As well as the major gases (O2, CO2, N2, CO) the submarine atmosphere contains trace concentrations of a large number of organic contaminants that arise from a variety of sources. Although air purification devices embarked for this purpose remove these contaminants, an equilibrium steady state is reached during the dived period and the men breathe this cocktail of trace contaminants throughout the patrol duration.These residual atmospheric contaminants must be monitored to ensure that the ambient concentrations are maintained ‘as low as reasonably practicable’ (ALARP) and that any variations from the norm are recorded and suitable corrective action taken. Real-time monitors are embarked to record ambient concentrations of the critical gases, and also trends in organic contaminants. Adsorption tubes are provided to take regular air samples for retrospective analysis at a shore based laboratory. An inventory of organic compounds typically found in submarine air has been used to produce a list of target compounds, which are required by the RN medical experts to be routinely quantified for each patrol.A simple method of sampling the air using custom-made Tenax tubes is used to pre-concentrate the contaminants prior to laboratory analysis. The concentrations of the target compounds are recorded on a database for assessment and medico-legal purposes. In addition, to enable the close control of the critical contaminants, action levels must be set to enable decisions to be made in real-time on compounds likely to have an acute effect on submariner health and performance.Consideration must also be given to compounds posing a possible longer-term risk to health, which can be more suitably monitored post-patrol. Limits for submarine contaminant control have been set based on current UK and international occupational exposure standards. It was considered more appropriate to apply action levels, known as maximum permissible concentrations (MPC), to those compounds likely to impair submariners' performance, and to keep time weighted average continuous exposure standards (CES) for compounds likely to have longer-term health implications. This paper presents the concepts behind the standards and the regimes adopted by the RN for monitoring the trace organic contaminants.