A theory that equates the time of ‘occurrence’ of a ‘long-tail’ injury, such as asbestosis, with the time of exposure to the cause of the injury for the purpose of a losses-occurring policy, the cause meaning harmful working conditions and/or exposure to harmful substances. All insurers on risk during this exposure period are deemed to be liable for the resultant liability in proportion to their time on risk. Alternative occurrence theories include the manifestation theory; the injury-in-fact theory; and the triple trigger theory.