A system for determining a service life for a filter, the service life being representative of a time period that the filter is exposed to the contaminant before the contaminant passes through the filter at the breakthrough concentration, the system comprising: a filter configured to be coupled to an air purifying respirator (104; 114; 116) to filter a contaminant; a filter sensor (212; 214; 216) disposed at least one of proximate to and internal to the filter to measure exposure data which is representative of ambient conditions to which the filter is exposed and includes a contaminant concentration; and a monitoring module for receiving the exposure data from the sensor (212; 214; 216) and environmental data representative of at least one of the ambient conditions from an input that differs from the sensor, wherein the monitoring module is configured to calculate a service life estimate based on the exposure data, characterised in that the service life estimate is representative of an estimated time that the filter is exposed to ambient conditions before the contaminant passes through the filter at a breakthrough concentration; and in that the monitoring module is further configured to establish a service life prediction based on the environmental data, the service life prediction representative of a predicted total exposure time that the filter is exposed to the ambient conditions before the contaminant passes through the filter at the breakthrough concentration; and determine the service life for the filter based on a comparison of the service life prediction and the service life estimate. a filter configured to be coupled to an air purifying respirator (104; 114; 116) to filter a contaminant; a filter sensor (212; 214; 216) disposed at least one of proximate to and internal to the filter to measure exposure data which is representative of ambient conditions to which the filter is exposed and includes a contaminant concentration; and a monitoring module for receiving the exposure data from the sensor (212; 214; 216) and environmental data representative of at least one of the ambient conditions from an input that differs from the sensor, wherein the monitoring module is configured to calculate a service life estimate based on the exposure data, characterised in that the service life estimate is representative of an estimated time that the filter is exposed to ambient conditions before the contaminant passes through the filter at a breakthrough concentration; and in that the monitoring module is further configured to establish a service life prediction based on the environmental data, the service life prediction representative of a predicted total exposure time that the filter is exposed to the ambient conditions before the contaminant passes through the filter at the breakthrough concentration; and determine the service life for the filter based on a comparison of the service life prediction and the service life estimate. calculate a service life estimate based on the exposure data, characterised in that the service life estimate is representative of an estimated time that the filter is exposed to ambient conditions before the contaminant passes through the filter at a breakthrough concentration; and in that the monitoring module is further configured to establish a service life prediction based on the environmental data, the service life prediction representative of a predicted total exposure time that the filter is exposed to the ambient conditions before the contaminant passes through the filter at the breakthrough concentration; and determine the service life for the filter based on a comparison of the service life prediction and the service life estimate.