According to the latest data from the United States Geological Survey, domestic consumption of asbestos in the U.S. has fallen from a peak of more than 800,000 metric tons in 1973 to a little more than 350 metric tons in 2015. But the long latency of asbestos-related diseases ensures that the health consequences of asbestos will be with us for years to come. And despite the well-known risks of exposure, some 2 million metric tons of asbestos were produced worldwide in 2015. 

The once-ubiquitous use of asbestos in products ranging from insulation and roofing materials to coatings and plastics, combined with continuing production in countries such as Russia and Brazil, ensure that sampling for asbestos will long remain relevant for industrial hygienists. In the U.S., renovation of older buildings containing asbestos and the thicket of regulations governing such projects will continue to require knowledgeable professionals for sampling and abatement. And for some of the laboratories that serve IHs, analyzing those samples remains a source of steady business. WHO'S COUNTING While many IHs are likely familiar with the sampling procedures for asbestos, they might not know what happens to those samples at the lab, and how labs arrive at the results they report to IHs. Most analysis of asbestos air samples follows NIOSH method 7400, which is a fiber-counting method. (NIOSH is currently revising the method; see the sidebar below for more information.) Microscopists prepare samples by opening up the cassette, cutting out a portion of the filter, and mounting it on a slide, typically through use of acetone vapor and triacetin resin. The method calls for phase-contrast microscopy (PCM), a technique that renders visible the changes in light waves as they pass through objects, at a magnification of 400x.

Because PCM can’t distinguish between asbestos fibers and non-asbestos fibers, and because exposure to other elongate mineral particles (EMPs) has been associated with adverse health effects, the method instructs microscopists to count all fibers in a field that have certain dimensions. The fibers must be at least three times as long as they are wide, with a minimum length of 5 µm (microscope calibration using the HSE/NPL test slide determines the minimum detectable fiber diameter of 0.25 µm). To identify fibers in the appropriate size range, microscopists use the Walton-Beckett graticule, which has gradations to assist with accurate sizing. 

The counting rules for the 7400 method might strike an uninformed observer as surprisingly complex. Microscopists are instructed to count fibers that cross once through the boundary of the circular field, but not fibers that cross twice—in other words, if you can’t see at least one end of a fiber, you shouldn’t count it. Asbestos fibers often occur in bundles of fibrils; if the fibrils seem to be part of the same bundle, the whole bundle counts as one fiber. According to the method, microscopists must count at least 20 fields. They are to stop counting when they reach 100 fibers, or when they’ve counted 100 total fields—whichever comes first. The average number of fibers per graticule field is used to determine the number of fibers on the filter.

Depending on the quality of the sample and the loading of fibers on the filter, the 7400 method can require a lot more time than the typically low cost of PCM analysis might suggest. 

“People probably think it doesn’t take very much time,” says Derek Popp, the head of quality control at the Wisconsin Occupational Health Laboratory in Madison. “But if the filter’s not heavily loaded, you’re counting 100 fields, and it just takes a lot of time to go through it.”

Popp knows about asbestos analysis from firsthand experience. He spent four years as an analyst in the late 1980s, when asbestos was front and center in the public consciousness. Congress had passed the Asbestos Hazard Emergency Response Act (AHERA) in 1986, which established inspection and abatement standards for asbestos in school buildings and led to a surge of asbestos-related work. (AHERA required a different method for counting fibers in air samples: transmission electron microscopy, or TEM.) At the time, Popp estimates that his lab was taking in approximately 50 bulk and air samples a day.

“It gets pretty tedious when you’re spending eight, nine hours on and off a microscope,” he says.

While the amount of asbestos work at Popp’s lab has tailed off since then, he still sees asbestos samples, usually following demolition projects.

Asbestos analysis still accounts for steady business at EMSL Analytical, Inc., according to Scott VanEtten, CIH, EMSL’s national director of industrial hygiene. EMSL currently has a number of analysts listed on AIHA’s Asbestos Analyst Registry (AAR), a program that establishes standards for quality control related to the practices prescribed by the 7400 method.

VanEtten is well aware of the challenges of fiber counting. “It takes a special person to do microscopy work,” he says.