Dust extinction of HII regions in NGC 598 and NGC 5457

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 Paschen emission lines covering a large wavelength range. Three homogeneous models of the geometrical distribution of the emitting sources and the obscuring dust have been tested. Only for low extinctions can the data be fit by a homogeneous slab. For most of the observed HII 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.