  We study the prospects of detecting magnetic helicity in galaxies by
observing the dust polarization of the edge-on galaxy NGC 891. Our numerical
results of mean-field dynamo calculations show that there should be a
large-scale component of the rotationally invariant parity-odd B polarization
that we predict to be negative in the first and third quadrants, and positive
in the second and fourth quadrants. The large-scale parity-even E polarization
is predicted to be negative near the axis and positive further away in the
outskirts. These properties are shown to be mostly a consequence of the
magnetic field being azimuthal and the polarized intensity being maximum at the
center of the galaxy and are not a signature of magnetic helicity.
