Various causes for soil pollution are detailed below. Of these causes, construction sites are important causes of soil pollution in urban area due to their almost ubiquitous nature. In general, any chemical handled at construction sites may pollute the soil. However, the higher risk come from those chemicals that may travel easier through air (as fine particulate matter) and which are resistant to degradation and bioaccumulate in living organisms such as PAHs. Additionally, construction dust may easily spread around by air and is dangerous due to its lower particle size (less than 10 microns). Such construction dust may trigger respiratory vilnesses, asthma, bronchitis and even cancer. The sites that invole demolition of older buildings may release asbestos. This may act as a poison in soil. It may be re-distributed by wind.