Fibre optics for distributed environmental condition and/or air quality sensing such as, but not limited to, sensing air quality in enclosed areas of an aircraft such as the passenger cabin. Fibre optics can be used for sensing and for communication and are therefore well suited to the distributed sensing of environmental conditions such as temperature, relative humidity, gas and particulate composition within spaces of the aircraft e.g. the cabin, cargo space, cockpit, electronics bay etc. and in related systems such as the environmental control system (ECS), the cabin recirculation system, bleed air system etc.