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7.1 Exposure Pathways and Significance

Michael Kottek

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Introduction

​The manufacture and installation of asbestos containing building materials (ACM) inevitably results in exposure to asbestos for employees involved in the manufacture of ACMs and construction workers installing ACMs. The serious risk of asbestos related disease (ARD) for workers in these sectors is well recognised, having been established in numerous epidemiological studies. (1, 2)

​Based on epidemiological and toxicological considerations, many bodies have concluded that all forms of asbestos are carcinogenic, without a documented threshold exposure free from the risk of disease. (3, 4)

​With regards to a threshold exposure free from risk of disease the British Thoracic Society, state that:

  • ​There is no evidence for a threshold dose of asbestos below which there is no risk. However, the risk at low levels of exposure is small. (5)

Based on the limitations of epidemiological studies, Hodgson and Darnton (2000) noted that:

  • ​Direct statistical confirmation of a threshold from human data is virtually impossible. (6)

The Australian Faculty of Occupational Medicine guide on Occupational Cancer,(7) notes that nearly all cases of mesothelioma are asbestos related and that:

  • ​The implication is that mesothelioma can arise from asbestos levels close to background levels (i.e., the low levels in the general environment to which all urban dwellers are exposed).

It is sometimes claimed that the ACMs in good condition pose no measurable risk to health. However, the measurability or not of a risk gives no indication of the public health significance of the risk. There is a very large population of people who live or work around ACMs and given the limited power of epidemiology to detect small increments in risk; a non-measurable risk to health may still have significance as a public health issue, particularly given ongoing deterioration of some ACMs such as external asbestos cement roofing and cladding.

Assessment of Exposure

​Lippman has comprehensively reviewed epidemiological and toxicological studies to identify critical fibre dimensions for causing ARD, which are summarised in Table 1. (8)

Table 1 Critical Fibre Dimensions for ARD

Occupational exposure to asbestos is typically assessed using phase contrast optical microscopy (PCOM). Using PCOM, it is possible to resolve fibres with diameters greater than about 0.2 µm (double the diameter of fibres implicated in mesothelioma induction), while fibres shorter than 5 µm (implicated in fibrosis) are ignored. Measurements of exposure made by PCOM are only an index of exposure to asbestos and may not be accurately measuring exposure to fibres with the potential to cause ARD. Indeed, it can be seen on Figure 1 that the fibres implicated in mesothelioma induction are almost totally ignored when PCOM techniques are used. This has been demonstrated for environmental exposures by Lanting and den Boeft (1983) who found that nearly all the chrysotile fibres detected in ambient air 400 metres away from an asbestos cement plant were too small to be detected by PCOM (9).

Table 1 Critical Fibre Dimensions for ARD

Occupational exposure to asbestos is typically assessed using phase contrast optical microscopy (PCOM). Using PCOM, it is possible to resolve fibres with diameters greater than about 0.2 µm (double the diameter of fibres implicated in mesothelioma induction), while fibres shorter than 5 µm (implicated in fibrosis) are ignored. Measurements of exposure made by PCOM are only an index of exposure to asbestos and may not be accurately measuring exposure to fibres with the potential to cause ARD. Indeed, it can be seen on Figure 1 that the fibres implicated in mesothelioma induction are almost totally ignored when PCOM techniques are used. This has been demonstrated for environmental exposures by Lanting and den Boeft (1983) who found that nearly all the chrysotile fibres detected in ambient air 400 metres away from an asbestos cement plant were too small to be detected by PCOM (9).

Figure 1: graphic showing different critical dimensions including fibres countably by PCOM

Control of Exposure in the Occupational Environment

As a carcinogen without a verified threshold exposure, exposure to asbestos should be minimised to the maximum extent. Control of exposure to asbestos in occupational contexts should be based on the hierarchy of control.

