Laminar Flow: Should it be Horizontal or Vertical?

Brent Lloyd, RN


Clients often ask me which type of hood they should use for their laminar airflow- a vertical or a horizontal one. The short answer is it depends. The better answer involves a series of questions that will help deliver a better clean process.


Laminar flow is the unidirectional flow of clean air in either a vertical or horizontal orientation. Clean air is usually defined as either HEPA (high-efficiency particulate air) or ULPA (ultra-low particulate air). The goal of laminar flow workbenches is to protect the process and product being produced under the hood.

The types of functions that require laminar airflow systems include criminal forensics, smart device production, microchip and electronics production, industrial applications, optics and lens manufacturing, pharmaceutical compounding, microbiology research, and medical device manufacturing, among others.


A laminar flow system has a stainless steel, powder coated steel, glass or acrylic structural frame (the hood) that houses the other components. The hood holds a HEPA or ULPA air filter, a motor to draw air into the filter and out over the work area, and a stand (fixed or adjustable) to hold these parts above and behind the work area.

Each airflow direction offers its own advantages. For example, horizontal laminar airflow allows a sensitive object to be placed closer to the air filter. The motor blows through the filter, producing clean air that protects the products from contamination by blowing particles off the surfaces of products and sending them out and away.


In contrast, the advantages of vertical airflow are that it is not blocked by large work objects such as microscopes, and that the vertical stream is not continuously blowing air directly into the technician’s face, which can cause discomfort (especially when the operator is in the hood for hours at a time).


Guidance for choosing the right airflow direction


There are five questions to ask when determining which airflow orientation is optimal for a particular process:


1. Are there any regulations within the industry that dictate a specific orientation? For

example, USP specifically references both vertical and horizontal airflow as being acceptable for sterile non-hazardous compounding, but most airflows have been horizontal in my USP 797 experience.


2. Are there any internal standard operating procedures that specify horizontal vs vertical? How SOPs are written determines how the operators are trained. This training is usually related to ‘first air’ and how critical the process is. First air is defined as the undisturbed air coming directly from the filter source. Anything inside the hood (IV bar, analytical scales, lab equipment, product staged to be processed, etc.) can disrupt first air and potentially be a source of contamination. Process steps and contamination prevention are taught differently depending on the laminar flow orientation.


3. Will equipment be used inside the hood? If so, what are its dimensions? Are there fans involved to keep the equipment cool? Can the laminar air flow around the equipment and still maintain the integrity of the unidirectional flow?

4. Will you use visualization equipment (smoke tubes, smoke test powder, smoke generators, etc.) to assess how your specific process and equipment impact the airflow inside the hood? This will enable best practices and be a valuable training tool for operators.


5. Has operator comfort been considered? A horizontal hood has the clean air blowing in the face of the operator the entire time they are working in the hood, which can last an entire shift. The vertical orientation has the airflow going out the front (and sometimes the back of the hood) but not directly in the face of the operator.


The table below lists some sterile processes that require laminar airflow, shown under the orientation typically used. Note that nothing is set in stone, and that some processes (USP 797) may use both orientations. Some applications use a preferred hood material. For example, USP 797non-hazardous compounding and semiconductor work are typically done under a stainless steel hood, whereas plant tissue culturing and agar plate preparation often use hoods with glass side walls.


Protect the product and the process


Unless standards or internal SOPs dictate which airflow direction is best, the choice may be based on previous experience with that critical process, or it may be an individual decision. Ultimately, the goal should be to use the direction that maximizes productivity and reduces contamination risk. However, it should also take technician comfort into account if possible. Considering worker comfort may also optimize the process in the long run since a comfortable technician may be more productive.


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