  We test emission models of circum-nuclear dust torii around quasars, at low
and high redshifts, by using a large collection of photometric data for an
unbiased sample of 120 optically-selected objects with millimetric and
sub-millimetric fluxes, including new unpublished data. Under the assumption
that the dust is heated by a point-like source with a power-law primary
spectrum, as defined by the observed optical-UV continuum, we infer the basic
model parameters, such as dust masses, temperature distributions and torus
sizes, by numerically solving the radiative transfer equation in the dust
distribution. In addition to the substantiated statistics, an essential
improvement over previous analyses comes from the use of optical-UV data to
constrain the primary illuminating continuum, which is needed to estimate dust
temperatures and sizes. Dependences of the best-fit parameters on luminosity
and redshift are studied and the contribution of dust in the host galaxy to the
observed fluxes is briefly mentioned. This analysis constrains the properties
of the enriched interstellar medium in the galaxies hosting the quasars. The
dust abundance does not display appreciable trends as a function of redshift,
from z ~ 1 to almost 5, and shows that dust and metals are at least as, and
often more, abundant at these early epochs than they are in local galactic
counterparts. This evolutionary pattern is remarkably at variance with respect
to what is expected for disk galaxies, like the Milky Way, slowly building
metals during the whole Hubble time. It rather points in favour of a much more
active phase of star-formation at early epochs, probably related to the
formation of massive spheroidal galaxies.
