A
Deaf
Spot
for Industrial Hygiene
The Problem of Impulse Noise
BY COREY BENDER
Editor’s note: The mention of specific products and services in this article does not constitute endorsement by AIHA® or The
Synergist®
.
Impulse noise, the noise created by the expansion of a gas, is commonly defined as a short energy burst lasting less than one second with a large increase in sound pressure from ambient. Weapons fire is a perfect example of impulse noise. Because of the extremely short duration of the impulse, often microseconds, the bang is not perceived as being as loud as continuous noise.
The involuntary flinch a gunshot invokes from bystanders overshadows the dangerously high peak pressure level. Weapons fire—or, more accurately, the hot gas escaping the end of the barrel—typically cracks a pressure change that results in a peak decibel level in the range of 140–185 dBP (ref 20 µPa).
For many military personnel, impulse noise is an ever-present hazard. Yet, as of January 2017, subject matter experts still have not agreed on damage risk criteria (DRC) for impulse noise. Each branch of the U.S. military evaluates the risk of impulse noise differently, complicating efforts to protect hearing.
According to the United States Army Aeromedical Research Laboratory (USAARL), DRCs are
[c]omprehensive statements of the relationships between critical parameters or hazardous entities . . . and the probability of injury of various degrees. They are characterized by their statements about the probability of specified injury resulting from specified exposure conditions in set proportions of the at-risk population.In the 1950s and 1960s, the Committee on Hearing, Bioacoustics, and Biomechanics (CHABA)—a group established by the National Academy of Sciences- National Research Council—performed initial work on a DRC for continuous noise and impulse noise. Validated and accepted results for impulse noise have lagged considerably behind continuous noise. In 1970 OSHA promulgated its General Industry Noise Standard, 29 CFR 1910.95. The standard mentions impulse noise just once in Table G-19, and only then as a footnote, which provocatively states, “Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level.” From a legal perspective, should is not the strongest word.
The U.S. military built on the CHABA recommendations, incorporating three distinct and different DRCs throughout the many versions of the military’s
Design Criteria Standard: Noise Limits
(MIL-STD-1474), which was updated in 2015 (1474E). The current Army medical risk criterion was adopted from MIL-STD-1474D. This criterion determines the allowable number of shots a person wearing single hearing protection can be exposed to in a 24-hour period. It requires a peak measurement and a waveform time duration measurement (B duration).
The Marines follow a different medical risk criterion for impulsive noise that calls for single hearing protection for exposures to peaks between 140 and <165 dBP and double hearing protection for exposures to peaks ≥165 dBP. NIOSH’s Occupational Noise Exposure Criteria, revised in 1998, touches briefly on impulse noise, recommending no exposure above a 140 dB peak. Currently, a project of the NIOSH National Occupational Research Agenda is attempting to quantify risk to hearing from impulse noise and update NIOSH guidelines.
THE CHALLENGE
What makes impulse noise so difficult?
First, while the damage done by continuous noise exposure amounts to a chemical poisoning of the sensory hair cells in the inner ear, the damage to the hair cells from high peak impulse noise is a more physical event and the target of continuing research. It still needs to be determined how much the peak pressure, duration, frequency spectra, kurtosis, and temporal spacing contribute to the damage mechanism. The three DRCs contained in various versions of MIL-STD-1474 differ significantly in their evaluation of risk. However, even one exposure above a 140 dB peak can cause permanent hearing loss and tinnitus, often when the impulse peak is unexpected. Perhaps impulse noise should get a little more attention, and maybe we should consider high pressure impulse noise as dangerous as a carcinogen.
Second, impulse noise peaks above 140 dB are difficult to measure accurately. A NIOSH study published in 2003 found that noise dosimeters are not suitable to characterize impulse noise. Most dosimeters and sound-level meters (SLMs) max out somewhere in the 140 dB range. A presentation at the 2016 annual conference of the National Hearing Conservation Association showed that even souped-up SLMs for measuring weapons fire lose reliability when peaks reach 155–160 dB and above. Recently, NIOSH and a private contractor developed an Impulse Noise Measurement Kit designed to take 102,400 samples per second (standard SLMs take around 50,000 samples per second). The Impulse Noise Measurement Kit captures the waveform (see Figure 1) of the impulse noise. The waveform is vital: it shows not only the peak but also time domains such as the aforementioned B duration.
