A pass box is a small, enclosed chamber used to transfer materials between areas of different cleanliness, such as from a normal lab to a cleanroom, without allowing contamination. It is typically made of stainless steel, has two interlocked doors (so both cannot open at the same time), and may include UV lights or HEPA filters for sterilization. Pass boxes help maintain aseptic conditions by reducing personnel movement and preventing the entry of dust, microbes, or other contaminants into the clean area.