Manganese and the new occupational exposure limit

Manganese and the new
occupational exposure limit
Léo Jr. Nicolas, M.Sc., CIH
Background
• In 2013, the American Conference of
Governmental Industrial Hygienist (ACGIH) has
lowered occupational exposure limit (i.e.
threshold limit value or TLV) for manganese.
• In Manitoba, TLVs have been adopted by
Workplace Safety & Health Regulation
What is Mn?
• Manganese (Mn) is a grey-white metal
resembling iron.
• Used extensively to produce a variety of
important alloys and to desulfurize and deoxidize
steel.
• Found in many welding rods and filler metals to
promote hardness.
• Mn oxide fume is formed when Mn metal is
heated and reacts with oxygen in air, such as
occurs during welding.
What is the new level?
• Prior to this recent reduction, the TLV-TWA for
Mn was 0.2 mg/m3.
• As of 2013, ACGIH has adopted a TLV-TWA of
0.02 mg/m3, based on the respirable fraction.
Why was the TLV reduced?
• The reduction of any TLV is based on current
research studies that reveal a link between
the current occupational exposure limit and
potential negative impact on the human body.
• For Mn, 5 studies were given consideration
that derived lowest-observed-adverse-effect
levels (LOAEL) among workers.
• LOAEL concentrations ranged from 0.03 to
0.04 mg/m3, measured as respirable fraction.
• A 6th study demonstrated increased
neurobehavioral changes among workers
exposed to 0.01 – 0.04 mg/m3 (respirable
fraction)
• TLV of 0.02 mg/m3 (respirable fraction) was
recommended for Mn and its inorganic
compounds
– To reduce the potential of preclinical, adverse,
neurophysiological and neuropsychological effects
in Mn-exposed workers
• TLV of 0.02 mg/m3 (respirable fraction) is 1.5 –
2.0 times lower than the range of LOAEL
values observed (0.03-0.04 mg/m3)
• According to one statistical model, a level of
0.02 mg/m3 (respirable fraction) would lead to
impaired hand steadiness in 2.5% of workers.
• Virtually all Mn is absorbed from particles
deposited in the fine, gas-exchanging regions
of the lungs.
• The particle of greatest concern are in the fine
respirable fraction (less than 4 µm).
• A supplementary TLV-TWA of 0.1 mg/m3,
inhalable particulate matter, is recommended
for conditions where particles greater than 4
µm are anticipated.
– Some occupational exposure profiles include
aerosols with a substantial fraction of particles
larger than 4 µm.
• An inhalable aerosol limit provides some
protection for intestinal absorption secondary
to inhalation exposure
• Protection for possible absorption from more
soluble
particles
deposited
in
the
nasopharynx.
• The ratio of inhalable to respirable mass may
vary from 1:1 (as for most forms of welding) to
10:1 or higher.
• TLV committee selected a mid-point ratio of
5:1, applied this to respirable exposure limit,
and estimated the inhalable limit as 0.1
mg/m3.
• N.B. if an inhalable aerosol limit for Mn is
used, it should be in addition to the respirable
aerosol limit.
• Insufficient data available to recommend a
TLV-STEL or skin/sensitivity notation.
• Mn classified as an A4 (Not classifiable as a
human carcinogen) due to absence of related
studies.
Definitions
• Threshold limit value-time weighted average:
– TWA concentration for a conventional 8-hour workday
and 40-hour workweek
– Believed that nearly all workers may be repeatedly
exposed, day after day, for a working lifetime without
adverse effect
• TLV-Short term exposure limit (STEL):
– 15-minute TWA exposure that should not be exceeded
at any time during a workday, even if the 8-hour TWA
is within the TLV-TWA
Definitions
• TLV-Short term exposure limit (STEL):
– 15-minute TWA exposure that should not be
exceeded at any time during a workday, even if
the 8-hour TWA is within the TLV-TWA
– Exposures above the TLV-TWA up to the TWA-STEL
should be:
• Less than 15 min
• Occur no more than 4x a day
• Should be at least 60 min between successive
exposures.
