  The thermal emission of dust is one of the most important tracers of the
interstellar medium: multi-wavelength photometry in the far-infrared (FIR) and
submillimeter (submm) can be fitted with a model, providing estimates of the
dust mass. The fit results depend on the assumed value for FIR/submm opacity,
which in most models - due to the scarcity, until recently, of experimental
measurements - is extrapolated from shorter wavelengths. Lab measurements of
dust analogues, however, show that FIR opacities are usually higher than the
values used in models and depend on temperature, which suggests that dust mass
estimates may be biased. To test the extent of this bias, we create
multi-wavelength synthetic photometry for dusty galaxies at different
temperatures and redshifts, using experimental results for FIR/submm dust
opacity, then we fit the synthetic data using standard dust models. We find
that the dust masses recovered by typical models are overestimated by a factor
2 to 20, depending on how the experimental opacities are treated. If the
experimental dust samples are accurate analogues of interstellar dust,
therefore, current dust masses are overestimated by up to a factor 20. The
implications for our understanding of dust, both Galactic and at high redshift,
are discussed.
