There is no doubt of the presence of dust in H II region galaxies and in Seyfert galaxies, nor that it modifies the spectra of these objects. However, to correct accurately for its extinction is difficult. Since very little is known of the properties of dust in these galaxies, the standard assumption is that the optical properties of the dust in emission-line galaxies are identical with the optical properties of dust in our Galaxy near the Sun. Therefore, the Whitford reddening curve as parameterized by Miller and Mathews (1972) is normally used (Table 1).
Moreover, we have to to estimate the dust content of the galaxies.
The most widely used method
is based on the relative strengths of the lower Balmer lines,
and 
.
The observed Balmer decrement is invariably
steeper than the calculated decrement and the difference is then
assumed to be due to be due to interstellar extinction.
The effect of reddening on the ratio 
/
can be written (see Appendix B for a derivation):
where C is the measure of the amount of reddening (E(B-V) =
0.77 C), I(
) indicated
the (unreddened) intrinsic flux and F(
) the flux measured.
For the intrinsic flux ratio for H II region-like objects
we adopted the case B Balmer recombination decrement
I(
)/ I(
) = 2.85 for 
K
and 
.
In AGN, however, the harder photoionizing spectrum results
in a large transition zone in which collisional
excitation is also important in addition to recombination collisional
excitation (see Appendix B).
Therefore, for the intrinsic ratio for AGN we adopt
I(
)/I(
)= 3.1
The reddening correction at the different wavelengths can thus be obtained :
