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.