WHO fiber release, workplace exposure measurement and
assessment
Elisabeth Heunisch, Volker Bachmann
Particulate Hazardous Substances, Advanced Materials
BAuA Berlin
What are we talking about?
20 µm
[Chrysotile_UICCA.TIF von http://usgsprobe.cr.usgs.gov/picts2.html]
[Creutzenberg, O., et al. (2005). Toxizität von Stäuben im Peritoneum der Ratte.]
[Anthophyllite_UICC von http://usgsprobe.cr.usgs.gov/picts2.html]
German Asbestos exposure limits (TRGS 519, VDI 3492)
Sample
Count
1
2
3 4
5
6
7 8
TRGS 519: exposure limits for asbestos fibers
• Tolerance level: 10
5fibers/m³
• Acceptance level: 10
4fibers/m³
• Clearance level: 10
3fibers/m³
VDI 3492: measurement of inorganic fibers by SEM
• Dimensions to be counted:
0.2 µm < width < 3 µm;
length > 5 µm; ratio l:w > 3
• Magnification: 2000:1 to 2500:1
• 2L/0.5 mm² area on filter to be evaluated for clearance evaluation
• count individual fibers
• If no fibers are found, less than
500 fibers/m³ were around.
German Asbestos exposure limits (TRGS 519, VDI 3492)
Lower counting limit at 0.2 µm width to stay comparable to light microscopy
Consequence for Nanofibers:
• No information, as they are not counted.
• Need for measurement strategy adaptation.
• Need for the analysis of nanofiber aerosols.
• What is released from nanofiber powders and how do the aerosols look like?
BAuA developed a low-energy impact dustiness
testing method for nanofiber powders, the BAuA shaker.
The Shaker Method
Equalization tank with baffle plate
Shaker Sample holder w. HEPA filter at air inlet
Function
generator Amplifier Sampling Measurement instruments,
e.g. SMPS, CPC, APS, collective samples
The BAuA Shaker: a low-energy impact method to test the dustiness of nanofiber powders.
[BA uA ]
Research results generated by Shaker Method
Broad variety in dustiness
• Set of CNTs with average diameters between 10 and 250 nm (one CNF material)
• Mainly industrial grade materials
• Variations over 3 orders of magnitude
Incr easi ng dus ti ness
CN T 1
CN T 2
CN T 3
CN T 4
CN T 5
CN T 6
CN T 7
CN T 8
CN T 9
CN T 1
0 CNF
CN T 1
1 CN
T 1 2
CN T 1
3 CN
T 1 4
CN T 1
5 CN
T 1 6 100
1000 10000 100000
Particle number concentration from fitted integral [#/cm³]
Research results generated by Shaker Method
Broad variety in morphology
Materials (top left to bottom right): MER Corp., Nanocyl, TCI C2158, n-tec, nanothinx, FutureCarbon
[BA uA ]
Fiber classification
Scheme of various morphologies of nano-scaled fiber objects
LAR = low aspect ratio < 3 HAR = high aspect ratio > 3 WHO = HAR, L > 5 µm, W < 3 µm
O = object
C = cluster (low number of objects distinguishable individually)
A = agglomerate (objects in higher concentration with overlap and predominantly not
distinguishable individually)
[BA uA ]
Morphological classification
Individual Fiber (HARFO) Fiber Cluster (HARFC) Fiber Agglomerate (LARFA) Particle Agglomerate (LARPA) Individual Particle (LARPO)
Visual classification of SEM images
SEM image of Arry ARIGM001 aerosol
[BA uA ]
Workplace Exposure Measurements – decanting process
12:00 12:14 12:28 12:43 12:57 13:12 13:26 2000
4000 6000 8000 10000 12000 14000 16000
particle number concentration [1/cm³]
time course
CPC - directly in the breating area SMPS
background measurement decanting process
SMPS stationary CPC 3007 [#/cm] handheld
particle number
concentration
standard deviation particle number concentration
standard deviation
background 6338 1421 6896 1562
decanting 4421 274 5200 1028
1 10 100 1000
0 2000 4000 6000 8000
dN/dlogdm [1/cm³]
mobility diameter dm [nm]
background decanting process
CPC and SMPS do not show any increase in particle concentration.
NAS samples and contact samples from protective clothing show big fiber agglomerates and single fibers.
[BAuA]
Workplace Exposure Measurements – polymer processing - extruder
Sudden increase of concentration at the start of the experiment, highest concentration at the platform above the extruder. (Emission from hot parts of extruder, no CNT.)
No difference if CNTs were handled (ZE_25 1101) or not (ZE_25 1100), significant reduction of exposition by exhaust ventilation
NAS samples on platform show big fiber agglomerates.
13:30 14:00 14:30 15:00 15:30 16:00 16:30 17:00 17:30 0
20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000
ZE_25_1100
ZE_25_1101
particle number concentration [#/cm³]
time course
SMPS- stage CPC 3007- PC worktable CPC 3007 - platform background
ZE_25_1101 with exhaust ventilation
SMPS stationary CPC 3007 [#/cm]
handheld
particle
number concentration
standard deviation
particle number concentration
standard deviation
background 28347 801 71270 14138
ZE_25_1100 51892 14795 146106 47872
ZE_25_1101 51297 24202 112428 35087
ZE_25_1101 with exhaust ventilation
14549 3013 38670 27157
night measurement
9516 14392 -- --
Workplace Exposure Measurements – conclusions
No significant increase in particle concentration due to handling of fibrous nanomaterials with appropriate protective measures.
