Non-metallic materials (plastics, elastomers and lubricants) are known to be the most critical materials in high-pressure oxygen applications. Such materials are typically found in valves (seats and stem seals), regulators (diaphragms and seats), and plastic-lined flexible hoses. The risk of ignition has been well known for many years, but more recently it was determined that toxic gases can be generated by the ignition or decomposition of some non-metallic materials. This risk shall of course be assessed for medical oxygen and any breathing gas application. New international standards, currently in preparation, recommend how to address this toxicity risk. In this paper, the toxicity risk related to the use of some non-metallic materials is reviewed. A test method to determine the potential toxic gases, which could be generated by combustion or decomposition is described. Results from the analysis of toxic gases are presented. Finally, we make practical recommendations to avoid this toxicity risk.