Impulse noise, ototoxic substances and special settings

Most of this course deals with continuous and fluctuating noise from machinery. Some hazards do not fit that pattern neatly. Impulse noise from gunfire, explosives, nail guns and impacts can damage hearing in an instant. Some chemicals damage the ear on their own or make it more vulnerable to noise. And some workplaces, such as construction sites, music venues, call centres and firing ranges, have noise problems that need special approaches. This lesson covers these situations and shows how the general principles from earlier lessons apply to them.

What impulse noise is

Impulse noise is a sudden burst of sound lasting less than about one second, often only a few milliseconds, with a rapid rise to a very high peak. Impact noise, from one object striking another, such as a hammer on steel or a press closing, is closely related. Examples include gunshots, explosions, nail guns, powder-actuated tools, forging hammers, pile drivers, metal stamping and dropping steel plates.

Impulse noise is especially damaging for two reasons. First, the rise time is faster than the acoustic reflex described in Lesson 1, so the middle ear muscles offer no protection. Second, very high peaks can cause direct mechanical damage to the hair cells and supporting structures, the acoustic trauma described in Lesson 1, rather than the slower metabolic damage of continuous noise. Above roughly 140 dB peak, a single exposure can cause permanent loss. That is the basis for the 140 dB peak limit used in many jurisdictions and in OSHA's guidance that impulse exposure should not exceed 140 dB peak.

Measuring impulse noise

Ordinary sound level meter readings with slow or fast time weighting greatly underestimate impulse peaks. Impulse assessment uses the peak sound pressure level, usually C-weighted, measured by a peak detector that responds in microseconds. For very high peaks, such as gunfire, special microphones and instruments with a high upper measurement range are needed, because ordinary dosimeters may saturate.

With a 3 dB exchange rate, the energy of impulses is included in the Leq and Lex,8, because an integrating instrument adds up all the acoustic energy. With a 5 dB exchange rate and slow time weighting, as in OSHA's rule, impulses are underweighted, which is one reason NIOSH recommends the 3 dB rule. The energy of a single impulse can be described by its sound exposure level, SEL, the level of a one-second steady sound with the same energy. The contribution of many impulses to the daily exposure is then:

where N is the number of impulses in the day and 28,800 is the number of seconds in 8 hours.

import math

def lex8_from_impulses(sel, n):
    """Daily exposure from n identical impulses with sound exposure level sel (dB)."""
    return sel + 10 * math.log10(n) - 10 * math.log10(8 * 3600)

def combine(levels):
    return 10 * math.log10(sum(10 ** (L / 10) for L in levels))

sel_nail = 108          # SEL of one nail-gun shot at the operator's ear (illustrative)
background = 82         # Lex,8 from the other noise of the day
for nails in [200, 1000, 3000]:
    imp = lex8_from_impulses(sel_nail, nails)
    print(f"{nails:>5} nails: impulses alone {imp:5.1f} dBA, with background {combine([imp, background]):5.1f} dBA")

  200 nails: impulses alone  86.4 dBA, with background  87.8 dBA
 1000 nails: impulses alone  93.4 dBA, with background  93.7 dBA
 3000 nails: impulses alone  98.2 dBA, with background  98.3 dBA

A framer driving a few thousand nails a day can exceed the daily limit from the nail gun alone, even though each shot lasts only milliseconds. Quieter tools, exhaust deflectors pointing away from the ear, and protectors suited to impulses help.

Protecting against impulse noise

The hierarchy of controls still applies. Engineering options include quieter tools, such as electric or cordless fastening tools in place of powder-actuated ones, padding and damping to reduce impact noise, enclosures for presses and forges, and baffles and absorption on indoor ranges. Administrative controls include limiting the number of rounds or impacts, keeping non-essential people away and scheduling blasting when few workers are present.

Hearing protectors work against impulses, but selection is different. Peaks above about 150 dB often call for dual protection. Electronic level-dependent earmuffs and earplugs are popular with shooters and blasters because they let the wearer hear speech and range commands between shots while limiting the peaks. Passive non-linear earplugs, with a small filter that attenuates more at high levels, offer a simpler option for intermittent impulses. Laboratory ratings for impulse attenuation, such as impulse peak insertion loss, are increasingly published.

Ototoxic substances

Some chemicals damage hearing directly, by harming the hair cells or the auditory nerve, or make the ear more susceptible to noise damage. These ototoxic substances matter in noise programs for three reasons: they can cause hearing loss at noise levels below the limit, the combination of noise and an ototoxicant can cause more damage than either alone, and audiometric results may be misinterpreted if chemical exposure is not considered.

No Canadian jurisdiction yet sets a specific occupational rule for ototoxicants, but ACGIH's documentation for noise recommends audiometric testing for workers exposed to ototoxic chemicals even below the noise limit, and several provincial guides mention the issue. Because ACGIH limits are adopted in many provinces, programs should identify ototoxicants in their chemical inventories and safety data sheets, control them through the hierarchy of controls, and include exposed workers in audiometric testing.

Skin contact matters for solvents, which can be absorbed through the skin, so gloves and skin protection are part of preventing ototoxic effects. Respiratory protection, covered in HSE-240, may be needed where airborne levels cannot be controlled. Audiometric results for workers exposed to ototoxicants may show shifts across a broader range of frequencies, and the reviewing professional should be told about the chemical exposure.

Hand-arm and whole-body vibration

Many noisy tools also vibrate: grinders, chipping hammers, rock drills, chainsaws and impact wrenches. Hand-arm vibration can cause vibration white finger, a form of Raynaud's phenomenon, along with nerve and muscle damage, and some research suggests that workers with vibration white finger have more hearing loss than similarly exposed workers without it. Vibration is a separate hazard with its own assessment methods and limits and is beyond the scope of this course, but noise and vibration controls often overlap: a quieter, better-balanced tool with vibration-damped handles reduces both.

Special settings

Some workplaces present noise problems that call for adapted approaches.

Construction in more detail

Construction workers often face the highest noise exposures of any sector, yet their exposures are among the hardest to manage. A worker may run a concrete saw for 20 minutes, then carry materials, then work next to another trade's jackhammer. Since 2016, Ontario's noise regulation has applied to construction projects, so constructors and employers on Ontario sites must meet the same 85 dBA Lex,8 limit and the same priority for engineering controls as factories. Practical tools include task-based exposure tables for common tools, quieter equipment specified in rental and purchase agreements, water-fed and shrouded cutting tools that reduce both dust and noise, scheduling loud work to limit how many people are nearby, and simple rules such as hearing protection required within a set distance of named tools.

Young and new workers

Young workers are often given the noisiest, least skilled tasks, have the least experience of using protectors and may already be exposed to loud music and recreational noise. Their hearing has decades of potential exposure ahead of it. Programs should make sure that new and young workers receive training and properly fitted protectors before their first noisy task, are enrolled in audiometric testing with a good baseline, and are supervised closely in their first months.