A good analogy to explain how hearing loss occurs is to visualize a thick grassy lawn.
As you walk across the grass, the grass bends down because of your weight. After you pass, the grass stands back up. The more you walk across the same area, the longer it takes for the grass to stand back upright. If you continue to walk across the same area, eventually the grass will die and the area becomes a dirt path,
The same thing can happen to your hearing. When sound vibrations enter your ear, tiny hair cells in the inner ear change the vibrations into nerve impulses. The nerve impulses are then transmitted to the brain where they are translated into the sound we hear. When the hair cells are subjected to excessive noise, they begin to lie down just like grass does when we step on it. After the noise subsides, the hair cells stand back up. Over time, the more noise the hair cells are exposed to, the longer it takes for them to stand back up. Eventually, they fail to return to normal, resulting in permanent hearing damage.
Wearing proper protective hearing equipment reduces the outside noise to below 85 decibels (dB). This level is considered safe to work in throughout an 8-hr day. Prolonged exposure to any noise above 85 dB can cause gradual hearing loss. The higher the decibel level of noise you are exposed to, the shorter the time you are allowed to work around the noise, CDC says that regular exposure to 110 dB for more than 1 minute risks permanent hearing loss. This is the level of sound an average chainsaw makes. An ambulance siren is about 120 dB.
When the noise levels vary, a mathematical calculation is used to determine a time-weighted average of the noise exposure (11 dB = 0.5 hour). If the sound level is a constant 95 dB, you would be able to work in the noisy environment for a total of 4 hr out of an 8-hr work shift. If the sound level was a constant 100 dB, you would be able to work a total of 2 hr.
Wear the Right Ear Gear
The noise level can be reduced by wearing appropriate hearing protection. All hearing protection must be labeled to show its effectiveness. The effectiveness is rated via the noise reduction rating (NRR). The higher the NRR, the more protection provided. If the outside noise is 110 dB, hearing protection with an NRR of at least 25 dB would be needed to keep the noise level at 85 dB (110 dB – 25 dB = 85 dB). Additional protection can be obtained by wearing an earmuff over earplugs. Don’t be fooled, however, into believing that the protection will be the total of both NRRs added together; the increased protection will only muffle about 2 to 5 dB.
The highest NRR is provided by moldable ear plugs——if they are worn correctly. They can be made of foam, wax, silicone, or other materials and fit directly in the ear canal. The earmuffs, which can be custom fitted, has the next highest NRR. The least effective are semi-insert plugs–two earplugs held over the ends of the ear canal by a rigid headband. But remember, there can be a wide range of NRRs for the same type of protection. Read the label and follow the manufacturer’s recommendations for wearing and maintaining the products, For more information, go to OSHA’s Hearing Protection Program website: www.osha.gov/SLTC/noisebearingconservation.