The present disclosure relates to methods and apparatus for detecting when respiratory cartridges of a respirator have reached their end-of-service-life. In some instances, two or more respiratory cartridges are removably connectable to a respirator housing, where each of the two or more respiratory cartridges receives ambient air and deliver cleaned air to the respirator housing. One of the respiratory cartridges may be configured to have a lower adsorption capacity than the remaining respirator cartridges. A gas sensor may be situated downstream of the reduced capacity respirator cartridge, and may detect an increased concentration of the targeted gas in the cleaned air delivered by the reduced capacity respiratory cartridge. When detected, an end-of-service-life indication may be provided to the user.