Definitions
• TLV-Ceiling (C):
– Concentration that should not be exceeded during
any part of the working exposure
• TLV chronology:
– Manganese:
• 1948-1959: TLV-TWA, 6 mg/m3
• 1960-1962: TLV-TWA, 5 mg/m3
• 1963-1969: TLV-Ceiling, 5 mg/m3
– Manganese and Compounds:
• 1970-1981: TLV-Ceiling, 5 mg/m3, as Mn
– Manganese Fume:
• 1977: proposed, TLV-TWA, 1 mg/m3, as Mn
• 1979-1994: TLV-TWA, 1 mg/m3, as Mn; STEL, 3 mg/m3,
as Mn
• TLV chronology:
– Manganese Dust and Compounds:
• 1982-1987: TLV-Ceiling, 5 mg/m3, as Mn
• 1986: proposed, TLV-TWA, 5 mg/m3, as Mn
• 1988-1994: TLV-TWA, 5 mg/m3, as Mn
– Manganese, Elemental and Inorganic Compounds
• 1992: proposed, TLV-TWA, 0.2 mg/m3, as Mn
• 1995-2012, TLV-TWA, 0.2 mg/m3, as Mn
• 2003: proposed, TLV-TWA, 0.02 mg/m3, as Mn,
respirable particulate matter (PM), withdrawn in 2004
• TLV chronology:
– Manganese, Elemental and Inorganic Compounds
• 2009: proposed, TLV-TWA, 0.02 mg/m3, as
respirable particulate matter, and 0.1 mg/m3, as
inhalable matter, withdrawn in 2011
• 2011: proposed, TLV-TWA, 0.02 mg/m3, as
respirable particulate matter, and 0.1 mg/m3, as
inhalable matter
• 2013: 2011 proposed values adopted
Mn,
Mn,
Mn,
Mn,
Other occupational exposure limits
• Australia:
– Mn dust, fume and compounds an Mn:
• TWA: 1.0 mg/m3; STEL – 3.0 mg/m3
• United Kingdom:
– Mn and it inorganic compounds
• TWA (8 hours): 0.5 mg/m3
• Germany
– Mn and it inorganic compounds (inhalable)
• MAK: 0.5 mg/m3
Other occupational exposure limits
• Sweden
– Mn and it inorganic compounds (respirable)
• TWA: 0.1 mg/m3
• USA
– OSHA PEL: C 5 mg/m3
– NIOSH REL: TWA 1 mg/m3; STEL 3 mg/m3
Health Effects
• High Mn exposure has been associated with
central nervous system effects, referred as
manganism.
• Symptoms similar to Parkinson’s disease
– Tremors
– Slowness of movement
– Muscle rigidity
– Poor balance
• Male workers also have a higher risk of fertility
problems.
• Effect on nervous system is believed to be
permanent.
Impact on Welding Industry
• High frequency of elevated Mn concentrations
measured in production welding manufacturers
that do not have local exhaust ventilation (LEV)
• Example of 48 production welders tested in large
facilities not equipped with LEV in 2013
• Dominantly MIG welding on mild steel
– Minimum value of 0.014 mg/m3 (70% of allowable)
– Maximum value of 0.951 mg/m3 (4755% of allowable)
– Average value of 0.224 mg/m3 (1120% of allowable)
1
0.9
Manganese Concentration (mg/m3)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
TLV-TWA
1
0.9
Manganese Concentration (mg/m3)
0.8
0.7
One worker had exposure
less than 0.02 mg/m3
0.6
0.5
0.4
0.3
0.2
0.1
0
TLV-TWA
1
0.9
Manganese Concentration (mg/m3)
0.8
LOAEL: 0.03 – 0.04 mg/m3
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
TLV-TWA
1
0.9
Manganese Concentration (mg/m3)
0.8
LOAEL: 0.03 – 0.04 mg/m3
0.7
0.6
43 workers had Mn exposure
exceeding LOAEL
0.5
0.4
0.3
0.2
0.1
0
TLV-TWA
1
0.9
Manganese Concentration (mg/m3)
0.8
0.7
Half face APR with
protection factor of 10
0.6
0.5
0.4
0.3
0.2
0.1
0
TLV-TWA
1
0.9
Manganese Concentration (mg/m3)
0.8
0.7
Half face APR with
protection factor of 10
0.6
17 workers (35%) remain overexposed
0.5
0.4
0.3
0.2
0.1
0
TLV-TWA
Impact on Welding Industry
• High frequency of elevated Mn concentrations
measured in areas samples collected near
(approx. 25 feet) and far from welding areas
(100+ feet)
• Eight area samples collected in large welding
facilities not equipped with LEV but have general
ventilation
– Average Mn concentration = 0.048 mg/m3 (240%)
• Data suggests respiratory protection for nearby
non-welders?
Area testing results
0.1
0.09
Manganese Concentration (mg/m3)
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
TLV-TWA
Area testing results
0.1
0.09
Manganese Concentration (mg/m3)
0.08
0.07
0.06
Half face APR with
protection factor of 10
would provide
adequate protection
0.05
0.04
0.03
0.02
0.01
0
TLV-TWA
Controlling Welding Exposures
• What is our Criteria?
• Effective
• Reliable
• Cost
• Convenience
Hierarchy of Controls
•
•
•
•
•
Elimination
Substitution
Engineering Controls
Administrative Controls
PPE
Welding Operations
Common types -Manual Welding -Presented in decreasing order of relative welding
fume generation rate.
