Transcript Development & applications of exposure Video

The value of microscopy
analysis
Garry Burdett
Health and Safety Laboratory
Harpur Hill, Buxton, UK, SK17 9JN
Asbestos Hazard
• Mortality (excess deaths) due to:
– Fibrosis of the lung (asbestosis)
– Cancer of the lung (bronchial cancer etc)
– Cancer of the lung lining (e.g. mesothelioma)
• Morbidity (lung changes/dysfunction)
– Pleural plaques;
– Diffuse pleural effusions
Fibre Related Risk:
• Depends on:
–
–
–
–
Dimensions
Durability / Type
Dose / Exposure
Time
(Measurable by microscopy)
EU Directive: Dimensions
• For the purposes of measuring in the air, only
fibres with a length of more than five
micrometres, a breadth of less than three
micrometres and a length to breadth ratio
greater than 3:1 shall be taken into
consideration.
(2003/18/EC amending to EU asbestos worker
protection directive (83/477/EEC) article 7.6 )
Durability / Type
Fig 1: Mesothelioma risk by fibre type
500
500
Risk factor
400
300
200
100
100
1
0
Crocidolite
Amosite
Asbestos type
Chrysotile
Dose / Exposure
• Exposure is used as a surrogate for dose by
sampling a known volume of air onto a
membrane filter and counting the numbers
of fibres present in a known area.
• This is used to calculate the fibre
concentration in f/ml or f/cm3 of air.
• Uses size criteria to determine ability to
reach pulmonary region of the lung.
Time
• Dose = exposure x duration (f/ml/years)
• Risk  cumulative dose (assume linear);
• Risk time since first exposure
(mesothelioma related by power law).
• Lag time 15 – 60 years from exposure to
disease symptoms
EU directive: Fibre Counting
• “Fibre counting shall be carried out
whenever possible by PCM (Phase
contrast microscope) in accordance with
the 1997 (World Health Organisation)
recommended method (16) or any other
method giving equivalent results.”
EU Directive: Requirements
• The 2003 amendment to 83/477/EEC
abandoned action levels and different control
levels for different types of asbestos and
introduced a single limit of 0.1 f/ml over an
8-hour time weighted average. (Article 8)
• Therefore the EU regulation for controlling the
risk to workers, require only PCM fibre
number information over a broadly defined
size range.
• Article 4 requires the type and quantities of
asbestos handled to be notified
Main types of microscopy for
fibre counting.
•
•
•
•
Phase contrast light microscopy PCM
Scanning electron microscopy SEM
Transmission electron microscopy TEM.
All have ISO or WHO standard methods
• Polarised light microscopy PLM for bulk
analysis.
PCM (X500 magnification)
SEM
TEM
SEM higher Mag & 3D
TEM high mag chrystotile
PLM : Dispersion staining
Discrimination of fibre type
• PCM: No discrimination or identification ;
• PLM Uses optical properties + refractive
index/ dispersion colours to identify fibres;
>0.8µm width;
• SEM uses Energy Dispersive X-ray analysis
to semi-quantitatively measure chemistry;
• TEM uses quantitative EDX and electron
diffraction to identify fibres.
Comparison of methods for fibre
counting
Operating Area (mm2)
examined
Mag
PCM
X500
(200 fov)
1.5
SEM
X2000
TEM
x5000
(50 fov)
0.15
(50 go)
0.5
Time
taken
(mins.)
20
Rate area
examined
mm2/hr
4.5
30
0.3
40
0.75
Likely upper limit of detection
(f/ml) by method and volume
WORK
VOLUME
(litres)
PCM
500
ENVIRON COST PER
MENTAL HOUR
3000
0.01*
0.0017*
30-50
SEM
0.015
0.0025
80 - 120
TEM
0.005
0.0007
80 - 120
Accuracy & Precision
• Accuracy not usually known.
• Precision includes:
– Random “Poisson” counting error
– Instrumental errors (QA / calibration)
– Human errors (QA / Proficiency testing)
Random counting errors (95%)
No. of Lower Upper No. of Lower Upper
fibres limit
limit
fibres limit
limit
0
0
2.99
5
1.624
11.669
1
0.025
5.572
6
2.202
13.060
2
0.242
7.225
10
4.795
18.39
3
0.619
8.767
20
12.217 30.889
4
1.090
10.242 50
37.112 65.919
Quality and accreditation
• Quality assurance and proficiency
testing is essential, particularly if
results from different analysts and
laboratories are being used or
compared.
• Accreditation of Laboratories to ISO
17025 mandated by the EU directive.
Type of
analysis
PCM
SEM
TEM
Fibre counting
of air samples
Belgium (25)
France (?)
Netherlands(2
0)
Spain (20)
UK
(200)
Germany*
France (20)
Not applicable
Fibre
Identification
of
bulk
samples
Not applicable
UK (5)
UK (17)
UK (250)
Fibre counting
and
identification
of air samples
Not applicable
Germany*
France (20)
Not applicable
* PT scheme run for one or two rounds in past a new scheme was planned
Note: UK schemes also include a number of EU countries who participate
PLM
RICE: Fibre counting PT
• RICE - 187 labs.
• MMMF – 12 labs.
• ERM – WHO major change
Numbers of laboratories
Chart 1. RICE Ratings
250
200
196
197
195
192
190
193
194
186
189
190
184
184
183
184
184
Rating 1
150
Rating 2
100
Rating 3
50
0
52
53
54
55
56
57
58
59
60
Round
61
62
63
64
65
66
AIMS: Fibre Identification PT
C h a rt 4 : D is trib u tio n o f cu m u la tive s co re s fo r ro u n d s 2 2 - 2 4 (U K L a b o ra to rie s )
1%
22%
25%
0 (N o E rro rs )
7 (1 Min o r E rro r)
8 - 20
21 - 47
19%
> 47
33%
Value of PCM counting
1. Workplace exposure can be controlled
using the WHO - PCM method;
2. Counts “regulated” WHO fibres &
meets EU criteria;
3. Relatively quick (on-site) and cheap;
4. Environmental exposure can be
screened by PCM.
PCM Limitations
1. PCM has limited detection due to
background counts on blank filters
(0.01 f/ml);
2. No knowledge of fibre types;
3. Only an “index of exposure”
Value of analytical SEM
• Can be adjusted to give regulated fibre
counts,
• Can give fibre size information,
• Classifies fibre types present using EDX
spectra data.
SEM limitations
•
•
•
•
Limited availability;
Off site analysis;
Costly
Can only be used for regulated fibre
counting,
• No definitive identification of fibres,
• Lowest area of filter scanned per field of
view.
Value of TEM
•
•
•
•
•
•
Can count regulated fibres + all fibre sizes.
Accurate size information,
EDX chemical analysis of all fibre sizes
Crystal structure information (SAED)
Identification of fibre type,
Can give complete information on all fibre
sizes if needed
Limitations of TEM
• Limited availability;
• Off site;
• Costly
Recommendations 1
• Use PCM for workplace control and
checking after clean –up (with disturbance
sampling).
• Indoor and near source environmental use
PCM screening and SEM or TEM analysis
(as necessary) on selected half-filters.
• Ambient air: Analytical TEM is best, SEM
is more limited, PCM not very useful.
Recommendations 2
• Use Polarised light microscopy (PLM) for
analysis of bulk samples for asbestos
present and type of asbestos.
• Use PLM to screen minerals and soils for
asbestos content.
• Asbestos in water usually analysed by TEM