Regulation 4 prescribes that each non-domestic building's "dutyholder", as defined in law, has a duty to manage the risk from asbestos and mandates that "a suitable and sufficient assessment is carried out". Risk assessment involves a combination of factors relating to the type and condition of the asbestos-containing materials and the likelihood of disturbance and exposure. Human exposure potential depends on the number of occupants in a building and the frequency and duration of use, and the nature of any disturbance: for example, intrusive maintenance activity within the building increases the risk of exposure - removing a number of asbestos-containing ceiling tiles to undertake works above the ceiling level would raise greater risks than changing a light bulb. Buildings may be ranked according to whether disturbance activity is undertaken: