Origin of IR radiation in living things ?

Hi,

I've been reading that in a relatively normal environment about 1/2 of human heat loss is due to radiation. Most of this seems to have a wavelength of 10-12 μm.

Could someone here speak to the molecular origin of this? I'm guessing it is mostly from water molecules going from a higher to lower energy state, but can't find any information on the specifics.

thanks in advance,