  We develop a dust efflux model of radiation pressure acting on dust grains
which successfully reproduces the relation between stellar mass, dust opacity
and star formation rate observed in local star-forming galaxies. The dust
content of local star-forming galaxies is set by the competition between the
physical processes of dust production and dust loss in our model. The dust loss
rate is proportional to the dust opacity and star formation rate. Observations
of the relation between stellar mass and star formation rate at several epochs
imply that the majority of local star-forming galaxies are best characterized
as having continuous star formation histories. Dust loss is a consequence of
sustained interaction of dust with the radiation field generated by continuous
star formation. Dust efflux driven by radiation pressure rather than dust
destruction offers a more consistent physical interpretation of the dust loss
mechanism. By comparing our model results with the observed relation between
stellar mass, dust extinction and star formation rate in local star-forming
galaxies we are able to constrain the timescale and magnitude of dust loss. The
timescale of dust loss is long and therefore dust is effluxed in a "Slow Flow".
Dust loss is modest in low mass galaxies but massive galaxies may lose up to
70~80% of their dust over their lifetime. Our Slow Flow model shows that mass
loss driven by dust opacity and star formation may be an important physical
process for understanding normal star-forming galaxy evolution.
