In eye surgery under local anaesthesia, a micro-climate develops under the operative field. Respiratory CO2 concentration, measured by an infrared capnograph, increases rapidly and becomes stable at a certain level. A three-dimensional relation has been established (CO2 concentration X weight of the patient X free space under the field). The effects of the accumulated CO2 are complex and partially depending on premedication. The air inhaled by the patient should be renewed by elimination of the excess CO2.