All the satellite test areas at ESTEC are cleaner than a hospital operating theatre, so they are called 'cleanrooms'.
A cleanroom is an area where the air contains very few dust particles. Since there is no dust in space, it is important that there should be none during assembly and tests. A speck of dust in a switch or circuit can cause a spacecraft to fail. Dust on mirrors or camera lenses can make the satellite blind and so must be avoided.
All of the air in the cleanrooms is filtered to remove as much dust as possible. But this alone is not enough.
Everyone entering the cleanroom has to walk across a sticky floor-mat to remove dirt from his (or her) shoes.
They then put on a dust-free 'bunny suit'. Elasticated covers are placed over their head and shoes. Finally, they may put on gloves and a facemask.
Humidity (water content of the air) in cleanrooms is also kept fairly low, and temperature is kept close to 20ºC.
One very useful feature of ESA's Test Centre is the interlinking of the cleanrooms. This means that satellites can easily be moved to different test areas without having to be packed in containers and risk dust contamination.