By Chuck Kardous, MS, PE; Christa L. Themann, MA, CCC-A; Thais C. Morata, Ph.D. and W. Gregory Lotz, Ph.D.
Noise-induced hearing loss (NIHL) is 100% preventable; however, once acquired, it is permanent and irreversible [NIOSH 1998]. Understanding and minimizing the risks associated with noise exposures are the keys to preventing noise-related hearing loss. NIOSH has a long history of leadership in conducting research, advancing control measures, and recommending noise-exposure limits to prevent job-related hearing loss. Sometimes, observers ask whether our recommended limits for occupational exposure can be applied to exposures in the general environment from sources such as street noise, consumer appliances, and recreational pastimes.
The answer, as we’ll explain below, is not exactly.
What is the NIOSH Recommended Exposure Limit?
NIOSH establishes recommended exposure limits (RELs) to protect workers against the health effects of exposure to hazardous substances and agents encountered in the workplace. These NIOSH limits are based on the best available science and practices. In 1998, NIOSH established the REL for occupational noise exposures to be 85 decibels, A-weighted (dB[A]) as an 8-hour time-weighted average. Exposures at or above this level are considered hazardous. The REL is based on exposures at work 5 days per week and assumes that the individual spends the other 16 hours in the day, as well as weekends, in quieter conditions. Importantly, the NIOSH REL is not a recommendation for noise exposures outside of the workplace in the general environment.
NIOSH also specifies a maximum allowable daily noise dose, expressed in percentages. For example, a person continuously exposed to 85 dB(A) over an 8-hour work shift will reach 100% of their daily noise dose. This dose limit uses a 3-dB time-intensity tradeoff commonly referred to as the exchange rate or equal-energy rule: for every 3-dB increase in noise level, the allowable exposure time is reduced by half. For example, if the exposure level increases to 88 dB(A), workers should only be exposed for four hours. Alternatively, for every 3-dB decrease in noise level, the allowable exposure time is doubled, as shown in the table below.
Average sound exposure levels needed to reach the maximum allowable daily dose of 100%: