The dust extinction towards bright HII regions in NGC 598 and NGC 5457
has been studied in detail by forming line ratios of Balmer and Pasch
en emission lines covering a large wavelength range. Three homogeneous
models of the geometrical distribution of the emitting sources and th
e obscuring dust have been tested. Only for low extinctions can the da
ta be fit by a homogeneous slab. For most of the observed H II regions
the Witt et al. (1992) 'dusty nucleus' model matches the observations
equally well as the usual assumption of a foreground screen, but the
former implies much larger actual dust contents. Spatial variations in
the optical depth in V of the order 0.3 across a region are found.