The inhalation of airborne asbestos fibres in the home or workplace can cause a variety of occupational diseases, which may ultimately lead to premature death. It strength and shape (or aspect ratio) of the fibres which means they tend to remain in the lungs for a long period of time that leads to respiratory diseases. The shape of the fibres does, however, make them distinguishable to the trained eye for counting during analysis.
Exposure can come from asbestos mining activity which, despite the known health risks, is prevalent in a small number of countries, or when asbestos-containing materials are disturbed e.g., during building maintenance, refurbishments, or demolition. Trades that are likely to disturb asbestos-containing materials include:
Where asbestos is known to be present, it is a legal requirement for licenced contractors to undertake the removal following strict regulations and guidance which exist to limit the potential release of invisible, airborne fibres, such is the extremely hazardous nature of this naturally occurring mineral, once ironically hailed a ‘wonder material’.
The guidance, depending on national practice, typically includes personal air sampling and/or a static (also known as area) air sampling, to ensure that there is no exposure during remediation work or that there is effective cleaning and clearance of a removal site. The so-called four stage clearance process, follows asbestos removal and involves:-
Stage 1 Preliminary check of site condition and job completeness
Stage 2 A thorough visual inspection inside the enclosure/work area
Stage 3 Air monitoring
Stage 4 Final assessment post-enclosure/work area dismantling
Stage 3 typically uses a static pump which has a much higher flow rate capacity than its smaller, personal counterpart. This type of test is to check that the airborne fibre levels in the asbestos work area are below the clearance level indicator, the limit of quantification in the UK of 0.01 fibres/cm3, prior to removal of the asbestos work enclosure and reoccupation of the area. Sampling may also be done prior to work being undertaken to establish a background level and during work on or near asbestos to confirm there is no leakage from the enclosure often called ‘reassurance’ sampling.
The use of asbestos dates back at least 4,500 years. Evidence found near Lake Juojärvi in Finland, shows that people used it to make pots and other cooking utensils. In Greece, from around 300 BC there was a reference to a material that is thought to be asbestos, the name derived from the ancient Greek word meaning “unquenchable” or “inextinguishable”.
Several hundred years later, ancient Roman scholar, Pliny the Younger (61-112 AD) wrote that slaves who mined and worked with asbestos became ill, though it would be nearly 2,000 years before scientists discovered the cause of this mysterious illness.
Once hailed as a ‘wonder material’, asbestos use was widespread as a building material because it was strong, cheap and, often only now thought of as a hazard in the construction industry, used for just about everything in the early 20th Century home, including hairdryers, toys, oven gloves, even clothing! Its excellent electrical insulating and heat resistant properties made it suitable for many diverse applications.
However, as far back as 1918, the U.S. Bureau of Labor released a report that revealed abnormally high risk of early death among asbestos workers and later, during the 1960s, further evidence emerged indicating that asbestos fibres posed a dangerous medical risk. Today, mention the word asbestos and it is probably safe to say that most people would know that it presents a danger to human health.
A clean site will eliminate future asbestos exposures and consequential risk to the health of occupants or workers. However, an HSE UK report [¹] found that were sampling deficiencies and in summary it said that “previous inspection studies and anecdotal evidence from UKAS [²] have indicated that independently audited four stage clearance (4SC) procedures produce more clearance failures compared with unchecked activities. Concerns have also been expressed about several other 4SC matters including: the extent of additional cleaning by analysts before a clearance pass, discrepancies in sampling and analysis procedures and personal protective clothing. There has also been concern over the quality of personal sampling carried out for licenced contractors.”
So, what are the sampling requirements? Generally, sampling involves drawing a known flow of air through a filter for a measured time (or volume) so that any airborne fibres are collected. The filter is then prepared for microscopical examination at an approved laboratory. The calculated total number of fibres collected on the filter is divided by the volume of air sampled to determine the fiber concentration.
Sampling requirements may differ depending upon the type of microscopical analysis employed but typically uses a 25mm MCE gridded filter, housed in a conductive cowl especially designed for asbestos sampling. In the UK, the detailed requirements are laid down in guidance document HSG 248 [³]; an up-issue has been in draft since 2015 and is expected to be released during 2021. Both HSG 248 and US method 7400 [⁴] uses phase contrast microscopy (PCM). PCM is a technique that converts phase shifts in the light passing through a transparent specimen to brightness changes in the image. Phase shifts themselves are invisible but become visible when shown as brightness variations. It facilitated the counting of all the fibers seen greater than 5µm and less than 3µm wide with a length to width (aspect ratio) of greater than 3:1 in a known number of graticule areas.
US method 7402 [⁵] uses transmission electron microscopy (TEM). This technique uses a beam of electrons transmitted through a specimen to form an image formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device.
Because of the hazardous nature of asbestos, there is extensive guidance on the removal, inspection, and air sampling but as the HSE pointed out, there is still a margin for errors.
- Use the correct method i.e., personal and/or static sampling. If in doubt check local guidance e.g., in the UK, those from the Asbestos Removal Contractors Association (ARCA) [⁶].
- Follow best practice with respect to your own exposure safety and use ‘remote control’ methods where available e.g., Bluetooth & phone app.
- Don’t use the results of a static test to determine an occupational/workplace exposure limit (OEL/WEL). Personal sampling should be used and for at least 1 hour.
- Ensure that equipment meets the relevant standard(s) and has the required flow-range capability e.g., up to 16 L/min
- Read the manufacturer’s handbook before using the equipment to ensure correct use and in order to get the most from its features.
- Ensure equipment has been maintained in accordance with the manufacturer’s instructions and don’t exceed the service life (modern instruments will have an indicator)
- Check that the sampling pump has been fully charged if mains power is not available.
- Position the sampling head at circa 1.6 m above the floor.
- Set the required flow rate, typically 12 L/min.
- Calibrate the flow before use, using a suitable gauge or flowmeter.
- Run for the full time and collect the required volume. Modern pumps have the ability to do timed or volume runs.
- Handle the sample with care using tweezers and transport it in a suitable container or if using a pre-loaded cassette, replace the cover and don’t forget to ship the required number of blank samples.
-Decontaminate equipment after use and select an air sampling pump that has a good Ingress Protection (IP) rating so it can be decontaminated easily.
The items mentioned above are a finite list and there are lots of other considerations, and there is lots of further reading and training available to get to a required level of competence. If unsure consult with a reputable association in your local country.
1. Report on the Asbestos Analyst Inspection Programme 2015, HSE, November 2018
2. UKAS, the National Accreditation Body for the United Kingdom www.UKAS.com
3. HSG 248 Asbestos: the analysts’ guide for sampling, analysis and clearance procedures, HSE 2005
4. NIOSH Manual of Analytical Methods. Method 7400 Asbestos and other fibers by PCM
5. NIOSH Manual of Analytical Methods. Method 7402 Asbestos by TEM
6. Guidance on Monitoring of Airborne Asbestos Fibre (GN001-V1015), ARCA, 2015.