Third, researchers face many obstacles when trying to study the effects of impulse noise on hearing. It’s not ethical to conduct research that exposes unprotected subjects to high peak impulse noise, but the research I’ve seen relies on capturing temporary threshold shifts after exposure to weapons fire, providing valuable data that could be used to validate or develop a DRC model. A NIOSH team is currently looking for research subjects so that they can obtain pre-shoot and post-shoot hearing tests, but military personnel are considered to be a vulnerable population, and it’s very difficult to get approval. The worry is that due to the rank and power structure of the military, service members will make coerced instead of voluntary decisions when considering participation in research such as this. My hope is that perhaps this article will inspire a military or law enforcement population to reach out to the NIOSH researchers (contact Chuck Kardous at
cyk5@cdc.gov
) and consider volunteering.
Figure 1. A waveform of a Glock handgun. Image provided by NIOSH.
TAKING ACTION
But even though the picture of impulse noise hazards is incomplete, we have enough knowledge to take action. Shouldn’t occupational hearing loss be eradicated by now? In the military, hearing loss and tinnitus traditionally have been the top 2 compensable injuries. While some military-related impulse noise exposures exceed 170 dB, a well-fitted set of hearing protective devices can at least protect those fragile ears from small arms fire, which has impulse noise in the range of 140–165 dB.
We have to consider the challenges faced by occupational users of weapons such as military and law enforcement. I would divide their exposure into two environments: the controllable training setting (such as firing ranges) and the battlefield. Have you ever seen a police officer fumbling with a set of foamies before drawing her weapon? What would be the pros and cons of law enforcement carrying weapons with noise suppressors? Hearing protection devices (HPDs) are not a broad-brush panacea, and more research and innovation is necessary, but I would propose that the industrial hygiene community has been operating with a “deaf spot” when it comes to impulse noise and the protection of exposed populations.
We can speculate how someone may suffer hearing loss from exposure to weapons fire. Consider a unit in the military that is scheduled to shoot an M9 handgun and an M249 SAW automatic at the outdoor range, weapons whose peak pressures have been documented as 157–159 dBP. The supply officer responsible for purchasing the HPDs acquires inadequate foam ear plugs because they are less costly than others and he doesn’t understand the options available. While well-fitted foamies are perfectly able to protect the user, one of the users at the range has a hard time communicating and hearing instructions when wearing the plugs. He “loosens” or discards them, a weapon is fired, and now he has permanent tinnitus. How can the IH community turn its ear to this problem?
IHs should stay in touch with research and ensure its application while promising the research community that its efforts will not be in vain. At the same time, IHs must sell the end user that the cost of HPDs is worthwhile and instill confidence in the solutions. The IH community must lobby industry, manufacturers, researchers, and hearing conservation stakeholders (like the military) to invest and commit to preventing noise induced hearing loss (NIHL) from impulse noise while motivating end users (like warfighters and law enforcement officers) to commit their resources and modify their behavior. In other words, the market won’t provide solutions unless there is a customer base. In my opinion, a large portion of the customer base doesn’t know yet that they should be demanding solutions from the market, so the IH community needs to guarantee return on investments.
Something similar was suggested by Fred Boelter, CIH, PE, BCEE, during his Cummings Award address at AIHce 2016 in Baltimore. As
reported
in The Synergist
, Boelter “pressed his audience to try to frame discussions on occupational risk factors more clearly and to work on presenting that information in more ‘compelling business terms.’” While Boelter’s words were meant to inspire the IH community to address non-occupational as well as occupational exposures, his focus on the big picture and guidance for engaging with the business side of things is the strategy I am advocating.
