Noise Dose Calculator (OSHA & NIOSH)
This Noise Dose Calculator helps Safety Managers and Acoustic Engineers determine if a worker’s daily noise exposure exceeds legal or recommended limits.
It calculates the Noise Dose (%) and 8-Hour Time Weighted Average (TWA), automatically adjusting for OSHA (United States Legal) or NIOSH (Scientific/International) standards.
🧮 How to Use
- Enter Noise Level (dBA): Input the A-weighted sound level measured by a dosimeter or sound level meter.
- Enter Duration: How many hours is the worker exposed to this noise? (e.g., 8 for a full shift, 2 for a specific task).
- Select Standard:
- OSHA (Occupational Safety and Health Administration): Use this for US legal compliance. Uses a 5 dB Exchange Rate.
- NIOSH (National Institute for Occupational Safety and Health): Use this for best-practice safety or international (ISO) standards. Uses a 3 dB Exchange Rate.
🧠 OSHA vs. NIOSH: What’s the Difference?
The key difference lies in the “Exchange Rate” (how fast the allowed time drops as noise increases).
| Feature | OSHA (Legal) | NIOSH (Recommended) |
|---|---|---|
| Limit (100% Dose) | 90 dBA | 85 dBA |
| Exchange Rate | 5 dB | 3 dB |
| Doubling Rule | Time halves every 5 dB increase | Time halves every 3 dB increase |
| Use Case | US Factory Compliance | Global Standard / Best Practice |
Example: At 95 dBA:
- OSHA: Allows 4 hours (90 + 5).
- NIOSH: Allows only ~47 minutes (85 + 3 + 3 + 3…). NIOSH is much stricter.
📐 Formulas & Calculations
1. Maximum Allowed Time ($T$)
How long can you safely stay at a specific noise level ($L$)?
$$T = \frac{8}{2^{(L - L_c) / ER}}$$
- $L_c$: Criterion Level (90 for OSHA, 85 for NIOSH)
- $ER$: Exchange Rate (5 for OSHA, 3 for NIOSH)
2. Noise Dose (%)
The percentage of your daily “noise budget” used up.
$$Dose % = 100 \times \frac{C}{T}$$
- $C$: Actual exposure time (hours)
- $T$: Allowed time (hours)
3. TWA (Time Weighted Average)
The equivalent steady noise level over an 8-hour shift.
- OSHA: $TWA = 16.61 \times \log_{10}(\frac{Dose}{100}) + 90$
- NIOSH: $TWA = 10 \times \log_{10}(\frac{Dose}{100}) + 85$
⚠️ Action Level vs. PEL (OSHA)
For OSHA compliance, there are two critical numbers:
- 85 dBA (Action Level): If TWA exceeds 85 dBA (50% Dose), employers must implement a Hearing Conservation Program (training, audiometric testing, providing earplugs).
- 90 dBA (PEL - Permissible Exposure Limit): If TWA exceeds 90 dBA (100% Dose), mandatory engineering controls or hearing protection is required.
🙋 Frequently Asked Questions
What does “100% Dose” mean?
It means the worker has reached the maximum allowable noise exposure for the day. Any further exposure without protection violates safety standards and risks permanent hearing loss.
Can TWA be higher than the measured noise level?
Yes, if the shift is longer than 8 hours. TWA normalizes everything to an 8-hour day. A 12-hour shift at 90 dBA results in a TWA higher than 90 dBA because the exposure duration is longer than standard.
Which standard should I use?
If you are in the US and need to meet legal requirements, use OSHA. If you want to ensure the best health outcomes for workers (or are in Europe/Canada), use NIOSH.
📚 References
- OSHA Technical Manual (OTM) Section III: Chapter 5.
- NIOSH Publication No. 98-126: Criteria for a Recommended Standard.