Touch temperature is based off the limitation of human skin. Human skin can bear some higher temperatures for brief periods of time before pain and cellular damage can set in. These two limitations, pain and damage, are the last things our users want to experience with our products. NASA put out an awesome summary of a new approach to touch temperature. NASA conducted some empirical data and research on previous studies to develop their own mathematical model of what temperatures are allowable for skin constant contact. Essentially, our pain threshold for heat depends on what area of the skin is heated, but in general that ranges between 42C (sensitive skin the chest area) to 45C (on our feet).

Contact Time

Another factor for determining the maximum touch temperature is how long a user might be in contact with the surface. Skin can handle higher temperatures for a few seconds before pain or skin damage occurs. Ecma International, a body responsible for creating standards for Information and Communication Technology and Consumer Electronics, has set some ergonomic limitations for skin contact time and temperature. The Standard ECMA-287 has a table the breaks down allowable temperatures based off use and the expected time of contact. //

Continuous hold ( < 8h) -- 43C
Short period touch ( < 10s) -- 55C metallic, 65C non-metallic
May be touched ( < 1s) -- 65C metallic, 85C non-metallic