Check out UL 1950 or UL 60950 (Underwriters Labs) standard. These are
specifically for electronics boxes, but the principle is the same. It gives different
max temperatures for different materials, and depending on how long you might
touch them. Metal surfaces that you have to hold with your fingers for a long time, like
control knobs, can't be higher than 55C. Glass and plastic limits are higher (65 and 75C, respectively.) Surfaces that you might just brush up against have different limits: 70C for metal, 80C for glass and 95C for plastic.
Metals, because of their good conductivity, can cause burns at lower temperatures than thermal insulators, like plastic or wood. You might have experienced this yourself when sitting on a wood bench that is held together with metal bolts. The whole bench that was sitting in the sun is all at one temperature, but it's the bold head that burns your bare leg.