A deposition of solid material on a surface causing resistance to heat and mass transfer. The material may be process materials, precipitates, and particulates that build up on the surface of heat exchangers, packing supports, and distillation trays, etc. Pipe scale, lime, carbon, gums, and other chemical deposits can restrict the passage of flow, reducing heat transfer, and increasing pressure drops. Extreme fouling can cause blockages or plugging. Periodically, the process equipment must be taken out of service and cleaned. A fouling factor is a number used in heat transfer calculations where fouling is likely to be an issue on a surface causing resistance to heat transfer. Scale, dirt, and other deposits may accumulate on the heat transfer surface resulting in additional resistance to heat flow. Fouling factors are usually specified to provide a margin of safety.