With an adequate supply of matter, the inward drifting of dust particles induced by the Poynting-Robertson effect can lead to the formation of a shell or disk-like structure around the source of radiation. In a steady state, the Poynting-Robertson effect produces a spatial density of dust in the form of 1/r which in turn gives rise to a mid-infrared spectrum where Fnu varies as 1/nu. It is believed that this report could be of interest for the interpretation of the infrared excess found in many extragalactic sources.