The purpose of a laminar flow cabinet or a laminar flow hood is to pass HEPA or ULPA filtered laminar air flow across the hood to protect the work from the laboratory environment. Laminar flow cabinets are used in microbiology and in the semiconductor, pharmaceutical, medical device and microelectronic industries. Laminar flow is defined as movement of a fluid (or gas) in parallel layers. Air cleaned by a HEPA filter passes across the work in the hood to protect the work from contamination by the worker or the laboratory environment. Laminar flow cabinets do not protect the worker from any hazardous material in the hood. Laminar flow cabinets are either vertical or horizontal. The vertical laminar flow carries filtered air down from the top of the cabinet out the front and back of the cabinet. Vertical laminar flow is best when large objects such as microscopes are to be used in the hood. Horizontal laminar flow cabinets pass the filtered air across the work and out into the room. Horizontal laminar flow hoods are best for electronic parts and microbiology where the work can be placed close to the filter in the back of the cabinet. Vertical air flow may force particles from the hand or sleeve of the worker on to sensitive parts or into cultures. The choice of a vertical or horizontal laminar flow cabinet is determined by the work to be performed.