In deciphering environmental monitoring program data, it is always surprising to find a wayward Listeria, Campylobacter, Vibrio, or some other foodborne pathogen residing on an electric motor, light fixture, or wall appurtenance. The microbe did not get there under its own mobility, but rather it was propelled into the air and found its way to the strange landing place via an air current.

Air and its physical characteristics are known to contribute to cross-contamination. The air bioburden is assessed by capturing bioaerosols, with a view to preventing recontamination. In doing so, basic questions can be answered:

• What is the origin of bioaerosols?
• How did they become airborne?
• What measures can be taken to mitigate risk of their generation or prevent them altogether?

Bioaerosols are the result of personnel movement, food processing, cleaning, and conditions created by building mechanics, structure, and anything else that we can see or imagine, including entry from the outside. Capturing and characterizing these bioaerosols is not an easy task. It is both time-consuming and costly, and may not provide the clues needed. When dust is taken into account, the picture becomes vastly more complicated.

The Conundrum of Dust

Regardless of what is said about it, dust can be a remarkable tool in assessing airborne cross-contamination, the presence of allergens, the operation and efficiency of a ventilation system, and tracking people activity. Since dust is a mixture of tiny particles of solid matter, it is fairly easy to sample and is stable for analysis.

Dust can include bacteria, smoke, volcanic ash, salt crystals from the ocean, microplastics, tiny fragments of plants and animals, dust mites (Arachnida), plant trichomes, pollen, fungal spores, and small bits of dirt or rock including sand. Dust that is generated in the indoor environment can also contain bacteria, skin, hair, fungal spores, cleaning chemicals, dry foods, fillers, and additives, along with airborne food allergens.

Dust particles range in size from 1–400 micrometers (μm; 1 μm = 0.001 millimeter [about 0.00004 in.]). Particles larger than 100 μm (0.1 mm [0.004 in.]), which is about the width of a human hair, are visible to the unaided eye. By identifying various dust particles and estimating their size and mass, we can infer their origin. While dust identification cannot be equated to disease transmission or pathogen contamination, it does give us a clue to the environmental conditions that may preclude illness and cross-contamination.

Sampling dust is straightforward and provides answers in real time, rather than requiring time for growth in a culture medium. Identifying the specific particles in dust to determine their source takes a bit of practice; once mastered, however, it makes the determination of contamination controls and preventive measures much easier. There are two basic approaches to sampling dust: air sampling and sampling horizontal surfaces. Both applications provide a wealth of information.

Dust Sampling Toolkit Essentials

Surveying, sampling, identifying, and classifying dust requires only a few additional elements in a sanitarian's inspection tool kit. Three additional components or tools are needed to complete a dust survey and to examine and catalog the samples.

The first tool needed is an air quality monitor/particle sampler. There are several on the market that meet or exceed requirements for dust sampling, are reasonable in cost (approximately $500), and are easy to operate. The preferred air quality monitoring system ideally measures two particle size parameters: those that are greater than 0.5 µm and those that are greater than 2.5 µm in size. The sampler should measure for both particle concentrations and mass concentrations, with the latter being the most frequently mentioned in the popular literature and most readily understood. A particle sampler should also provide flexibility of both continuous and intermittent operation. This is particularly useful in monitoring dust loading generated during construction and maintenance, as well as operations during shipping and receiving when goods are moved and the facility is open to the outside. These features, in addition to allowing the inclusion of a "correction factor" that is provided by the closest Environmental Protection Agency (EPA),¹ provide a wide range of sampling and interpretive options depending upon the environmental conditions (both activity and structural) found at the site.

The second essential component is an illustrated particle atlas.² A good-quality, used copy can be found through online book sellers. The atlas provides images of dust particles and insights into sampling methods and classification of dust particles by using various, basic optical and simple spot test analyses.³ While it is not necessary to be exacting in identifying specific dust particles, the particle atlas helps categorize dust into classes (biological and chemical), from which the origin of the dust can be inferred and its potential for contamination/cross-contamination postulated.

