindemann and Brusis reported the level of noise both inside and outside race cars for professional racing drivers. (7) They measured peak noise levels of 130 dB(A) outside and 125 dB(A) inside the cars. Their audiologic testing of drivers and pit employees suggested that those working in pit areas or at other motor test sites were more likely to show hearing impairment than the drivers.
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Noise at stock car races may also contribute in an additive manner to NIHL from other environmental and occupational exposures, as well as to age- or disease-related hearing loss. While the risk for permanent hearing loss from exposure to a single race seems low, it remains advisable for fans to protect their hearing with commercially available noise-reducing earplugs or protective headphones. Based on our findings, a noise-reduction rating of at least 25 dB is recommended. The ratings for such products typically range from 19 to 33 dB, although devices may differ in their ability to block different frequencies.
As part of our effort to study the noise at stock car races, sound pressure levels were also measured at four specific frequencies in addition to the A-weighted scale. Our aim was to characterize the frequency spectrum associated with such noise, in addition to the other primary factors--intensity and duration. These results indicated a greater intensity of noise at lower frequencies (125 and 500 Hz) in comparison to higher frequencies (2,000 and 8,000 Hz). In an investigation of noise transmitted to the fetal cochlea, Huang et al demonstrated a shift in auditory brainstem responses in fetal sheep exposed to low-frequency noise in utero. (8) While others have raised concerns regarding the transmission of noise and the possibility of NIHL during fetal life, adequate studies of this risk with recommendations on the safe limits of noise exposure for pregnant women or young children are still needed.
In addition to intensity, frequency, and duration of exposure, genetic susceptibility may also play a role in NIHL. In an animal model of NIHL, Luebke and Foster demonstrated a correlation between the susceptibility to noise damage in guinea pigs and the expression of acetylcholine receptor (AChR) subunit alpha 9, suggesting a possible protective mechanism at the hair cell level. (9) The human AChR subunit alpha 9 gene has been localized to chromosome 4p15. (10) Further work in the area of genetic susceptibility to noise may significantly improve our understanding of the pathogenesis of NIHL, as well as aid in its prevention.
Our report describes the noise associated with stock car auto racing--a source of recreational noise not previously reported in the medical literature. Future studies of NIHL associated with stock car racing may include audiologic testing of fans, drivers, and track employees to demonstrate any related temporary or permanent threshold shifts.
The authors wish to thank John H. Grose, MSc, PhD, of the Department of Otolaryngology-Head and Neck Surgery, University of North Carolina at Chapel Hill, for his assistance in the review of our data and preparation of this manuscript. The corresponding author, Austin S. Rose, MD, had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
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Austin S. Rose, MD; Charles S. Ebert Jr., MD; Jiri Prazma, MD, PhD; Harold C. Pillsbury III, MD
From the Department of Otolaryngology--Head and Neck Surgery, University of North Carolina, Chapel Hill.
Corresponding author: Austin S. Rose, MD, Department of Otolaryngology--Head and Neck Surgery, University of North Carolina, CB #7070, Chapel Hill, NC 27599-7070. Phone: (919) 966-3342; fax: (919) 966-7941; e-mail: firstname.lastname@example.org