A filter simulation system that includes a simulation filter (11) with a sensor (13) such that the volume of air passing through the filter can be determined. There is also a control means (16) with stored data relating to level of protection the simulation filter provides against a simulated toxic substance. In this way the filter simulation system can determine whether a correct simulation filter has been selected by a user and whether or not that filter has expired, in which case a dead or incapacitated result can be indicated. In the event of expiry a simulation end of life service indicator (ELSI 22) may be activated.