How much noise is too much noise, and what can we do to protect employees working in extreme noise environments?
Often at noisy facilities, there is one process or area that generates extremely loud noise. It typically is task-specific (such as tank cleaning, motor run-up tests or demolition projects) or tool-specific (a bead blaster, arc welder or jackhammer, for example). All the workers recognize it, even those who work several departments away.
Engineering controls often are infeasible, and it's common to hear the safety manager describe the noise level in words such as, “We can manage all the other hazardous noise levels at our facility, but not THAT one!”
When faced with noise exposures over 105 dBA, well-intentioned safety managers often ask which of the various contradictory de-ratings should be applied to the noise reduction rating (NRR) on the hearing protector package. “To estimate the noise protection, do we divide the NRR by 2, subtract 7 or de-rate the NRR by 70 percent?” (All three methods have been proposed at one time by various government bodies.)
After applying de-rating schemes, earplugs or earmuffs alone often are not enough to protect against extreme noise. Safety managers resign themselves to their most costly administrative control: limiting the number of minutes a worker is allowed to work on the extreme-noise job. They then rotate other trained workers into the area to supply a new pair of “fresh ears.”
To add to the confusion, hearing experts typically disagree with the regulatory definitions of safe noise. Published noise-dose tables defining safe, time-weighted noise exposures simply fall apart for extreme noise. Using its 5 dB exchange rate and 90 dB criterion level, OSHA1 defines maximum permissible unprotected noise exposure as 115 dBA for 15 minutes — an overexposure most hearing conservation professionals consider to be unacceptable. In contrast, NIOSH2 recommends that no worker be exposed to more than 28 cumulative seconds of noise at 115 dBA (and no other noise) in any workday.
Even the audiometric results may not correspond with extreme noise exposures. Among hearing conservation professionals, it is common to hear anecdotal reports of higher rates of hearing loss among workers exposed to moderate noise (90-95 dBA, for example) than extreme noise (over 105 dBA).
“My workers in 112 dB know they've got to take it seriously: They fit the hearing protectors well and wear them religiously,” one safety manager surmised. “My workers in 92 dB figure it's okay to cheat the system and remove their protectors for a few minutes here and there throughout the shift without any consequence. They're wrong.”
What hearing protectors can defend against continuous extreme noise levels over 105 dB? Is there any way to leave a skilled worker safely in extreme noise for several hours? And most importantly, how can we document that with any degree of certainty?
CONVENTIONAL METHOD: DUAL PROTECTION
Dual protection, using earplugs and earmuffs simultaneously, is sometimes the only available method to achieve maximum protection from hazardous noise. While dual protection typically lengthens the allowable exposure time, it comes at a cost: Using earplugs and earmuffs concurrently significantly can isolate the wearer, making it difficult to communicate or hear warning signals.
Dual protection is not required in OSHA regulations for general industry, but it is required in mining operations governed by MSHA3 for noise exposures over 105 dBA (8-hour, time-weighted average). Similarly, a guideline document from NIOSH2 recommends dual protection for any exposures over 100 dBA (8-hour, time-weighted average). This recommendation is viewed by some to be overly cautious, perhaps based on the blanket assumption that noise-exposed workers do not wear their hearing protectors properly.
Some companies have adopted internal safety policies requiring dual protection in specific work locations or for certain noisy tasks. OSHA4 even has affirmed an employer's right to require double protection for particular employees, specifically when the audiogram indicates a decline in hearing thresholds despite normal protective measures.
In noise attenuation, there is a ceiling effect, whereby more and more protection does not offer any additional noise reduction. We hear noise through two channels: the air-conducted noise (sound through the air) and the bone-conducted noise (vibrations amplified through teeth and skull). Bone conduction is the pathway used when you bite down on crunchy food and hear the sound amplified through your teeth and skull louder than air-conducted sound heard through the ears. Hearing protectors effectively can stop only the air-conducted noise, but not the bone-conducted signals. For many people, the noise-blocking ability of the outer ear for air-conducted noise maxes out around 40 dB. Above that, you will still hear sound, but it's coming through your skeletal system, and you would need acoustically sealed body armor to stop that.
In terms of estimating the amount of protection while wearing earplugs and earmuffs concurrently, OSHA recommends simply adding 5 dB to the higher NRR (typically, this means adding 5 dB to the NRR of the earplug). Attenuation in the low frequencies will be a bit more, and in the high frequencies a bit less. The benefits of dual protection heavily rely upon a good fit of the earplug.
