  The halo of NGC 891 has been the subject of studies for more than a decade.
One of its most striking features is the large asymmetry in H-$\alpha$
emission. We have taken a quantitative look at this asymmetry at different
wavelengths for the first time. We propose that NGC 891 is intrinsically almost
symmetric, as seen in Spitzer observations, and the large asymmetry in
H-$\alpha$ emission is mostly due to dust attenuation. We quantify the
additional optical depth needed to cause the observed H-$\alpha$ asymmetry. A
comparison of large strips on the North East side of the galaxy with strips
covering the same area in the South West we can quantify and analyze the
asymmetry in the different wavelengths. From the 24 $\mu$m emission we find
that the intrinsic asymmetry in star-formation in NGC 891 is small i.e.,
approximately 30%. The additional asymmetry in H-alpha is modeled as additional
symmetric dust attenuation which extends up to ~ 40'' (1.9 kpc) above the plane
of the galaxy with a mid-plane value of $\tau$=0.8 and a scale height of 0.5
kpc. This observational technique offers the possibility to quantify the
effects of vertical ISM disk stability as an explanation for dust lanes in
massive galaxies Dalcanton et al. (2004).
