We propose to use the mapping, photometric and spectroscopic
capabilities of ISO to perform a detailed observational study of
dust in the low density components of the Galaxy. Dust grains
in low density gas undergoes drastic changes,
mainly through sputtering and shattering in fast supernovae
shocks. Although there has been extensive modelling of the dust
destruction, still little is known about the grains
which survive ? The proposed photometric and spectroscopic observations
will be sensitive to changes in abundance, size distribution and
spectroscopic properties relative to dust in
dense clouds.  Part of the observations also aim at studying the
density structure of the low density gas and its volume filling factor
in the Galactic halo. To achieve these goals we will use B stars
located within the low density gas as a local heat source of
the interstellar dust around them. The proposed observations will
provide fundamental data on the dust properties in the low density
warm components of the interstellar medium and new insights
on the structure of the low density gas in the disk and in the halo.
They should have important implications on the understanding of
the dust life-cycle and the evolution of interstellar gas in between
the different phases of the interstellar medium.