  We present a detailed model capable of explaining quantitatively the
temperature fluctuations observed in luminous, large HII regions. The model is
based on two assumptions which we justify on the basis of observations: that
the major fraction of the hydrogen in the clouds that form the HII regions is
not photoionized and is essentially HI, this HII is lightly ionized by fluxes
of low energy cosmic rays (CR) produced by processes originating in the hot
stars which illuminate the regions.
