Safe Noise Exposure Time Calculator

Noise-induced hearing loss is one of the most common occupational hazards and recreational risks worldwide.

NIOSH (National Institute for Occupational Safety and Health) recommends a maximum exposure of 85 decibels (dBA) for 8 hours.
For every 3 dB increase above 85 dB, the safe exposure time is cut in half.

Safe exposure time formula (NIOSH):

T = 8 / (2 ^ ((dB - 85) / 3)) hours

Common noise levels:

• Normal conversation: 60-70 dB (safe)
• Vacuum cleaner: 70 dB (safe)
• City traffic from inside a car: 80-85 dB (8 hours at the top of that range)
• Lawn mower: 90 dB (about 2.5 hours)
• Concert or nightclub: 100-110 dB (minutes only)
• Chainsaw: 110 dB (about 1.5 minutes)
• Fireworks at close range: 140-160 dB (immediate damage)

OSHA (Occupational Safety and Health Administration) sets the legal US limit at 90 dBA for 8 hours, with a 5 dB exchange rate, so its times run much longer.
NIOSH uses a 3 dB exchange rate, which is considered more protective and is used in this calculator.

Hearing protection, and why this page uses smaller numbers than the packet

The NRR (Noise Reduction Rating) printed on a box of earplugs is measured in a laboratory, on a subject whose plugs were fitted by a technician. Real people roll foam plugs badly and push them in half way, and muffs leak around the arms of glasses.
This page uses OSHA’s field derating, the same one the site’s other noise calculators use. It takes 7 dB off the label, which converts it to the A-weighted decibels a sound meter reads, and then halves what is left:

real-world reduction = (NRR − 7) ÷ 2

A foam plug rated 32 comes out at 12.5 dB. Muffs rated 26 come out at 9.5, which surprises people who assumed the bulky option was the stronger one.
NIOSH has a derating of its own by protector type (a quarter off for muffs, half for foam plugs and 70% for other plugs, then the 7 dB), which comes out lower for foam and higher for muffs. Both agree on the part that matters: fit moves the result more than the rating does.

That is not pessimism, it is the difference between a calculator that tells you the truth about a 95 dB workshop and one that tells you what you would like to hear.
Doubling up plugs under muffs does not add the two ratings together either. Derate the higher of the two and add 5 dB for the second layer, because bone conduction sets a floor on what any protector can achieve.

Key facts:

• Hearing damage is cumulative and permanent.
• Ringing in the ears (tinnitus) after exposure is a warning sign.
• Regular hearing checks are recommended for anyone exposed to loud environments.