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Dangerous fiber
Asbestos fibers may cause lung cancer. New types of nanoparticles also present a high health risk. What kinds of fibers are contained within specific materials? How many particles get into the air we breathe? Within an occupational safety context, the technical office for dangerous occupational substances of a professional liability insurance association in Düsseldorf, Germany pursues these and other related questions. A transmission electron microscope, MegaView III digital camera and the iTEM image analysis platform are the investigators' tools.
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Avoid contact Since the 1930's, the carcinogenic effects of asbestos have been known. When asbestos fibers get into the lungs and get lodged there they may cause lung cancer. Even the tiniest amounts are dangerous. Fibers and dust may also cause asbestosis (a kind of 'black lung' disease caused by asbestos). The field of occupational safety today is thus doing everything possible to prevent any sort of human contact with asbestos. Preventing asbestos particles entering the air we breathe is a particular concern.
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Figure 1: Tracking down suspicious fibers: the side-mounted MegaView III digital camera transmits TEM images and diffraction images directly to the PC. The iTEM image analysis platform controls both camera and TEM and provides convenient processing, evaluation and archiving of all images and data at the PC. This greatly simplifies definite identification of fibers.
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SEM, TEM, EDX and moreWhen investigating materials that may contain asbestos, the standard approach is via Scanning Electron Microscopy (SEM). When determining the shape and chemical composition of suspicious fibers, an EDX unit is used in conjunction with Scanning Electron Microscopy (SEM) methods. The EDX unit and SEM make energy dispersive X-ray microanalysis possible. In some cases, this method is not sufficient for distinguishing various types of fibers. Then a Transmission Electron Microscope (TEM) is required due to its greater magnification and resolution. The TEM that we use in Essen, Germany is an EM 420 by Philips. Our investigative department is equipped with a scanning unit (STEM) and an EDX analyzer. The use of the TEM is advisable when, for example, the fibers in the material being investigated are very thin (in liquids usually only 30 – 80 nm in diameter). This makes them impossible to be seen while using a SEM. If the EDX spectra of the asbestos and other types of fiber in the specimen are almost completely identical, the TEM is required to detect the asbestos – in conjunction with electron diffraction (see below).
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Figure 2: Does an asbestos disease result from one's occupation? If the air at one's place of work (left) and in the lung tissue of an employee who contracted lung cancer (right) both contain the same type of asbestos fibers, this is a powerful indicator that there is a causal connection. TEM images thus assist to prove that the disease was caused at one's place of work and is thus to be classified as an "occupational disease". (for more detail on the preparation method used, see the main text)
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Recording and archiving digitally We use a MegaView III for digital acquisition of the TEM images. This CCD camera is side-mounted on the wide-angle port of the TEM column. The camera has a prism that is moved into the beam path of the microscope and intercepts the electrons in the intermediate image plane. The software for reading out the image data and for operating the camera and microscope is an integrated part of the iTEM image-analysis platform – and includes an autofocus routine. The camera and image analysis platform are by Olympus Soft Imaging Solutions. Images are immediately available in digital form and thus require no physical developing in a photo lab as in the past. Before acquiring, acquisition parameters can be optimized on the PC screen via live image. During acquisition, images are automatically calibrated. Once acquired, image quality can be checked immediately and archived in the well structured iTEM database along with all acquisition parameters. If necessary, evaluation data is saved there as well.
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Solid, liquid and gaseousWithin an occupational safety context, we inspect solid, liquid and gaseous specimens to determine potential health hazards due to asbestos and other fibers. Objects investigated include insulation materials, dust, chemicals, drinking water, air samples from work areas and lung tissue. All these substances require special preparation for a TEM investigation. This is the reason why we initially dissolve solid materials and dusts in liquid and ensure an even dispersion via ultrasound in fiber-free water. This solution is then filtered through a capillary pore filter with a pore width of 0.2 µm. Air samples and most liquids are directly impacted onto the capillary pore filter. The filter is then vapor-coated with carbon and removed. The asbestos fibers are thus located on a layer of carbon for the TEM acquisitions.
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Figure 3: Talcum powder is used in many applications industrially and in our day-to-day lives. It consists of the silicate mineral talc and may contain asbestos. Our analyses ensure that non-asbestos-containing talc is used and processed. The asbestos fibers are much more distinct than the lamellar talc crystals within the TEM images (a, b, c). This makes it easy to distinguish them from a talc fiber (d).
