We present a detailed model capable of explaining quantitatively the temperature fluctuations observed in luminous, large H II 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 H II regions is not photoionized and is essentially H I, this H I is lightly ionized by fluxes of low energy cosmic rays (CR) produced by processes originating in the hot stars which illuminate the regions.