Sick Building Syndrome (SBS) is an imprecise term used to describe those buildings in which there is a prevalence of a range of symptoms causing discomfort and a sense of being unwell rather than a distinct illness. There is a particularly high incidence in certain types of buildings, especially offices which are sealed and mechanically ventilated or air-conditioned. SBS is a complex phenomenon, and although a number of potential contributory factors have been suggested, much of the evidence is circumstantial, and no single underlying cause has been found. It is probable the cause is multi-factorial. Clinically-diagnosed illnesses which can readily be attributed to a particular cause, such as humidifier fever, Legionnaires' Disease, or exposure to a toxic agent in the workplace environment are not usually regarded as SBS.