• Flux Core Arc Welding (FCAW) consists of a wire electrode with arc shielding
provided by flux contained within the electrode. One FCAW process variation uses
an inert gas fed through the welding gun to provide additional shielding of the arc.
• Shielded Metal Arc Welding (SMAW), most common type. A short electrode with
a coating (also know as stick).
• Gas Metal Arc Welding (GMAW) also known as MIG, second most common type.
A wire electrode with an inert gas (e.g. argon) which is fed through the welding
gun to provide a shield against oxidation.
• Tungsten Inert Gas Welding (TIG) uses a non-melting tungsten electrode and in
some cases a metal filler that the welder introduces into the arc. An externallysupplied inert gas (e.g. helium, argon) is fed through the welding gun to shield the
arc.
Welding Processes and Fume
Generation Rates (g/min)
FCAW-CO2
>1
FCAW-Ar/CO2
0.6
GMAW-Steady
0.5
SMAW
0.4
GMAW-Pulsed
0.2
GTAW
<0.1
SAW
<0.1
Different Type of Welding
Relative Fume Generation Rates of Common Processes
High
Flux-cored Arc
Welding
Shielded metal Arc
Welding
Moderate
Low
Gas Metal Arc Gas Tungsten Arc
Welding
Welding
(MIG)
Submerged Arc
Welding
Arc Gouging
Source: Jerome Spear, Spears Consulting, LP presentation at 2010 AIHCE
Control Measures - Substitution
• Most cost effective way to control exposure is
to substitute welding rod or wire with one
that is lower in Mn.
• As much as 95% of the welding fume from
MIG welding originates from the welding wire
rather than the metal being welded upon.
• Some welding wires have excessive amount of
Mn.
High Manganese Wire
Worker Exposures at Initial Survey
Average Exposure is 192% of Allowable Exposure
Worker
Activity
Controls
Exposure
before
PPE
A
Frames
none
176 %
B
Production Booth
HFAPR
101 %
C
Platforms
none
163 %
D
Production Booth #1
none
157 %
E
Frames
none
403 %
F
Headers
none
156 %
Note: HFAPR = half face air purifying respirator
Same Place but with Low Mn Wire
Exposures at Resurvey
Average Exposure is 59% of the Allowable Exposure
Worker
Activity
Controls
Exposure
before
PPE
A
Frames
none
61 %
B
Tacking on mild
steel
none
35 %
G
MIG Welding on
mild steel
none
58 %
D
Production Booth
#1
HFAPR
113 %
F
MIG Welding on
mild steel plates
none
27 %
Note: HFAPR = half face air purifying respirator
Effect of Low Mn wire
Metal
Original Wire
Low Mn Wire
Iron Oxide
48%
32%
Manganese
168%
57%
Mn / Iron Oxide
3.5
1.78
Lower production day as suggested by lower iron
oxide results but much lower exposure to manganese
– almost halved the exposure
Administrative Control of Welding
Parameters
•
Electrical current:
–
•
In general, the fume generation rate is exponentially
proportional to the current.
Electrode diameter:
–
•
The electrode diameter has a modest effect on the fume
generation rate because of the differences in voltage
and current. In general, a small diameter electrode has a
higher fume generation rate than a large diameter
electrode.
Electrode angle:
–
The angle of the electrode to the workpiece has a slight
(unpredictable) affect on the fume generation rate.
Administrative Control of Welding
Parameters
•
Arc voltage:
–
–
–
The fume generation rate generally increases when
the arc voltage increases.
Increasing arc voltage tends to increase puddle
fluidity, flatten the weld bead, increase edge wetting
and increase spatter.
Higher voltages also reduce penetration and may
cause additional loss of alloying elements.
Effect of Arc Voltage
20 – 26 V
Effect of Arc Voltage
26 – 29 V
Effect of Arc Voltage
30 – 36 V
Effect of Arc Voltage
Relative Comparison
20-26 V
26-29 V
30-36 V
J Occup Environ Hyg. 2007 Dec;4(12):903-12.
Effects of voltage and wire feed speed on weld fume characteristics.
Hovde CA, Raynor PC.
Division of Environmental Health Sciences, University of Minnesota, Minneapolis, MN
Abstract
Welding generates high concentrations of ultrafine particles. Fume characteristics
were measured in a controlled apparatus as a function of voltage level and wire feed
speed. Particles were sampled close to the welding process on mixed cellulose ester
membrane filters and analyzed for iron, manganese, and total particulates.
Submicrometer particle number concentrations and iron, manganese, and total
particle mass concentrations all depended on voltage levels but not on wire feed
speed at a constant voltage. Ultrafine particle concentrations were more than three
times greater at 23.5 V than at 16 V. Manganese concentration was 1.7 mg/m3 at
16 V vs. 6.4 mg/m3 at 23.5 V. The data suggest that welders should use lower
voltage levels whenever possible.
