Measuring Asbestos and Nikon

I started researching asbestos in 1974. The Industrial Safety and Health Law stipulated that industrial sites using asbestos must measure the concentration of asbestos present in the work environment and keep a record of their findings for the next 30 years.

When buildings collapsed as a result of the Great Hanshin-Awaji Earthquake in January 1995, we became aware that asbestos had been used in many building materials. And now, many buildings that were built around 1975 are reaching the end of their durability period. Asbestos became a problem in Japan at the same time that the national government was working on establishing domestic laws governing asbestos. Now the government is accelerating its measures to tackle this problem.

Asbestos was an extremely convenient material. The story of Hiraga Gennai (1729-1779), a Japanese inventor and scientist, having discovered asbestos in the mountains of Chichibu, northwest of Tokyo, back in 1764 and making cloth that didn't burn is well known. Going further back in history, asbestos was found in cloth that was used to wrap mummies in Egypt. Asbestos was also once used in the protective clothing of firefighters. Asbestos has been an indispensable material throughout much of human history.

Asbestos is a natural mineral that has been widely used as a building material due to its superior characteristics and low cost. Japan stands on strata of serpentine rock that includes chrysotile, a type of asbestos; so we have always been around such materials. Because of this, I believe there was little awareness that asbestos should be handled as a dangerous substance.

The International Agency for Research on Cancer classifies asbestos in Group 1, “Carcinogenic to humans.” Since the average diameter of mineral wool fiber is between 3 and 5 micrometers, up to 100 times thicker than asbestos fiber, it does not enter the respiratory system as easily and is classified in Group 3, “Not classifiable as carcinogenic to humans.”

Glass fiber has a similar shape to that of asbestos and also has the potential to pierce the lungs. At present, glass fiber is not categorized as hazardous material, but it certainly seems to fall into a gray area. I think glass fiber will be inspected more aggressively from now.

The JAWE was founded in 1979 as a specialized technical group concerned with the examination and control of working environments. Its main activities include carrying out working environment research and examination, holding various seminars, and providing technical guidance.

Inquiries skyrocketed after problems concerning asbestos were reported by the mass media. There were so many inquiries that our staff was overwhelmed.

Some time ago, we were asked to measure concentrations of asbestos in a particular working environment. We used an optical microscope (phase contrast microscope) and the count analysis method to measure the concentration of asbestos fibers in a given area, and the figures we came up with were extremely high. We thought these figures were strange considering the conditions of the workplace, so we asked another laboratory to check the samples. It turned out that the material was actually talc.

With an optical microscope we can only observe shapes. At that particular factory, a large quantity of fibrous talc was mixed with the asbestos, so all of it was included in the count. Talc and asbestos are two varieties of the same mineral. Talc's primary component is a hydrous silicate of magnesium.

Yes, exactly. An ordinary optical microscope will only let us see shapes. Measurements using X-ray diffractometry, on the other hand, work in the opposite way. They let us use qualitative analysis to identify the material, but we can't identify the shape.

In a directive concerning the method for determining the ratio of asbestos contained in sprays and other fire-resistant building materials issued in 1996, the shape of asbestos fiber must satisfy three conditions: Have a length of at least 5 micrometers; have a width of less than 3 micrometers; and have an aspect ratio between length and width that is at least 3:1. This means that even if the fibers are truly asbestos they are not counted, unless they meet all those conditions.

To take measurements through microscopic observation, we need to be able to determine the composition and shape of the fiber at the same time. Asbestos refers to six types of natural silicate mineral fibers. Three of these, namely chrysotile, amosite, and crocidolite, were mostly used in building materials in Japan. Not only has the dispersion staining method made it possible to measure asbestos, it has also made it possible to identify chrysotile, amosite, and crocidolite.

It was quite difficult, but we did take measurements with a phase contrast microscope and a polarizing microscope. The directive concerning specific dust concentrations related to asbestos issued in 1989 stated that a phase contrast microscope should be used to count fibrous material of over 5 micrometers in length and with an aspect ratio of at least 3:1. With this method, we can tell that the fibers are asbestos but we can't identify whether they are chrysotile, amosite, or crocidolite.

