The handling of high quality optics requires cleanroom facilities to prevent even microscopic dust particles from entering the system. These dust particles would eventually be attached to the mirror surface and burnt into it by the intense laser radiation of about 3 kW/cm2. As the handling of the high quality optics naturally takes place in the vicinity of and even in the tanks at least the surroundings of the tanks have to be of cleanroom quality.

The GEO600 cleanroom facilities are arranged in nested levels. The entire building is pressurised with filtered air reducing the particle count by about an order of magnitude. The Central Building visitor gallery and electronics areas can be entered in regular "street" clothing only wearing shoe covers to keep outside the worst dirt from the street.

The inner part of the building is sealed and again pressurised with filtered air, with respect to the visitor gallery. In addition the inner air conditioning system includes High Efficiency Particulate Air (HEPA) filter stages reducing the particle count by about two orders of magnitude.

When scientists are working inside the cleanroom, an additional venting system circulating air through HEPA filters further reduces the particulate contamination level. Full cleanroom garments and gloves are required for work inside the cleanroom.

When opening a vacuum tank for work on the optics inside, an additional moveable tent is placed over the tank and HEPA filtered air is blown into this tent at a rate of about 1 cubic metre per second. This reduces the particle count inside the tent by another few orders of magnitude below the detection threshold of the particle counters used.

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