In fact, as long as the earplug is properly fitted, it makes little difference which earmuff is used over it, as long as it has decent attenuation (10 dB or more) in the low frequencies. An earmuff with moderate attenuation, for example, has just the same effect as a high-attenuation earmuff when either is worn over a well-fitted earplug. But when a poorly fitted earplug is worn with an earmuff, the resulting dual protection is little more than the earmuff alone.
Is dual protection worth the hassle for just a few extra decibels of protection? In extreme noise, the answer often is yes. The decibel scale is a logarithmic scale, not a linear scale. This means that every 3 dB drop in noise level (that is, 3 dB more attenuation in the hearing protection) reduces incoming noise by half. To jump from 30 dB of protection (with earplugs alone, for example) to 35 dB of protection (earplugs plus earmuffs) represents reduction to nearly one-third of the noise exposure at the eardrum — and the risk of damage to hearing.
DOCUMENT PROTECTION WITH FIT-TESTING
For workers in extreme noise, it is critical to document adequate protection. But documentation traditionally has been difficult to achieve, especially when earplugs and earmuffs are rated with idealized laboratory ratings rather than real-world usage values.
There is a trend toward fit-testing of hearing protectors in the field, allowing employers to document exactly how much protection is achieved by each worker in real-world usage. Fit-test systems that can be used with any earplug are available from several manufacturers. Some of the systems also include on-screen training, showing workers the correct procedures for optimal fit of each type of earplug or earmuff.
For workers in extreme noise, fit-testing serves two valuable purposes. First, it ensures and documents an optimal fit of the earplug. As noted above, when wearing dual protection, the bellwether protector is the earplug: A good fit of the earplug means good dual protection, often resulting in total noise reduction near 40 dB. But if the earplug is poorly fit, dual protection offers few benefits over earmuffs alone. Fit-testing can train a worker in how a “good fit” sounds and feels.
Secondly and more importantly, fit-testing often documents there is no real need for dual protection. Anxious to err on the side of over-protection, some companies have policies requiring dual protection for any anticipated noise over 105 dBA. But such policies forget the downside of dual protection: acoustic isolation from the work setting (moving equipment, machinery noises), inability to hear communication (radio, speech, shouted warnings) and inability to hear warning signals. Ideally, we don't want to block all noise. We do want to reduce noise to safe levels while still giving the worker critical situational awareness.
Fit testing often documents that well-fit earplugs alone are enough to provide safe protection up to 110 dBA or higher. If fit testing proves that a worker's earplugs are providing 35 dB of protection in 110 dBA of noise, a safe noise level of 75 dBA is reaching the eardrum. Dual protection is not necessary, and the documentation showing safe exposure levels is captured.
While fit testing documents that a worker is capable of obtaining an adequate fit of hearing protection, it merely is a snapshot in time. Whether the worker continues to fit the protectors that same way each day on the job is an open question, resolved by one of the most recent technology advances in hearing protection: in-ear dosimetry.
In-ear dosimetry uses dual miniature microphones to measure noise exposures under the hearing protectors — effectively at the eardrum. This real-time measurement continuously monitors the effectiveness of the protectors (proper fit and wear time). A poor fit, or removal of the protector even if “just for a minute or two,” is captured real-time, and the user (and supervisor) is alerted to the overexposure.
A research study using in-ear dosimetry among coal miners found that with properly fitted hearing protectors, there was no need for dual protection even in ambient noise levels of 107 dBA TWA.5
When a high attenuation earplug or earmuff is properly fitted and the user is motivated to use it correctly, some hearing professionals say the need for dual protection is rare. The effort expended by a safety manager to enforce dual protection often is better spent in ensuring best fit of single hearing protection, and documenting it.
OSHA Noise Regulation, 29 CFR 1910.95.
NIOSH Publication No. 98-126:1998, “Criteria for a recommended standard: Occupational noise exposure: Revised criteria, 1998.”
MSHA, 30 CFR Part 62 — Health Standards for Occupational Noise Exposure.
OSHA interpretations letter, May 8, 1984: “Can the employer mandate a combination of a plug and a muff?”
Dr. Kevin Michael, AIHce 2002, “Upstream Prevention of Occupational NIHL Via Individual Exposure Management.” http://www.dosebusters.com.