During the first training session I attended in 2000 and in various articles I have seen that an incurable burn threshold is 205 degrees F for 0.1 seconds with a curable burn threshold at 175 degrees F for 0.1 seconds. I have always wondered about these figures. Last week I began to investigate an accident in a sauna and found that saunas normally operate as 160 to 190 degrees f and people stay in a sauna for about 15 minutes. So, where did the curable and incurable temperatures come from? Aare these temperatures called out in any standards?

"Just curable" is a term for 1st degree burn. "Incurable" is anything worse than that. I don't know who came up with these words but they are very misleading. Leaving aside modern medical technology when I was about 8 years old I fell onto a wood burning stove. I received a 3rd degree burn across the bottom of my belly about 6 inches long. Today, 40 years later, there is no scar or any sign I ever did what I did. I never received medical treatment beyond first aid. I have had first and second degree burns in the past from welding and various industrial processes. They all go away eventually. I have not had to make a trip to a burn unit either so nothing I have received is anything close to a severe burn. I know people who have. I can appreciate what "incurable" looks like, but a 2nd degree burn is not "incurable".

AVOID figuring temperature into it. When it comes to heat transfer, it is not a good way to measure things because it doesn't give you enough information. If that was the case, try cooking a steak in an oven set to 140 F, stopping when it reaches 130-135 F (medium). Taste it. Trust me, that will be the last time you ever cook a steak this way. This is cooking by conduction. It's good for some things but not meat. And as you described in your sauna example, even 140 F which is advertised as the threshold for a 1st degree burn is obviously inaccurate.

Humans have a hard time working for any length of time when the air temperature reaches around 130-140 F. This is simply because their internal cooling systems have a hard time maintaining body temperature at 98 F, which works by evaporative cooling. AND humidity plays a big role in this. Here in the Southeast once we get above 100 F or so with 90%+ humidity, the sweat just lays on you and gets sticky. You get little to no cooling from it. But playing golf in Arizona at 120 F is a nice day outside.

In a sauna you are dealing with heat exposure by convection and conduction. Heat transfer in those conditions is proportional to the temperature difference. When we dump water on the rocks, we are actually cooling the heat source but moist air contains more heat and transfers it more efficiently via conduction than dry air. This still has absolutely nothing to do with heat transfer at industrial conditions.

Arc flash is heat transfer via radiation. This is proportional to the 4th power of the temperature difference. Radiation is vastly more efficient in terms of heat transfer and it is the dominant heat transfer mechanism in most industrial processes. Put another way you can stand outside in 30 F air next to a bonfire and feel perfectly comfortable in a short sleeve shirt but take away the bonfire and most people would consider that air temperature to be very uncomfortable. The fire isn't heating the air except the air above it. It is thermal radiation that makes standing near it comfortable.

Live burns are also definitely not linear. Human skin has a pretty good built-in cooling system as well as different layers with different heat transfer coefficients. The result is that heat transfer is a nonlinear curve.

And you have to figure area into it. Some experiments involving lasers give numbers quite a bit different from larger affected areas so they are not directly comparable. Also the skin has a cooling system so that as you extend the exposure times, you get different results. For instance we can pretty easily exceed the "1.2 cal/cm2" standard on a summer day at the beach from thermal radiation for the sun, and not even get to a 1st degree burn with enough sun screen.

The standard is the Stoll curve. Alicia Stoll did an experiment decades ago in which she exposed a small area of skin on a live test subject to a variable amount of heat for a variable amount of time. She then looked at the result (1st or 2nd degree burn). She also calibrated the heat source by measuring the heat using a copper calorimeter. From this data she was able to determine that with a certain amount of heat (measured with a copper calorimeter) how long it took for the onset of a second degree burn. The heat is expressed in cal/cm2.

A better comparison: Take a standard cigarette lighter. Hold your palm face down 1 inch from the flame for one second. If you actually do this (not recommending!) you will get a blister. That is a second degree ("incurable") burn.

Forget temperature. It will only lead to frustration trying to make the connection. The only advantage of temperature is that it is easy to measure but when it comes to burns you are basically measuring the wrong thing.

You need to consider skin temperature, not environmental temperature. While you can enjoy hot dry 190F sauna for a prolonged period of time (your skin temperature never reaches anywhere close to the air temperature due to heat dissipation and perspiration), I bet you wouldn't enjoy as much swimming in 190F pool where you skin would reach the environmental temperature in very short period of time.

Can't resist - I will take our couple of 115 to 120 F degree days each summer in Arizona any time over 100 F with 90% humidity.

In considering that the skin temperature must rise to the curable level, I will be testing the sauna in question and will measure the temperature at several locations.

Does anyone have a good idea as to how I can simulate skin for measuring the rise in skin temperature? I don't want to sit in a sauna for an hour long test that I will probably perform.

just turn on a stove, wait until it gets hot red, enjoy the warm feeling while standing beside it. Now, touch it and hold on to it for as long as you can, and you'll figure out very quickly the impact of skin temperature (vs source temperature). I'm sure you'll have very memorable and convincing experience (I had one when I was about 4 years old).

PaulEngr
thanks for the great explanation of the curable burn level. I have always had trouble with the numbers. With respect to my initial question a person passed out, or something like that, in a sauna and was not found for 3.5 hours. the person suffered massive burns which ended up in death. Thus, I am trying to find a way to establish a maximum temperature for a sauna.