  The dynamics of relativistic magnetized mass loaded outflows carrying
toroidal magnetic field is analyzed in the context of Pulsar Wind Nebulae
(PWNs). Mass loading is very efficient in slowing down super-relativistic
magnetized flows and weakening of relativistic shocks. We suggest that
weakening of relativistic reverse shocks by mass loading in PWNs is responsible
for the low radiative efficiencies of the majority of the PWNs. Mass loading
may also result in a shock transition near the fast magnetosonic point; this is
unlikely to happen in majority of PWNs. The evolution of magnetized mass loaded
flows beyond the reverse shock is complicated: after initial deceleration to
the minimal velocity required to transport the magnetic flux, the mass loaded
flows have to {\it accelerate}. In order to be able to expand to infinity,
magnetized flows should either become time dependent or destroy the toroidal
magnetic flux by developing internal instabilities. Destruction of the magnetic
flux initiated by mass loading may allow for the flow to slow down to
sub-relativistic velocities and resolve the $\sigma$ paradox of the pulsar wind
nebula.