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Example: talcum powder Talcum powder is often used in industrial fields: for lubrication or as a parting compound or as a carrier medium for other substances. In day-to-day life we also run into talcum, eg, in cosmetics and medical products. Talcum consists of the silicate mineral talc. Depending on the source of the talc, talcum powder may contain traces of asbestos – sometimes as much as several percent. These asbestos fibers may get into the air and from there into the human body. The investigations we conduct are meant to ensure that no talc containing asbestos is used.
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Not all fibers are equal Talc is a layer lattice silicate and has lamellar crystals. On the TEM images these appear as plate-like, semitransparent regions (fig. 3a, b, c). The long, fibrous structures are chrysotile fibers (white asbestos fibers). There are talc fibers also (fig. 3d) which are significantly different than the asbestos fibers in shape, transparency and where they break. This talc thus contains asbestos and is not suitable for the above-mentioned industrial or day-to-day usage
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Figure 4: Chrysotile or crocidolite? Figure a is ambiguous. Alongside the EDX spectrum, the diffraction image of the crystalline structure provides a further identification indicator. This is particularly helpful when the specimen contains other fibers whose spectra are similar to or identical to asbestos. Fig. b shows the diffraction image of crocidolite. Chrysotile (c) looks different.
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Possible nanoparticle danger? Nanotechnology is viewed as one of the key technologies of the future. Nano particles that are just a few nm in size can present a health hazard, however. The toner particles shown in fig. 5 are used in laser copiers. They consist of a carbon center with a polymer casing. When a toner cartridge is replaced improperly, particles may escape to the surrounding environment and may be inhaled by people. As is readily visible on the images, these tiny particles have a strong tendency to agglomerate. Despite subjecting them to powerful ultrasound treatment, we were not able to separate these particles.
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Figure 5: Some new kinds of nanoparticles may be very dangerous when inhaled. This ink toner used in laser copiers consists of carbon nuclei with a polymer coating. The TEM images show how strongly the particles stick together.
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Using diffraction patternsAlongside images and EDX spectra, diffraction patterns can be generated via TEM. SAD (Selected Area Diffraction) electron diffraction makes it possible to draw inferences regarding the crystalline structure of a fiber. Diffraction patterns of asbestos are different than those of talc and other comparable silicate minerals. This makes it possible to clearly distinguish between asbestos and fiber-shaped particles which are similar in appearance and have a similar EDX spectrum. They behave differently if they make it into the lungs and have different consequences health-wise. This is what makes it so important to differentiate these fibers – for those in the occupational safety field, and for being able to assess asbestos-caused ailments resulting at the workplace.
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Asbestos – a dangerous matterThere are various types of asbestos. Some occur naturally such as amphibole-silicate minerals with a fibrous structure (including blue and brown asbestos) as well as chrysotile (white asbestos) a sheet silicate mineral with a serpentine fibrous structure. Due to its chemical stability, heat resistance and high electrical and thermal insulation capacity, asbestos was popular up through the 1970's. It was used a lot for construction – despite the fact that its carcinogenic effects have been known since the 1930's. It has been used for heat insulation, for fire resistance in construction, as a sealant material and as insulation and in brake pads. Asbestos may also occur naturally or as a production-related impurity in other materials. It also occurs in certain chemicals due to production processes.
Asbestos is dangerous when fibers or dust thereof are released and inhaled. The fibers of natural asbestos are so dangerous because they cannot be broken down biologically or at best, very slowly. This means they cause continuous irritation to lung tissue once inhaled. The critical dimension for fibers inhaled into the lung are longer than 5 µm, thinner than 3 µm and their length/diameter ratio is greater than 3:1. When asbestos fibers get into the lungs and get stuck there, they can cause lung cancer and asbestosis (a kind of 'black lung' disease caused by asbestos). Asbestos fibers also cause the rarely curable mesothelioma (malignant tumor of mesothelial tissue): the fibers penetrate the outer layer of the lungs and cause irritation to the pleura which ultimately results in cancer. Technical usage or processing of asbestos causes asbestos dust and microfine fibers may become separated. This makes protecting employees from any exposure to asbestos a job for occupational safety.
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Authors
Dr. Gisela Binde, Department for Prevention, "Dangerous Substances“ Unit, Confederation of the Professional Liability Insurance Associations for Mechanical Engineering and Metallurgy; and for Smelting and Milling [German original: Präventionsabteilung, Fachstelle „Gefährliche Arbeitsstoffe“ | Verwaltungsgemeinschaft Maschinenbau- und Metall-Berufsgenossenschaft und Hütten- und Walzwerks-Berufsgenossenschaft], Hoffnungstrasse 2, D-45127 Essen, Germany
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