Contaminants of biological origin have been shown to cause outbreaks of building-related illness (BRI) which have a specific etiology and can be diagnosed clinically. These include hypersensitivity pneumonitis (HP), humidifier fever (HF), Legionnaires disease, and asthma. Both viable and non-viable contaminants of biological origin have been reported to cause BRI such as HP, HF, legionnaires disease, and asthma. HP and its European relative HF represent building-related health problems which have a characteristic symptomatology and an etiology which can be confirmed by clinical laboratory testing. Asthma is a severe respiratory disease which is induced as a result of exposure to a variety of causal agents, most importantly antigenic materials from organisms such as dust mites and mold. Asthma has only rarely been associated with exposures in office and institutional workspaces. Cases of asthma can be expected in building environments where heavy infestations of mold have occurred.