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From: Food Quality & Safety magazine, April/May 2005

Separating the In from the Out

Laser particle counter helps food plant managers measure and control airborne contaminants.

by Eric Hudson

More than the people or the equipment, the one element virtually guaranteed to contact food during processing is the air inside the plant. Indoor air can become the vector that delivers contaminants to food and food contact surfaces. HVAC systems are critical control points (CCPs) in ensuring that food production remains safe from pathogens, allergens and other airborne contaminants.

In-plant air quality must be managed as part of the HACCP program. As FDA advises in its 21 dFR 110.20 document, the food processing plant and facilities shall “locate and operate fans and other air-blowing equipment in a manner that minimizes the potential for contaminating food, food-packaging materials, and food-contact surfaces.”

Best practices incorporate a consistent program to test and monitor interior air quality, including regular testing for levels of airborne particulates. Airborne particles may include allergens or other food contaminants, such as dust from deteriorating lead paint, and particles that can carry disease-causing microorganisms.

Regular air quality testing, including the use of hand-held particle counters to monitor particulate levels, can help ensure that changing conditions are detected early and emerging problems are dealt with before production is disrupted or, even worse, contaminated product reaches consumers.

Keeping the Outside Out

Sanitary design to control airborne hazards starts outside of the facility. Any contaminants found outside of the plant, such as insects, rodents, birds and windborne microbes contained on dust particles, are liable to enter. Outside air may contain high counts of particles carrying microbes, and particle concentrations indoors tend to “track” those outside.

The FDA identifies maintenance of the plant and grounds (trimming weeds, maintaining roads and parking lots and providing adequate drainage) as an important factor in preventing contamination. Best practices in food plant operation prevent the intrusion of outside air. Doors and windows must be kept closed and loading doors must be closed or, during operations, sealed to limit the entry of contaminated air, insects, birds and rodents.

In addition, air inside the plant should be maintained at a higher pressure than outside. The HVAC system is managed to draw in some outside air, condition and filter it and pump the filtered air into the facility to maintain positive pressure. Inside air will prevent contaminated outside air from entering through any gaps, or when doors or windows are opened.

Best practices in sanitary design and operation of HVAC systems emphasize maintaining positive air pressure, air filtration to remove particulates, and maintaining the system to prevent microorganism growth.

Incoming air must be filtered. Many plants use standard spun-glass or low efficiency filters, which only capture up to 35 percent of particles 50 microns (µm) or larger. Particles 10 µm or greater in size typically settle out of the air stream, so such filters do little to remove smaller airborne particles.

Increasingly, food plants use HEPA filters that are 95 percent efficient at 5 microns (µm). This level of filtration will remove many dust particles carrying microbes and will also trap most allergens in recycled air.

The HVAC system must be properly maintained to prevent air quality problems. Particles generated inside a processing plant are often organic in nature, providing a food source for microorganisms. These particles may settle in ductwork or collect on fan blades and cooling coils. The right combination of temperature and humidity can contribute to the rapid growth of bacteria and/or mold, and turn the HVAC system into a breeding ground for contaminants.

Indoor Air Diagnosis

Identifying and solving indoor air quality problems before they become costly require a consistent and methodical approach to air quality testing and diagnosis.

The process starts with collection of accurate data detailing the levels of airborne particulates. One tool that has proven highly useful is the hand-held laser particle counter..

A particle counter operates by pumping an air sample of known volume-generally one liter-past a laser beam. As dust particles in the air stream pass through the beam, each one reflects or “scatters” the laser light. A photodetector senses the scattered light and generates an analog electrical signal. Larger particles scatter more light and create higher-voltage electronic “hits.” Onboard electronics track the hits and count the particles in six size categories, ranging from .3 µm to 10 µm. Particles larger than 10 µm generally settle out of the air.

Particle counters can take new counts as often as needed, from every second up to once a day, and store 5000 records of date, time, counts, relative humidity, temperature, sample volumes, alarms and location label records in its onboard memory. Stored readings may then be downloaded to a personal computer. As an alternative, the particle counter can be connected directly to a PC for real time downloads.

The accuracy, speed and compact size some particle counters make them useful for baseline particulate testing, problem detection and remediation and system maintenance.

  • Baseline Testing: This determines where particles come from and what they are. How does the HVAC system perform in controlling contaminants from inside or outside of the facility? It is a good practice to test particulate levels both inside and outside. Ideally, inside air will be cleaner and contain fewer particulates than outside air. In addition, plant operators must determine the source of indoor particulates. Do particles come from outside, from plant operations, from the building interior, equipment or occupants, or from a contaminated HVAC system?

    In addition to identifying the number and source of particles, it is important to know what the particles consist of. Using a portable sampling device, airborne contaminants are captured on a biological medium (a petri dish or agar strip), then analyzed for type, frequency and size. Cultures can be grown and analyzed in the laboratory to determine what kinds of bacteria, molds or yeast are present, what particle size each is associated with, and what health problems they may pose.
  • Tracking Down Contaminant Sources: A hand-held counter is easily moved around the facility to quickly test particle levels and identify problems at multiple locations. If levels are higher by the loading dock, loading procedures should be refined-for instance, reducing truck idling time or minimizing the time doors are open. If specific machines or operations produce high levels of particles, dust collectors or modified processes may cure the problem. If particle levels are high where HVAC systems supply conditioned air, air ducts or cooling coils may need cleaning, or filters may need maintenance.
  • Maintenance Testing: This is done periodically to check particle levels, identify any emerging problems and verify that remedial measures remain effective. An increased particle count could show that the HVAC ducts have developed a leak, filters need renewal or staff members have strayed from standard operating procedures. Higher counts could even indicate that mold or bacteria are growing inside the HVAC system.

If baseline testing has shown that a specific pathogen is associated with a specific size of particle, an increase in that particle size could provide early warning that the organism is reproducing. Verification would require sampling and lab analysis.

In conclusion, airborne contaminants pose significant risks to food producers. Productivity, profitability and reputation all could be harmed by an incident of airborne food contamination, especially a food recall. Laser particle counters provide plant operators a convenient, low cost tool to monitor air quality, identify sources of airborne particulates and provide early warning regarding the emergence of contaminant sources. Given early warning, managers can resolve underlying problems and reduce the company’s risk and liability.

Eric Hudson is marketing manager of Fluke Corp.’s Indoor Air Quality Division (Everett, Wash.). Reach him at 425-446-5334 or



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