Monitoring and assessment of exposure to elongate mineral particles and fibres Dr Garry Burdett
The Problem The commercial mining and use of asbestos fibre has resulted in widespread early mortality for workers worldwide. Para-occupational and environmental exposure (e.g. Witternoon, Crocidolite mine) show others are at risk. Large areas of land have metamorphic and sedimentary deposits with some asbestos (NOA) and elongate mineral particles (EMP).
Main issues PCM Regulatory fibre count definitions for asbestos too broad so they will include EMP s. Toxicity based on the 3D s: dose, dimension, durability. Is the asbestiform habit alone responsible for toxicity or is it a continuum of elongate shape? Role of chemistry/dimensions/surface properties?
Ten years since NIOSH Roadmap
NIOSH Roadmap: Goals How to assess the relative health hazards to the full spectrum of elongate mineral particles? Develop understanding of the determinants of toxicity of asbestos and EMPs; Develop knowledge on occupational exposures; Develop improved sampling and analytical methods
National academies roadmap review To establish a small number of in-vitro tests to screen in-vivo toxicity for several responses. Highest standard is inhalation (90 day & lifetime) exposures of animals (Rat) at several doses below overload. Fibrosis = hazardous. Intra-pleural and intra-peritoneal may be useful for certain types of mechanistic studies but is not an acceptable substitute for inhalation.
10 years later? Some new in-vivo Inhalation tests with EMPs, Some in-vitro tests with EMPs; Step change in in-vitro testing of nano-objects Ability to synthesize fibres (e.g. CNTs, nanofibres, nanowires etc.) with greater control of surface properties and shapes.
Amosite study, Davies et al. 1986 Long term inhalation 1 year period of dust inhalation at 10 mg/m3 Short fibre amosite failed to produce either pulmonary fibrosis or pulmonary tumours within the lifetime of the laboratory rat. The long fibre amosite sample with 30% of fibres > 5 um and 11 % of fibres > 10 um gave pulmonary tumours in over 30% of animals and large amounts of peribronchiolar fibrosis and alveolar interstitial fibrosis. IP gave mesothelioma rates for long: short fibre of 95%: 4%
Libby Amphibole (Gavett et al., 2016) Rat inhalation of relatively short Libby fibers for 13 weeks produced inflammatory, fibrogenic, and tumorigenic effects in rats which replicate essential attributes of asbestos-related disease in exposed humans.(l= <1% >20 µm, 30% >5 µm). (Note: Davies Long fibre amosite had 30% of fibres >5 µm long and short fibre amosite had 1% >5 µm long.)
How much does chemical composition matter for mineral fibres? USG analysis of Libby deposit
IARC Classification of Carcinogenicity Class Asbestos Fluro edenite 1 X X 2A SWCNT * MWCNT * 7(Mitsui) Other * MWCNT 2B X X RCF MMVF Insulation wools 3 X X X 4 1= Carcinogenic, 2A =Probably 2B = Possibly 3 = Not classifiable 4 = Probably not * No relevant human data available for classification. MWCNT-7 (length, 1 19 (median, 2) μm; diameter, 70 170 (median, 90) nm;
How much does fibre number / fibre habit matter? IP injection of tremolite fibres
Differences in habits: 3D images of Tremolite fibres used for in-vivo tests
Cause of Mesothelioma: Long fibres unable to clear through stomata of the parietal pleura. 1986 Kane et al. & Viallat et al. 1996 Boutin Black spot parietal pleura. 2010 Donaldson et al. Kind of sieve that selectively retains long fibres 2011 Murphy et al. 2011 Nanowires & CNTs suggest ~>5 µm fibres cause inflammation
Inflammation Response in Mice µm Ni Nw long Ni Nw short CNT tangle2 CNT L1 Mitsui CNT L2 length 24 1-5 5-20 13 <56 %>15 85 nd nd 24 84.3 width 0.20 0.20 0.0104 0.085 0.16
TEM appearance of asbestos ISO methods count parallel or stepped sided fibres Evidence of parallel aligned fibres/fibrils in the EMP Individual fibres with regular rectangular shaped ends Sections of individual fibres have same electron scatter / density
TEM appearance of EMPs & asbestos Low magnification (x500) Regulatory sized fibres seen NIOSH7400 & 7402 Range of aspect ratios for > 5 µm long fibres Shorter and thinner fibres present Asbestos morphology parallel or stepped sides Rectangular ends to individual fibres POPULATION mean aspect ratio > 20:1 for >5 µm long fibres is typical of commercial asbestos. This EMP mean aspect ratio was 18.3 (n= 46)
TEM appearance of cleavage fragments EMP cleavage fragment often show: Acicular habit Non-parallel sides Irregular angles at ends Irregular patches of different intensity of electron scatter from cleavage planes No evidence of being a bundle of smaller fibres / fibrils
Additional criteria for discrimination Water and air samples are on filters, so the 2 D image limits what can be viewed. Possible to tilt to get more of a 3 D appreciation or use of stereo pairs, SEM, tomography etc. Increased 3 D information still requires subjective judgement if you are discriminating fibres from EMP cleavage fragments. 2 D size information length, width & aspect ratio is easier to handle and to apply for comparing and characterising a population of EMPs (e.g. Chatfield yellow box display). Individual particle discrimination can be improve by: Looking for evidence of being formed from finer fibres; Measuring angles of cleavage; Assessing breakage morphology at fibre ends.
Example of aspect ratio plot used for bulk TEM size analysis (after Chatfield)
Summary 1 The classification of the habit and chemistry of amphibole minerals can be challenging and often there is a continuum. An appropriate index of exposure may suffice if a sensible balance between regulatory and industrial reality can be struck. Limiting the width of fibres counted to <1 µm will give an improved index of asbestos exposure, when extending regulation from asbestos to EMPs and better reflect what reaches and is retained in the lung. There is no strong evidence for changing the regulatory fibre length from >5µm.
Summary 2 Readily available low cost PCM offers the best chance of regular assessment that controls are adequate to protect workers. EM analysis to be carried out if issues arise (e.g. Client industry, consumer and environmental etc.). TEM methods (especially US methods: NIOSH 7402 & D7200/D6281) need updating and standardising to give better agreement on method and include more guidance on how to present and interpret the measurement data for clients and regulation.
Summary 3 Engineered nanomaterials such as CNTs, nanowires of specific size have further informed us of the mechanisms for mesothelioma and for fibrosis. The high levels of funding into nano research are bringing more understanding due to increasing sophistication of in-vitro models etc. and hopefully more understanding of the mechanisms, so we can have even more targeted measurement and regulation for the exposure and risk from elongate mineral particles and fibres.