In this module
Objectives covered: LO 1.8.1. By the end you will be able to:
- Explain how noise damages hearing.
- State the OSHA action level and permissible exposure limit.
- Calculate protected noise levels using the NRR and derating.
- Fit earplugs and earmuffs correctly.
8.1 How noise damages hearing
Loud noise damages the tiny hair cells in the inner ear. They do not grow back. Noise-induced hearing loss is permanent, happens slowly and does not hurt, so people often notice it only when it is too late. Ringing in the ears (tinnitus) after a shift is a warning sign.
Rule of thumb: if you have to shout to be heard by someone about 1 metre away, the noise is probably 85 dB(A) or more.
8.2 Noise limits
- Action level: 85 dB(A) as an 8-hour time-weighted average (TWA). At or above this, the employer must run a hearing conservation programme.
- Permissible exposure limit: 90 dB(A) 8-hour TWA, with a 5 dB exchange rate (95 dB(A) for 4 hours, 100 dB(A) for 2 hours).
- NIOSH and most international guidance (including the EU and ILO) use 85 dB(A) as the limit with a 3 dB exchange rate: every 3 dB increase halves the safe time (88 dB(A) for 4 hours, 91 dB(A) for 2 hours). This is more protective and is good practice.
A hearing conservation programme includes noise monitoring, a baseline hearing test (audiogram) and then annual audiometric testing, hearing protectors offered free, training, and records.
8.3 Typical noise levels
| Source | Typical level, dB(A) |
|---|---|
| Normal conversation | 60 |
| Sewing floor in an RMG factory | 75 to 85 |
| Weaving shed with shuttle looms | 95 to 105 |
| Captive power gas generator room | 100 to 110 |
| Angle grinder on steel | 100 to 105 |
| Jackhammer or rock drill | 110 or more |
These are typical values only. Always measure with a sound level meter or noise dosimeter.
8.4 Types of hearing protector
- Foam earplugs: cheap, high protection when inserted deeply; single use.
- Pre-moulded (reusable) earplugs: washable, come in sizes.
- Banded earplugs: good for people who go in and out of noisy areas.
- Earmuffs: easy to fit and check; can be hot; glasses arms break the seal. Helmet-mounted muffs are available.
- Dual protection (plugs and muffs together): for very high noise, generally above 100 dB(A) TWA.
8.5 Noise reduction rating (NRR) and derating
The NRR on the package is measured in a laboratory. In real workplaces protection is lower, so OSHA advises derating:
Protected level ≈ TWA dB(A) − (NRR − 7) ÷ 2
Worked example: a weaving shed is 100 dB(A). Foam earplugs have NRR 29. (29 − 7) ÷ 2 = 11. Protected level ≈ 100 − 11 = 89 dB(A). That is still above 85, so use dual protection, reduce exposure time, or better, reduce the noise at source.
For dual protection, add 5 dB to the higher NRR before applying the formula. Aim for 70 to 80 dB(A) under the protector. Too much protection can make it hard to hear alarms and forklift horns.
8.6 Fitting a foam earplug
- With clean hands, roll the plug into a thin, tight cylinder.
- Reach over your head with the other hand and pull the top of your ear up and back.
- Push the plug well into the ear canal.
- Hold it in place for 20 to 30 seconds while it expands.
- Check: your own voice should sound deeper and louder, and noise quieter.
Spinning and weaving sections in jute mills often run at 95 to 105 dB(A). One mill measured each section, marked "Hearing protection required" zones with Bangla signs, issued banded earplugs to people moving between sections and foam plugs to fixed operators, and started annual hearing tests. It also replaced worn gears and fitted rubber mounts on looms, which brought several areas below 90 dB(A). Hearing protectors protect workers while those engineering fixes are being made.
Learning Check
Answer these two questions to check your understanding. You can try again as many times as you like.