Left: S₂₄ vs. S₁₀₀ flux densities for the whole GOODS-H sample (black circles) as well as for the 24   μm dropout sources (red arrows). For the dropouts we consider a 3σ upper limit for the S₂₄. Dropouts as well as some 24   μm detected sources tend to depart from the bulk of the GOODS-H population, exhibiting redder S₁₀₀/S₂₄ colours. The cyan line corresponds to S₁₀₀/S₂₄ = 43. As discussed latter in the paper, sources with S₁₀₀/S₂₄  >  43 are classified as silicate-break galaxies. Middle: detection limits as a function of redshift for the GOODS-N and GOODS-S PACS 100   μm and MIPS 24   μm observations. Red squares correspond to the drop-out sources. Right: S₁₀₀/S₂₄ as a function of L_IR as derived by Herschel for the whole GOODS-H sample (circles) and lower limits for the 24   μm dropouts (arrows). Both samples are colour coded based on their redshift. Sources with a black cross are AGNs based on their X-ray emission. Filled symbols denote sources in the GOODS-H sample with spectroscopic redshifts while open symbols sources with photometric redshift. Similarly, yellow circles on top of the arrows indicate that a spectroscopic redshift is available for that 24   μm dropout source. The horizontal black dashed line corresponds to S₁₀₀/S₂₄ = 43.