It helps these computers didn't use much power, probably didn't even have much of a heatsink on the CPU. Someone in this thread quoted 3 watts for the CPU. Of course with this many of them you might be using something like a kilowatt (for all the components) which is enough to make a room quite warm, but not damaging. Think about a kilowatt space heater - it's not really dangerous to things that don't directly touch it.