  We study the conserved quantity associated with the dual symmetry of the
Maxwell equations, called the optical helicity, by means of transverse Hertz
vectors. In the presence of charges, its evolution yields the integral of
$\bm{E}\cdot\bm{B}$ which is the anomaly term for chiral fermions. We also
discuss the helicity change in condensed matter systems where topological
magnetoelectric effect emerges. An alternative expression of the optical
helicity is also found. Lastly, a dual symmetric Hertz Lagrangian is
constructed and its conserved charge is derived.
