Abstract: A system (100) for controlling the environment in a reaction box (300) comprises a controller (150) configured to control a gas multiplexer (130) to switch between applying an under pressure in the reaction box (300) from a vacuum pump (140) and applying a gas flow from a connected gas source (200) to the reaction box (300) multiple times in a cyclic manner. A particle monitor (160) generates particle information representing a concentration of particles in the reaction box (300). This particle information is stored as a GMP clean room classification notification for the reaction box (300).