  The blue compact dwarf galaxy I Zw 18 is one of the most metal poor systems
known in the local Universe (12 + log(O/H) $=$ 7.17). In this work we study I
Zw 18 using data from {\it Spitzer}, {\it Herschel Space Telescope} and IRAM
Plateau de Bure Interferometer. Our data set includes the most sensitive maps
of I Zw 18, to date, in both, the far infrared and the CO $J=1\rightarrow0$
transition. We use dust emission models to derive a dust mass upper limit of
only M$_{dust}\leq1.1\times10^4$ M$_{\odot}$ ($3\sigma$ limit). This upper
limit is driven by the non-detection at 160 $\mu$m, and it is a factor of 4-10
times smaller than previous estimates (depending upon the model used). We also
estimate an upper limit to the total dust-to-gas mass ratio of
M$_{Dust}$/M$_{gas}\leq5.0\times10^{-5}$. If a linear correlation between the
dust-to-gas mass ratio and metallicity (measure as O/H) were to hold, we would
expect a ratio of 3.9$\times10^{-4}$. We also show that the infrared SED is
similar to that of starbursting systems.
