Laminar Flow Hood

A laminar flow hood consists of a filter pad, a fan and a HEPA (High Efficiency Particulates Air) filter. The fan sucks the air through the filter pad where dust is trapped. After that the prefiltered air has to pass the HEPA filter where contaminating fungi, bacteria, dust etc are removed. Now the sterile air flows into the working (flasking) area where you can do all your flasking work without risk of contamination.
That's why you should nv block the top of flasks with hand.

shaded areas are filters