I'm looking for guidance to define how hot is too hot when related to human safety. I have heard of a general limit from OSAH of 60 °C (140 °F) being ok for contact up to 5 sec. Are there better sources for defining this limit for different temperatures and time limits?
Since this is a safety issue, you'll need to carefully follow relevant standards such as:
- ASTM C1055-20: Standard Guide for Heated System Surface Conditions that Produce Contact Burn Injuries.
- ISO 13732-1:2006 Ergonomics of the thermal environment — Methods for the assessment of human responses to contact with surfaces — Part 1: Hot surfaces
These can be traced back to a seminal series of papers in 1947 on "Studies of Thermal Injury" by Moritz and Henriques. Figure 1 in an 1963 article by J.P. Bull in the British Medical Journal summarizes their temperature-time threshold curve for injury and you can see where the "60°C for 5 seconds" threshold criterion for partial thickness burns comes from. According to the figure, the damage takes only about 1 second at 65°C, and about 100 seconds at 50°C.
For a more recent review, check out the 2017 article "A review of the evidence for threshold of burn injury" by Martin and Falder in the journal Burns. According to that article, burn injury occurs when the basal epidermis reaches 44°C and the rate of tissue damage increases logarithmically with temperature, until by 70°C the damage is so rapid that "interpretation can be difficult". This is roughly consistent with the figure mentioned above.
EN 60335-2 family of regulations addresses this issue. Tough read though. Possibly interesting source of information: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiUgvDF3rP7AhUpgP0HHZSWCu8QFnoECA4QAQ&url=https%3A%2F%2Fec.europa.eu%2Fdocsroom%2Fdocuments%2F5263%2Fattachments%2F1%2Ftranslations%2Fen%2Frenditions%2Fnative&usg=AOvVaw3ukF_Rq6zm0vu-DHNuQtpA
Non-functional hot surfaces project.