TL;DR: NIOSH would recommend limiting the 8 hour exposure to less than 85 dBA. At 100 dBA, NIOSH recommends less than 15 minutes of exposure per day. NIOSH also recommends a 3 dBA exchange rate so that every increase by 3 dBA doubles the amount of the noise and halves the recommended amount of exposure time.
15 minutes is 7 doublings away from 32 hours, i.e. constant exposure. So, 100-(7*3)= 79dBA is safe, and 82dBA is safe for long-term exposure, and 85dBA is a limit at which you should have hearing protection.
https://www.osha.gov/SLTC/noisehearingconservation/
TL;DR: NIOSH would recommend limiting the 8 hour exposure to less than 85 dBA. At 100 dBA, NIOSH recommends less than 15 minutes of exposure per day. NIOSH also recommends a 3 dBA exchange rate so that every increase by 3 dBA doubles the amount of the noise and halves the recommended amount of exposure time.
15 minutes is 7 doublings away from 32 hours, i.e. constant exposure. So, 100-(7*3)= 79dBA is safe, and 82dBA is safe for long-term exposure, and 85dBA is a limit at which you should have hearing protection.