Morphologic analysis of collective samples however show release of fibers and agglomerates.
A new measurement strategy to determine the exposition towards fibrous nanomaterials is required.
[BA uA ]
New measurement strategy for fibrous nanomaterials
BekGS 527: Occupational nanofiber concentrations with WHO dimensions
< 10,000 F/m³ (< 0.01 F/cm³)
Online Measurement devices
Resolution > 1 #/cm³
No morphological discrimination
Collective sample systems
Collecting of aerosol samples
Evaluation by SEM
• Sampling
• Sampling behavior of different sample systems
• Sampling time
• Sampling rate
• SEM image analysis
• Sample preparation
• Resolution pixel size
• Number of images for detection limit
• Fiber classification
• Fiber identification
New measurement strategy for fibrous nanomaterials
SEM evaluation according to VDI 3492 / BGI/GUV-I 505-46 needs to be adapted
[BA uA ]
Sampling time for the Partector, the TP and the ESP Nano is to high to reach the threshold limit of 10,000 fibers/m³.
The NAS cannot be used due to its deposition behavior.
Electrostatic Precipitator (NAS)
Deposition of fibers is dependent on their charge,
deposited fibers are standing vertically on the sample
Partector with TEM sampling grits
Deposition efficiency at 3 % with a flowrate von 0,45 L/min
Thermal Precipitator (TP)
low flowrate of 2 mL/min
ESP Nano
Deposition efficiency at 40 to 60 % , flowrate of 0,1 L/min
New measurement strategy for fibrous nanomaterials
Sampling behavior of different sample systems
Gold sputtered filter (pore size 0.2 µm) for sampling.
First results show a deposition efficiency of > 99 % at a flowrate of 4 L/min.
Gold-filter with 0.2 µm pore size NAS sample
[BAuA/BAM] [BAuA]
SEM image resolution necessary for fibre counting for nanofiber exposure assessment
Image Pixel Size
Images of 12 Mega Pixels
Images of 20 Mega Pixels 1 nm 400000 25000 3 nm 44400 2750 5 nm 16000 1000
10 nm 4000 250
20 nm 1000 63
50 nm 160 10
100 nm 40 3
200 nm 11 2
Asbestos Clearance Measurements
max. 1 fibre per 2 litres on 0.5 mm² filter area (VDI 3492)
3 nm 6 nm 12 nm
25 nm 50 nm 100 nm
Rigid fibres to count Flexible fibres to ignore
Manageable number of SEM images for rigid fibers
New measurement strategy for fibrous nanomaterials
SEM images of 60 and 20 nm thick fibers at different resolutions
[BAuA]
New measurement strategy for fibrous nanomaterials
Reasons for counting of WHO- and high aspect ratio objects and agglomerates
with a width > 20 nm
Fibers with a width
lower than 20 nm and a low probability of being rigid are not counted.
Image pixel size of 20 nm is sufficient, limiting the number of images to be evaluated.
Objects with a width
> 20 nm can be visualized on all state of the art SEM.
hypothesis
convention
Category I:
Single fibers identifiable with 1 µm < L < 5 µm
Category II:
closed clusters or agglomerates with L:W > 3:1 and 1 µm < L < 5 µm
Category III (WHO):
Single fibers identifiable with L > 5 µm
Category IV (WHO):
closed clusters or agglomerates with L:W > 3:1 and L > 5 µm
Fiber classification
New measurement strategy for fibrous nanomaterials
Category I and II are counted, but not evaluated, category III and IV are counted and evaluated.
[BA uA ]
New measurement strategy for fibrous nanomaterials
Is it a WHO fiber?
… yes
[BA uA ]
… a non-compact fiber composed of bundled SWCNTs !
New measurement strategy for fibrous nanomaterials
To be counted and evaluated as a single WHO fiber agglomerate.
[BA uA ]
New measurement strategy for fibrous nanomaterials
Fiber identification
How to identify manufactured nanofibers and discriminate them from other organic fibers?
Can they be discriminated by their morphology?
[BA uA ]
[BA uA ]
Size at monitor
New measurement strategy for fibrous nanomaterials
SEM image evaluation
100 images (20 MP) at pixel size of 16 nm for clearance measurement
For visual evaluation at the monitor image is divided in 16 small images
1600 images to be evaluated!
[BAuA]
[BAuA]
Automatic fiber identification and classification of
• Fiber – Agglomerate
• Number concentration of WHO fiber, WHO fiber
agglomerates, HAR objects, and HAR fiber agglomerates
• Mass concentration estimate for LAR fiber agglomerates
• Curvature probability distributions of fibers – a measure of rigidity?
Software-aided classification of SEM images
New measurement strategy for fibrous nanomaterials
[BAuA] [BAuA]
