Contaminant

An undesirable substance or material. This could be soil contaminated with, for example, a toxic heavy metal element such as Cadmium; Broken grass within the soil; or an Oil or fuel spillage.
For artificial surfaces this can be fine particles, leaf or grass remains, moss or algae on a surface, silt from surrounding area which have washed onto a surface, soil blown onto a surface from an adjacent agricultural field, as well as any human elements such as skin, hair, vomit etc.