As having the appropriate protective equipment is critical in any experiment, investing in a laminar fume hood is essential to keep contaminants contained and out of the laboratory. This can be achieved by pulling pollutants from the hood's working environment away from the user, thus reducing inhalation and exposure.

When working with microbes, RNAi shuttles, or other compounds that can pose harmful effects on your health similar to a variety of common chemical reagents, you'll need the additional layer of protection that only a specialised type of hood can provide. By keeping harmful bacteria behind an air protective shield, a laboratory hood protects you from being exposed to them as well as keeping other toxic substances, from cross-contaminating and reacting with anything else. 

Air flowing at the same velocity in the very same way, with no or minimum cross-over of air streams or ‘lamina’, is referred to as laminar airflow. The upstream area nearest to the filter face will always be the purest due to the structure. This non-contaminating architecture is prevalent in the compounding of sterile products and applicable to both horizontal and laminar flow hoods. Depending on the factors such as clearance requirements, design of the work surface, process area, and the safety of the operator, there are reasonable grounds to use either airflow system. 

Basic differences between horizontal and laminar flow hoods

Vertical laminar flow hoods

Vertical laminar flow hoods are popular because, on a limited scale, they replicate the architecture of a laminar flow cleanroom, where the fan or the filter units are normally located in the ceiling. Vertical laminar flow enhances the influence of gravity and blows particles out of the confinement, usually through a front access area, by directing the laminar flow downward. Although micro-contaminants have small dimensions, most of them are likely to settle on a worktop or a room's floor, and vertical movement helps them get there quicker. When space is limited, a clean bench with a vertical laminar flow arrangement is often preferred. A vertical laminar flow hood can be mounted on a conventional lab bench since the fan or the filter unit is stacked on top of the hood in this design.

Pros 

• Since the hood isn't very  deep, it takes up less floor area
• It's simple to locate the filter on top
• Air does not blow directly towards the operator, and the sash acts as a barrier in front of the operator's face
• Cross-contamination of objects on the work surface is reduced
• Air striking huge objects or processing equipment produces less turbulence

Cons 

• Items or hands cannot be placed on top of other items as it can obstruct the movement of air
• A step ladder may be required to change filters or service the unit because of the overhead clearance limitation

Horizontal laminar flow hoods

To accommodate the rear-mount filter or fan unit, a horizontal-flow hood requires more depth as well as more backside clearance to facilitate air into the system. Due to the extreme clearance requirements, a horizontal flow system requires a deeper bench and more floor area than a vertical flow system. Even though both airflow designs produce excellent sweeping action near the filter face, their different flow patterns eventually encounter interruptions. The work surface is an evident major obstacle in a vertical flow hood. When working with liquids or small items, a perforated or rod-top work surface allows the laminar air stream to pass through the hood with minimal obstruction. A horizontal flow design may be preferred if you're working with these materials and wouldn't want to pick up items off the floor.

Pros

• Vertical airflow striking perpendicular to the work surface is eliminated by laminar airflow, which is parallel to the work surface and eliminates turbulent effects
• Because they are downstream of the sample, hands and gloves are often less contaminated
• It's easier to place and scatter sterile equipment and materials closer to the filter face in  the work area

Cons 

• Replacing or servicing filters frequently requires moving the hood for rear access
• When used without a sash, it has the potential to blow fumes or powders into the operator's face
• Large samples block laminar airflow and may contaminate samples further downstream

Conclusion: 

Both horizontal and vertical laminar flow hoods circulate filtered air throughout the work area, sheltering the specimens from contaminants. Laminar Flow hoods are ideal for a range of applications that require clean air and a sterile environment to operate properly, such as numerous medical and research laboratories, electronic device assembly as well as other sterile equipment.