This paper describes initial work to develop an optical system to detect small concentrations, less than 1 ppm, of chemical or biological agents by monitoring the fluorescence emission from a hollow optical fiber. This class of sensors would enable highly sensitive detection of chemical or biological agents by a small and lightweight sensor. The sensors would also be readily adaptable to different types of analyte by changing the fluorescent coating. Hollow core optical fibers can be filled with a sol-gel matrix that incorporates a fluorophor. Current work has established the sensitivity of an amine dye to weak concentrations of formaldehyde. Formaldehyde can be introduced into a spacecraft crew environment by out-gassing, crew activities, experimental payloads, and human and bacterial metabolism. Formaldehyde is an important trace contaminant to monitor because it is detrimental in small doses and long term exposure to low concentrations causes hypersensitivity. These issues become increasingly important in a closed-loop environment such as the crew habitat for astronauts on a long term mission.