The pinkish film often appears during or after construction or remodeling, when dust and dirt containing Serratia bacteria are stirred up. Once the bacteria are airborne, they will seek a moist location where it can proliferate. Some people have reported that the pink residue only appears during certain times of the year, especially when their windows are left open for most of the day. This type of bacteria is present in a number of environments and wind can carry the airborne bacteria or stir up dust in which the bacteria are present.
The appearance of the pink residue can be intensified by the use of activated carbon filters, which remove chlorine from the water. The absence of the normal levels of chlorine in tap water allows Serratia to thrive. Because chlorine naturally dissipates from water that is allowed to collect on surfaces, Serratia may proliferate in these areas.