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Laminar Flow: Should it be Horizontal or Vertical?

The nature of contamination in clean manufacturing

A wide variety of manufacturing industries need to prevent particles from entering their processes. For pharmaceutical production, viable (organic) particles affect sterility and compromise patients. In aerospace and automotive industries, particles cause visible defects in painted surfaces. Semi-conductor industries notice decreased yields due to particle contamination.

Technicians are the worst source of particles because their bodies constantly shed even while fully gowned. The faster people move, the more they shed.

In contrast, the contamination introduced by foot and wheel-borne traffic includes a plethora of organisms that are not as easy to eliminate with general-purpose disinfectants or sanitizers as the human-borne flora is. Some spore-forming bacteria and many mold spores are hard to kill even with sporicidal disinfectants.

In recent years industries and their regulators have become more aware of mold contamination. Some molds, like penicillium or aspergillus, can be killed by general purpose disinfectants and sanitizers such as phenolic and quaternary ammonium compounds and 70% IPA. On the other hand, many mold species that enter clean manufacturing plants through soil, or that reside on cardboard or pallets, are harder to eliminate.

Personnel and materials from uncontrolled areas and warehouses can transport contamination on shoes or the wheels of material carts. Leaving contamination outside the cleanrooms has become one of the key elements of contamination control. Particles, including microorganisms, can be easily dispersed in the air due to the turbulent flow of clean zones or controlled environments. Eventually any particles present will either be removed from a clean area through the function of the air-handling system, or deposited onto a surface because of gravity, convection, or diffusion. Once contact has been made with a surface, particles will adhere to the surface either “reversibly” (temporarily) or “irreversibly” (permanently) through a combination of chemical or electrostatic forces.

The purpose of sticky mats

The goal of sticky mats is not just to remove dirt and debris from operator shoes or cartwheels, but also to ensure that the removed dirt stays on the surface and does not become airborne. This is achieved with at least three footfalls or rotations of the cartwheels.

There are two groups of sticky mats: temporary (tacky mats or peel-off mats) and semi-permanent (polymer flooring systems). Tacky mats are made of a stack of adhesive plastic film layers that are periodically peeled off and discarded. They have an adhesive surface that is placed at the entrances or exits to certain areas to remove contaminants from the bottoms of footwear and wheeled carts.

In contrast, semi-permanent mats consist of polymer layers with a mesh backing. They do not use adhesives and are cleaned and dried with a mop and squeegee. Polymer mats work via Van der Waal’s force. When two surfaces come into contact, there is an attraction. When the shoe leaves the polymer mat, the mat is more attractive than the shoe, which leaves the debris “stuck.” Polymer mats also curl slightly around the shoe tread to attract contamination from more surface area.
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What the studies show

Dr. C.S. Clibbon at GSK performed a side-by-side study determining the effectiveness of each type of mat (published in Cleanrooms in 2002). With three footfalls, tacky mats showed 27% effectiveness of cleaning shoes and wheels, while polymer mats showed 99% effectiveness.
Why was there such a difference? Tacky mats collected particles, but the violent action of peeling off soiled tacky mats put previously trapped particles back into the air column. In fact, the 27% figure above did not account for recontamination after peeling, so this effectiveness may be questionable. Regardless, this study illustrated why polymer mat systems are the better choice. whe 

In addition, Dr. Tim Sandle's study in 2012 demonstrated a reduction in air particle counts when using polymer mats in gowning areas. This achieves the goal of collecting contamination before it enters the controlled areas.  

Human behavior plays a significant role in the successful collection of wheel and foot-borne contamination. For example, the small size of the tacky mat, usually 2’ x 4’, allows personnel to avoid the surface, which they may do because they find it annoying (the adhesive pulls the mat up with the shoe). People can also be careless or hasty or may skip rocking back and forth to achieve three footfalls. Adding to the challenge, the effectiveness of tacky mats is reduced when they become saturated (they need to be regularly peeled to achieve a fresh surface).

Polymer flooring systems have also been shown to reduce air particle counts in cleanrooms by trapping particles that could become airborne through disturbance or turbulence. They also reduce transference of hard-to-kill flora from soil, cardboard, and pallets that get onto shoes and wheels from uncontrolled areas to controlled areas.

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In my next blog, I’ll discuss best practices for the optimal placements, sizes, and maintenance of polymer flooring systems.



2. Austin P. Encyclopedia of Clean Rooms, Bio-Cleanrooms and Aseptic Areas. Livoria, MI: Acorn Industries, 2000.

3. Sandle, Tim. Examination of air and surface particulate levels from cleanroom mats and polymeric flooring. European Journal of Parenteral & Pharmaceutical Sciences, 2012; 17(3):110-119]

4. Abraham, Z and Polen, M, eds, Cleanroom Contamination Prevention & Control: A Practical Guide to the Science. PDA, 2021, Chapter 12: Foot and Wheel-Borne Contamination Control by Bell, Krishna, and Abraham, Ziva.

5. Clibbon, Dr. Caroline. GlaxoSmith Kline Study: Reducing wheel and footborne contamination. European Journal of Parenteral Sciences, 2002; 7 (1): 13–15.

6. O’Hanlon, John F., McGowan, Colleen E., Gustafson, Lisa M. Particle Shedding from Tacky Mats. Final Report For: Litto DeGuzman, Clean Line Corporation. University of Arizona, Department of Electrical and Computer Engineering. January 17, 1995.

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