The reliable and effective use of chemical-cartridge respirators requires a knowledge of the remaining service life of the cartridges. To date, the primary indicators that a cartridge was no longer serviceable were contaminant taste or odour, and respiratory tract irritation. Both these approaches are dangerous. In the first two phases of this research, a prototype of a waterproof service-life indicator was developed using porous optical fibres. This phase of the research will improve the sensitivity of the fibre-optic system and complete the development of the surface treatment needed to ensure waterproofing. This optimization of the indicator will render it suitable for marketing, and thus help better protect.