  We discuss the effects of collisional enhancement of Balmer lines on the
determination of the primordial helium abundance. To this aim, we present a
photoionization model of the metal-poor extragalactic HII region SBS 0335--052.
We show that the derived helium abundance (Y) of this HII region depends on the
amount of collisional excitation affecting the Balmer lines, both directly
(through an underestimation of the actual He/H ratio) and indirectly (through
an overestimation of the interstellar reddening). We detail how each of these
effects affects the derived value of Y.
