The REL (Recommended Exposure Level) duration reported in VenueDB is based on National Institute for Occupational Safety and Health (NIOSH) Publication No. 98-126. The full report can be accessed here. This report states that daily 8-hour exposure to 85 dBA causes a small risk (10-15%) of experiencing a material level of hearing loss. A 3-dB exchange rate was also used as recommended by the report; this means that the REL duration decreases by half for every 3-dB increase in sound level. Importantly, the REL duration does not reflect what is safe for a single exposure. Instead, it is based on industrial noise exposures for 40 hours per week over many years.
Important Note: the dBA measurement (on which the REL is based) is only as accurate as the microphone used and noise level can vary significantly from location to location in a room and over time.  Therefore the reported REL should serve only as a general guide and not a substitute for dedicated noise measurement equipment and test procedures.