Shielding Gas
• Shielding gas:
– In gas-shielding arc welding, the fume generation rate tends
to be greater when 100% carbon dioxide (CO2) is used as
compared to argon for the shielding gas.
– In practice, using 100% CO2 will require a procedure increase
of 1-2 volts compared to Argon blends. This adds energy to
the arc, boiling off more metal and creating more fume.
Steady Vs. Pulsed Current
• Steady/current pulsed current welding:
– With pulsed gas metal arc welding, less fume is typically
produced than with a conventional constant voltage
power source.
– In this mode, the arc is controlled by pulsing the current
from a background level to a peak level at a specified
frequency.
– This reduces the average arc voltage and decreases the
amount of metal that is vaporized, which leads to reduced
fume generation.
Studies have shown that using a pulsing current during
welding generates less fumes than under steady current
welding process.
Steady Vs. Pulsed Current
Case Study
MIG Welding on Mild Steel
Half day welding using steady current
Half day welding using pulsed
•
Same day
•
Same work
•
Same welder
•
Same bench
Steady Vs. Pulsed Current
Case Study
MIG Welding on Mild Steel
Steady Current = 168% of allowable
Pulsed Current = 108% of allowable
35% reduction in exposure
Still a good weld
Change Work Position
• The exposure is in the plume
• In come cases, it is possible to orient the
process so that the worker is out of the
plume
• This can significantly reduce exposure
• Works best for small to medium sized parts
Different Work Positions
Different Exposures
Change Work Position
Exposure Reduced by 80% by changing
Horizontal vs. Vertical Surfaces
Horizontal
Vertical
Maximum
38.0 µg/m3
2.5 µg/m3
Median
4.5 µg/m3
0.8 µg/m3
Measures
Example of Rotating Jig
Different Clamps for Different Parts
Local Exhaust Ventilation
• Captures fumes at source
• Low volume of air
• Some require repositioning
Local Exhaust Ventilation
Control – LEV Retest
•
•
•
Overall average exposure of 139% reduced
to 53%.
This was achieved by expanding local
exhaust system to more workers.
Assistance with the local exhaust system
design reduced the exposure to workers
already using the system by 20%.
Control – General Ventilation
•
•
•
•
•
Uses large volume of air
Purges and dilutes air
No direct effect to breathing zone
No repositioning
Can be effective
General Ventilation
General Ventilation Retest
•
•
•
Study that introduced general ventilation
with air introduced blowing down onto
welding benches
Significant costs associated with modifying
ventilation system
Upgrade reduced average exposures from
130% to 66%
General Ventilation Retest Example
•
•
•
Modest upgrade to general ventilation in
worst part of shop.
Reduced exposures from an average of 80%
(3 out of 10 overexposed) to 56% (0
overexposed).
During retest, highest result = 80%.
Gun mounted extraction system
•
Performed study in which isolated welder was MIG
welding in a large rectangular shaped bin (8ft wide
x 8ft high x 20ft length), open at one end
Welder was asked to weld a total of 70 feet on a
piece of mild steel using following welding wire
with and without gun mounted vacuum, using
same positioning:
•
–
–
Regular welding wire (1.44% Mn)
Low Mn wire (0.96% Mn)
Gun mounted extraction system
Results:
Wire
LEV
Concentration
(mg/m3)
Exposure
(as a % of allowable)
Regular
No vacuum
0.36979
1849
Regular
Vacuum
0.15196
760
Low Mn
No vacuum
0.19507
975
Low Mn
Vacuum
0.08842
442
Changing of the wire resulted in 42% to 47% decrease in Mn exposure.
Gun mounted extraction system resulted in 54% to 58% decrease in Mn
exposure.
Plus the last resort - PPE
•
•
•
•
Effective
Semi-reliable if not diligent
Cost is ongoing so is it cheap?
Low comfort and convenience
Calculation of the Approximate Cost of Providing 5 Welders with Masks for One Year
Control Option
% Reduction
Comment
Lower Mn welding wire
20-70
Essentially zero cost, proven
effective
Local exhaust ventilation
80
High initial cost, high level of
control
Gun extraction system
55-60
Somewhat less effective than
above
Pulse setting
30-40
Zero cost, still gives good weld
Voltage Reduction
20-40
Zero cost
Change work position
80
Rearrange work station so worker
is out of plume
Respiratory protection
90
High ongoing costs and comfort
issues
Overall Conclusions
• Implementation of more than one control
will likely be required in order to achieve
worker Mn exposure below 0.02 mg/m3.
• Success will be achieved on trial and error
basis.