In observations through a polarizing microscope, we use a characteristic known as birefringence of asbestos. Fibers appear to have color or no color depending on the direction of each fiber's crystal. The observer needs to be highly skilled when carrying out polarizing observation. Also, because polarizing microscopes are specialized devices, they were not in widespread use.

Guidelines issued by the Ministry of Health, Labour and Welfare in June 2005 use the dispersion staining method with a phase contrast microscope for qualitative analysis. Then, for samples where dispersed colors have been confirmed with a 10x dispersion objective lens, the lens is switched to a 40x dispersion objective lens in order to count the fibers with aspect ratios of at least 3:1 and the total number of particles.

The point is to use the dispersion staining method and a 40x dispersion staining objective lens. The technology for the dispersion staining method was established by Dr. Walter C. McCrone of the McCrone Research Institute in the United States. McCrone Microscopes and Accessories, Inc. sells 10x dispersion staining objective lenses, and we have been combining these with cargill oil to perform qualitative analyses on asbestos in bulk samples.

Since dispersion objective lenses were not available in Japan at the time, we approached Nikon. I think it was around 15 years ago that Nikon completed prototypes of 10x and 40x dispersion objective lenses and started selling 10x dispersion objective lenses. In the beginning, we used them for observations with a polarizing microscope, but when we tried using the 10x dispersion objective lens with a phase contrast microscope, we discovered we could also simultaneously confirm the dispersion color of asbestos irrespective of the direction of the fiber crystal. This was a major breakthrough. When we used a prototype of the 40x dispersion objective lens at the same time, we were also able to identify airborne asbestos fibers and count them. This led to the possibility that considerable progress could be made with the count analysis method.

First, a filter is used to collect airborne dust. The filter is then fixed onto a glass slide for observation, and then the filter alone is burned with a low-temperature ashing device known as a plasma reactor so that the temperature can be held as low as possible. Due to the high fire-resistance efficiency of asbestos, it will remain on the glass slide.

The refractive index for chrysotile, amosite, and crocidolite is slightly different. After dripping refractive index liquid with the same refractive index as each of the three materials onto a slide and placing a cover glass over it, and observing it in the dispersion staining method, the color adheres only to fibers with the identical refractive index, making them visible.

Light has a property of dispersing when it passes through media with different refractive indices. This is due to the fact that the shorter the light's wavelength, the higher the refractive index, and the longer the wavelength, the lower the index. Sunlight is split into seven colors when it passes through a prism. Since the refractive indices of a prism and air are different, dispersion occurs at the boundary where they meet.

With a phase contrast dispersion microscope, only light of a wavelength that matches the sample (asbestos to be observed) and the refractive index liquid continues straight ahead without being dispersed. In the dispersion staining method, only the light that is not dispersed but goes straight ahead is removed, and the sample is observed in the combination colors of the spectrum of remaining wavelengths. Therefore, it is possible to differentiate between chrysotile, amosite and crocidolite, allowing individual fibers to be identified and selected for observation.

I am very grateful for Nikon's development of the world's first 40x dispersion staining objective lens, providing the idea of the rotating analyzer, as a researcher working at the frontline of these developments. To further improve measuring accuracy in the future, I would like an auto-stage to become standard equipment.

At the moment, we make observations of a single sample while changing the view field, but I think it would be very handy if it were possible to reproduce the process exactly. This would be especially useful when teaching observation methods to a number of people. In other words, when observing the same sample, if counting the same number of asbestos fibers in the same view field cannot be repeated, the data cannot be considered reliable. This feature could also be useful at seminars and in other situations where training in observation methods takes place.

Also, it would be nice if the auto-stage could keep the temperature of the sample at 25°C. The refractive index liquid is extremely sensitive to temperature change. I humbly urge Nikon to work on this. Besides the auto-stage, I look forward to the development of microscopes that are easy to use and meet the needs of the workplace.