Figure 2: Hierarchy of control graphic CDC

Considering the availability of substitutes, substitution should always be considered the most appropriate control measure. Relatively effective engineering controls can be implemented in manufacturing workplaces. However, it is very difficult to implement effective engineering controls and/or use PPE while ACMs are installed during construction work. Even with control measures in place, workplaces with compliant airborne fibre levels (as measured by PCOM) can still have significant levels of asbestos fibres with dimensions capable of inducing ARD.

​Ongoing Exposure Hazards

Wherever ACMs have been installed, their very presence creates an ongoing risk of exposure to asbestos and consequent ARD. Relatively high levels of exposure can occur during direct active disturbance to ACMs during remodelling of structures (10) or following natural disasters. (11)

​It is very difficult to implement effective control measures in remodelling and disaster situations as ACMs as their presence may not have been identified and/or the implementation of effective control measures may hamper urgently required humanitarian works.

​In addition to intense exposure from active disturbance to ACMs, the weathering of installed asbestos cement roofing (ACR) results in ongoing emissions of asbestos into the environment and exposure to members of the public. The ongoing contamination of ambient air from the weathering of asbestos cement roofing was first demonstrated in 1979, (12) and has been reported on many occasions since. (13-16)

Figure 3: After Teichert 1983. Graphic showing elevated asbestos fibre levels above an asbestos cement roof

Spurny measured asbestos fibre levels by scanning electron microscope and measured fibre emission rates from wind erosion of up to 14,000,000 fibres per square metre per hour for fibres > 5 µm long; most of the fibres would not have been visible using PCOM. (17, 18) In addition to weathering by wind erosion, significant quantities of respirable asbestos fibres are dispersed by runoff after rainfall events. This leads to local contamination of soils and hard surfaces, on drying fibres can be re-aerosolised from hard surface, while asbestos contaminated soil can be tracked indoors.

​Emissions of asbestos from weathering of asbestos cement roofing have been estimated for each administrative district in South Korea; and it is calculated that each year, such weathering releases almost a million tonnes of asbestos fibres into the environment. (19)

​It is important to note that chrysotile fibres released by weathering has not been significantly altered, (20) and in animal testing weathered chrysotile fibres from asbestos cement retain their carcinogenic potential. (21)

Health Risks from Environmental Emissions

​There are a number of epidemiological studies that indicate that there is a measurable risk to health from asbestos cement roofing materials.

​In 2000, Magnani reported a statistically significant increased risk of mesothelioma for people domestically exposed to asbestos, including six cases where the only apparent source of exposure was from residing in home with an asbestos roof. (22)

​In 2001, Magnani reported an elevated risk of mesothelioma for living in home with asbestos cement roofing, the elevated risk approached statistical significance. (23) The cases included in this study were different from the 2000 Magnani study.

​Ferrante has reported a statistically significant increase in the risk of mesothelioma for living in a building with an asbestos cement roof, or for living near large asbestos cement clad buildings. (24) Once again, the cases included in this study have not been included in the above two studies.

​In 2018, Kang demonstrated an association between living in an area with a high density of asbestos roofing and lower lung fibrosis and pleural disease; however, this study was not controlled for potential confounders. (25)

While the above papers make no reference to the condition or disturbance of the relevant asbestos containing materials, there is nothing in the papers that suggest that the asbestos cement roofing materials implicated in the increased risk of mesothelioma were necessarily in a bad condition or subject to ongoing disturbance. In addition to the retrospective studies above, a number of workers have attempted to estimate the risk of disease from environmental emissions from asbestos roofing.

Precautionary Responses

​Research to date has demonstrated an increased risk of disease to residents likely to be affected by emissions from asbestos cement roofing. While some studies are complicated by the presence of other potential sources of exposure, given the demonstrated extent of environmental emissions of asbestos caused by the weathering of asbestos roofing and the absence of a verified threshold for ARD induction; it is biologically plausible that asbestos roofing can cause ARD in members of the public. There appears to be limited opportunity to control the environmental emissions of asbestos roofing, and given the availability of alternative roofing materials a precautionary response would avoid using asbestos cement roofing.

 

 

References

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