Industrial hygienists engaged with a population exposed to impulsive noise need to:
Become familiar with the research needs of NIOSH and others, and support their work. They may need research subjects, or they may need to test equipment or HPDs.
We can push for action from inside our organizations while the researchers pull from the outside.
Encourage exposed populations to collaborate and pool their resources to invest in equipment for measuring noise exposures. For example, pitch the idea to a group of county sheriff’s departments that they should acquire and share the equipment and hire a trained individual to make those measurements. Then informed, defensible decisions can be made when selecting hearing protection. In order to recommend hearing protection, we need accurate, useful measurements.Advocate for EPA to require HPDs to be evaluated on and labeled with an Impulse Peak Insertion Loss (IPIL) rating.
Developed in ANSI S12.42 (2010), IPIL describes the protection provided against a high peak impulse. It involves using an Acoustic Test Fixture (ATF), a realistic mannequin head, and microphones that measure the sound pressure levels at the ear drum. The IPIL is the difference in peak sound pressure level measured with and without the HPD in place. Three rounds of testing are performed at nominal peak levels of 132, 150, and 168 dBP. This test is particularly useful for evaluating non-linear passive earplugs, which have a filter that allows conversation-level noise to pass through while clipping high peak pressures. Since conversation-level noise is not filtered out, the Noise Reduction Rating (NRR) for these earplugs would be very low and misleading. This type of HPD improves communication and situational awareness while still providing protection. You may see HPDs evaluated by both the NRR method (ANSI S3.19-1974) and the IPIL method, so understanding the difference is important. The U.S. Military has performed several evaluations on various HPDs, including IPIL. Those reports can contain valuable information that can be used along with an accurate impulse peak decibel level measurement to recommend an HPD.
Understand that a population exposed to impulse noise may be reluctant to admit to hearing loss or to advance the effort.
These populations often face the hard truth that their job requires them to be exposed to this hazard and protection is a challenge. They may fear that a hearing test will reveal they are unfit for the job and that they’ll lose their livelihood. They may also consider themselves “young and invincible,” so we must be creative to accept the importance of hearing protection.
Let’s take advantage of our unique and often misunderstood positions as industrial hygienists and take the fight to the NIHL battlefield. Impulse noise is a multi-layered, complicated hazard with many challenges, including lack of research, technological hurdles, regulatory neglect, marketplace expense due to low demand, and detrimental behaviors and attitudes. There is work to be done.
COREY BENDER, CIH, CSP,
is Northern Virginia regional safety director for Virginia Occupational Safety and Health. He can be reached at corey.bender@doli.virginia.gov
.RESOURCES
Applied Occupational and Environmental Hygiene
: “Noise Exposure Assessment and Abatement Strategies at an Indoor Firing Range” (August 2003).Military Medical Research
: “Impact of Noise on Hearing in the Military
” (February 2015, citing U.S. Dept. of Veterans Affairs Records from 2012).
National Hearing Conservation Association Annual Meeting: “Measuring Gunshots with Commercial Sound Level Meters” (February 2016).
NIOSH:
Development and Validation Testing of an Impulse Noise Meter
(September 2013).The Synergist
: “‘Great Works’ Abound at AIHce
” (June/July 2016).
United States Air Force Research Laboratory: Performance Assessment of Passive Hearing Protection Devices (October 2014).
United States Army Aeromedical Research Laboratory: Assessment of Four Passive Hearing Protection Devices for Continuous Noise Attenuation, Impulsive Noise Insertion Loss and Auditory Localization Performance (November 2014).
United States Army Aeromedical Research Laboratory: Controlling Impulse Noise Hazards: Programmatic Model for Developing Validated Exposure Standards (June 1986).
United States Army Center for Health Promotion and Preventive Medicine: “Noise Levels of Common Army Equipment” (
PDF
).
United States Army Research Laboratory: “
Impulse Noise and the Cat Cochlea
” (September 2010).
United States Marine Corps: Order 6260.3 (March 2015).
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