The third essential element is a basic optical microscope (any type) with a polarizing lens or polarized light. A dust sampling field kit consists of an ATP fluorescence detector and surface testing swabs. The ATP kit measures biological "dirt" and is an essential tool for classifying dust, among other applications.^4,5 Ultraviolet (365 nm) and LED white light (> 200 Lumens) flashlights show contrast and shadows of dust on surfaces, as well as airborne dust particles. Transparent adhesive tape is used for lifting dust from the surface and affixing it to a microscope slide for counting and characterization. A thread counter/linen tester with a scale plate is used to measure actual area on the slide. A good-quality magnifying glass or loop magnifier can also be used. Finally, add an inspection extension mirror and some means to check airflow and air current. These tools help characterize the environment by interpreting air quality monitor data.

Sampling Dust

To take a dust sample from any solid surface, press a short piece of cellophane tape to the surface and then mount the tape (sticky side down) on a glass slide (or another strip of tape, if a slide is not available). Characterize the dust by shape (thin, round, filamentous, granular, cubic) and count visible dust particles using a 10x magnifier (thread count/linen tester). Since linen testers have a 0.5-in. × 0.5-in. (1.27 cm × 1.27 cm) scale plate field for counting, it is a reproducible template and, therefore, enables real-time objective comparisons before and after cleaning, or for any other parameter under study.

To aid in characterizing dust on the slide, illuminate the slide with the white and ultraviolet light. These slides also serve to characterize the dust using a microscope. Further characterization of dust particles can be done under white light, a polarizing filter or polarized light, and an ultraviolet light to provide contrast. Simply characterize what you see using the particle atlas comparison.

When taking initial samples, try to find the cleanest and the dustiest places (strictly subjective) to give an idea on dust mass-concentration limits. These limits can subsequently be used to develop a gradient in establishing cost-effective cleaning protocols, ventilation maintenance schedules, or contamination control strategies. Use an inspection mirror to locate dust accumulation on high sills and dust bunnies (yes, that is the proper term) in areas with little air movement. For simplicity after counting, set either a two-, three-, or four-point gradient of the count to illustrate strategies and establish cleaning and maintenance protocols. Gradient(s) are a good indication of air patterns and people movement. They provide information on the air cleaning capability of ventilation systems over time and verify where the most dust is generated and, hopefully, from where it originates.

Upon further sampling, use either a simple stratified or random (MIL-STD-105E)⁶ pattern for a reasonable statistical assurance of repeatability.

Putting the Data to Work

Due to ventilation and activity, dust counts should not be used to measure effectiveness of cleaning; rather, they are meant to demonstrate where extra cleaning efforts should be directed. These data can be useful in developing an acceptable labor and time-saving cleaning strategy in an effort to lower the risk of airborne contamination. Like learning any skill, once mastered, dust sampling becomes a valuable quality control tool.

In a future column, I will detail and discuss air quality standards and requirements in food production, including air filtration, to match the type of production, best practices for ensuring air quality, and air quality monitoring recommendations. Including dust analyses will aid in compliance efforts and determining need for capital expenditure to control aerosol generation and movement, staff training, and further development of control strategies including cleaning and disinfection.

References

1. U.S. Environmental Protection Agency (EPA). "AirNow Fire and Smoke Map: Extension of the US-Wide Correction for Purple PM2.5 Sensors Webinar Archive." May 19, 2021. https://www.epa.gov/research-states/airnow-fire-and-smoke-map-extension-us-wide-correction-purpleair-pm25-sensors.
2. McCrone, W.C., J.G. Delly and S.J. Palenik. The Particle Atlas. Second Edition. Ann Arbor Science, 1979.
3. Jungreis, E. Spot Test Analysis: Clinical, Environmental, Forensic, and Geochemical Applications. John Wiley & Sons, 1996.
4. Powitz, R.W. "ATP Systems Help Put Clean to the Test." Food Safety Magazine June/July 2007. https://www.food-safety.com/articles/4080-atp-systems-help-put-clean-to-the-test.
5. Powitz, R.W. "The 'M' in ICM: Using ATP to Evaluate Sanitation." Food Safety Magazine April/May 2009. https://www.food-safety.com/articles/3933-the-me-in-icm-using-atp-to-evaluate-sanitation.
6. U.S. Department of Defense. "MIL-STD-105E: Military Standard: Sampling Procedures and Tables for inspection by Attributes." May 10, 1989.