Damage Risk Criteria and Noise Exposure Standards

There are probably two closely related key questions that concern people about noise exposure and hearing loss: First, what are the chances of getting a hearing loss from being exposed to some noise for some period of time? Second, how much noise exposure is acceptable before it becomes damaging to hearing? These questions are addressed by damage risk criteria (DRC), which are standards or guidelines that pertain to the hazards of noise exposure. However, this apparently straightforward issue actually involves many interrelated questions, the answers

to which are complicated, not necessarily understood, and often controversial. Some questions deal with the noise and predicting its effects: How should noise be quantified for the purpose of assessing damage risk, and how should we handle different kinds of noises (continuous, impulsive, intermittent, and time varying)? Are there valid and reliable “predictors” of NIHL, and if so, what are they and how are they related to hearing loss? Other questions deal with the amount of NIHL and how many people are affected: How much hearing loss is acceptable or at

least tolerable, and at which frequency(ies)? Since people vary in susceptibility, what is the acceptable/ tolerable percentage of the population that should be “allowed” to develop more than the acceptable/tolerable amount of NIHL? Still other questions pertain to distinguishing between different sources of hearing impairment: Can we separate NIPTS from other sources of hearing impairment (e.g., pure presbycusis and disease), and if so, how? Can we distinguish between the effects of occupational noise exposure and nonoccupational causes of hearing loss, including nonoccupational noise exposures? The latter is an important issue because we are usually concerned with industrial exposures. For this reason, we often use the term industrial noise-induced permanent threshold shift to refer to the part of a hearing loss actually attributable to industrial (or other occupational) noise exposures. These questions should be kept in mind when dealing with the effects of noise exposure in general, and particularly when dealing with hearing conservation programs and assessing hearing handicap for compensation purposes.

The Committee on Hearing and Bioacoustics and Biomechanics (CHABA) published damage risk criteria intended to limit the amount of industrial NIPTS to 10 dB at ≤ 1000 Hz, 15 dB at 2000 Hz, and 20 dB at ≥ 3000 Hz among 50% of workers who are exposed to steady or intermittent noises for 10 years (Kryter, Ward, Miller, & Eldredge 1966). The CHABA DRC

was based on the amount of TTS2 that occurs after an 8-hour noise exposure, and relied on the notion that this value seems to correspond to the amount of NIPTS that is present after 10 years of occupational exposure. However, it has never been proven that TTS validly predicts NIPTS, and this notion has several serious problems (e.g., Melnick 1991; Ward 1991). Damage risk criteria for impulsive noises using similar criteria were introduced by Coles, Garinther,

Hodge, and Rice (1968). The CHABA damage risk criteria required noises to be measured in third-octave or octave-bands, and expressed maximum exposure levels for durations up to 8 hours per day. The maximum allowable octave-band levels for an 8-hour exposure were ~ 85 dB for frequencies ≥ 1000 Hz but were higher for lower frequencies, because they cause smaller threshold shifts than do equally intense higher-frequency exposures. The allowable noise levels also became higher as the duration of the exposure decreased below 8 hours, because the amount of threshold shift is related to exposure duration. Botsford (1967) developed equivalent values that make it possible to apply the CHABA damage risk criteria to noises measured in dBA instead of octave-band levels.

OSHA noise exposure criteria Noise exposure standards are particularly effectual when they carry the weight of law. Numerous examples are found in state and local ordinances and in military regulations, but the most influential are federal labor regulations found in the Walsh-Healey noise standard and OSHA Hearing Conservation Amendment (HCA)

(DOL 1969; OSHA 1983). These noise exposure limits are shown in (first and second columns), where we see that the maximum noise exposure limit is 90 dBA for 8 hours. In addition, impulse or impact noises are not supposed to exceed 140 dB peak SPL. If the noise level exceeds 90 dBA, then the exposure duration must be reduced by one half for each 5 dB increase. In other words, the maximum exposures are 8 hours at 90 dBA, 4 hours at 95 dBA, 2 hours at 100 dBA, down to one-quarter hour at 115 dBA. However, the noise level is not permitted to exceed 115 dBA even for durations shorter than one-quarter hour. This trade-off of 5 dB per doubling

of time is called the 5 dB trading rule or exchange rate, and is based on the premise that sounds that produce equal amounts of TTS are equally hazardous.

This is the same equal-TTS principle discussed previously for the CHABA damage risk criteria. The major alternative approach is to reduce the intensity by 3 dB for each doubling of duration (the 3 dB trading rule or exchange rate), which is employed by the military, the EPA, and many foreign countries. The 3 dB trading rule is based on the equal-energy concept that considers equal amounts of noise energy to be equally hazardous, and is more strongly supported

by scientific evidence than the 5 dB rule (e.g., Suter 1992b; NIOSH 1998).

The maximum allowable noise exposure is known as the permissible exposure level (PEL) and is considered to be one full dose of noise. Using the OSHA (1983) criteria, a person has received one dose (or a 100% dose) of noise regardless of whether he was exposed to 90 dBA for 8 hours or 105 dBA for 1 hour.

Noises that have been measured in terms of octave-band levels need to be converted into 0 dBA

so that allowable exposures can be determined for them. The methods used to convert octave-band levels into overall level in dBA.

However, a simplified approach for compliance with the OSHA noise standard can be achieved by using the conversion chart shown in The procedure involves plotting the octave-band levels

of the noise on the chart, and then comparing them to the conversion curves. The highest curve that is crossed by any part of the noise spectrum constitutes the equivalent noise level in dBA. This value in dBA can then be used to assign noise exposure limits according to the first and second columns of for OSHA purposes.

NIOSH noise exposure criteria

We are very interested in the OSHA HCA because it continues to be the dominant force in occupational hearing conservation in most industries as the legally enforceable federal regulation. However, the National Institute of Occupational Safety and Health (NIOSH 1998)

developed revised criteria and recommendations for occupational hearing conservation programs that have considerable efficacy because they reflect the preponderance of scientific evidence. With regard to noise exposure limits, NIOSH called for

(1) replacing the PEL of 90 dBA TWA with a recommended exposure level (REL) of 85 dBA TWA,

(2) changing theexchange rate from the 5 dB rule to the 3 dB (equal

energy) rule, and

(3) setting a ceiling exposure of 140 dBA regardless of the type of the noise.

The 85 dBA REL and 3 dB exchange rate used by NIOSH result in different and more protective exposure values than those used by OSHA, which is seen clearly by comparing the two sets of maximum exposure durations in Table. For example, notice that a 90-dBA exposure is permissible for 8 hours by OSHA but for only 2½ hours by NIOSH, and that OSHA permits a 2-hour exposure to 100 dBA compared with only 15 minutes according the NIOSH recommendations. Noise exposure dosages are also considerably different according to the two sets of criteria. Referring to the plot labeled “NIOSH (1998).  we see that a TWA of 85 dBA corresponds to one dose or a 100% dose according to NIOSH; OSHA’s 90 dBA PEL is considered to be two doses or a 200% dose, and 105 dBA constitutes 100 doses or a 10,000% dose. Although the graph only goes up to a 10,000% dose, a

120 dBA TWA exposure would correspond to about a 316,000% dose (roughly 316 doses).