Nikolai Budarin, flight engineer for ISS Expedition 6, poses near the Potok 150MK air decontamination equipment in the Zvezda Service Module. Early evaluation of the internal environment during the first years aboard the ISS from air, water, and surface samples provided a baseline of the contaminant environment and potential health hazards. Maintaining a safe, healthy, and habitable environment on board the ISS has been one of the ongoing challenges over the years. Essential health and environmental systems are critical to life in orbit, and the ability to assess these systems--and make repairs and software updates--while simultaneously keeping everything running, is a difficult-to-master skill.