132. Special Problems in Measuring Sulphuric Acid Mist

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Journal of Environmental Monitoring c2em10659k
PAPER
1
Sulfuric acid at workplaces—applicability of the new
Indicative Occupational Exposure Limit Value (IOELV) to
thoracic particles
Dietmar Breuer,*Petra Heckmann, Krista Gusbeth,
Gregoria Schwab, Morten Blaskowitz and Andreas Moritz
The paper presents a first comprehensive study of sulfuric acid
measurements at workplaces using a high flow thoracic sampler.
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Sulfuric acid at workplaces—applicability of the new Indicative Occupational
Exposure Limit
1Value (IOELV) to thoracic particles†
Dietmar Breuer,*Petra Heckmann, Krista Gusbeth, Gregoria Schwab, Morten Blaskowitz and Andreas Moritz
2
Received 12th August 2011, Accepted 19th December 2011
DOI: 10.1039/c2em10659k
Until 2009, the limit values for airborne sulfuric acid in Europe were based on the inhalable particle
fraction (e.g. MAK (Maximum allowed concentration at workplace) value 0.1 mg m
3
as the inhalable
fraction). With the publication of the Commission Directive 2009/161/EU, an Indicative Occupational
Exposure Limit Value (IOELV) of 0.05 mg m
3
for sulfuric acid aerosols was based for the first time on
the thoracic particle fraction. To permit a comparison of the measured values for the inhalable fraction
with those of the thoracic fraction and to quantify the thoracic fraction, a cyclone was fabricated out of
sulfuric-acid-resistant stainless steel that achieves suitable collection characteristics (PM
10
) at a flow
rate of 5.34 L min
1
. 49 measurements were carried out in parallel in 21 companies. At concentrations
well below the IOELV, there is little difference between the thoracic and inhalable particle
concentrations. At higher concentrations (>0.1 mg m
3
inhalable aerosol), larger droplets have
a marked effect on the measured values and the thoracic fraction accounts for only 32.1 12.5% of the
inhalable fraction. The EU’s IOELV and the proposal of the MAK Commission therefore provide
a comparable level of protection. In the transposition of the IOELV into national law, an air limit
of 0.1 mg m
3
could therefore be implemented for the inhalable fraction.
Introduction
Sulfuric acid is the most important industrial acid, and world
production was over 195 million tonnes in 2008.
1
The largest
share (roughly 60–70%) is used in the fertiliser industry, although
there are numerous other sectors in which sulfuric acid is
employed.
Sulfuric acid is an aggressive inorganic acid and causes severe
burns. IARC has classified occupational exposure to dense
inorganic mists containing sulfuric acid as being carcinogenic to
humans. Epidemiological studies have shown increased rates of
laryngeal cancers in exposed workers and some links to lung and
nasal cancers.
2
At the end of 2009, the EU published the third
supplement to the list of Indicative Occupational Exposure Limit
Values (IOELVs)
3
comprising 19 substances and, as per
requirements, the member states of the EU have to transpose the
directive into national law within two years. One of the new limit
values concerns the most important industrially used acid,
sulfuric acid.
Particularly striking at first sight is the low numeric value of
0.05 mg m
3
, although the two footnotes in the IOELV list are
also highly revealing. The first footnote points out that the
measuring method has to be examined for cross-sensitivities to
other sulfur compounds. This takes account of the fact that, for
instance, sulfates very often occur in association with sulfuric
IFA, Institute for Occupational Safety and Health of the German Social
Accident Insurances, Alte Heerstrasse 111, Sankt Augustin, 53757,
Germany. E-mail: dietmar.breuer@dguv.de; Fax: +49 2241 2312234;
Tel: +49 2241 231 2533
† Presented at the Seventh International Symposium on Modern
Principles on Air Monitoring & Biomonitoring, June 19–23, Loen,
Norway.
Environmental impact
With the publication of the Commission Directive 2009/161/EU, an Indicative Occupational Exposure Limit Value (IOELV) of 0.05
mg m
3
for sulfuric acid sampled as thoracic aerosol, it was necessary to develop a sulfuric-acid-resistant high flow thoracic sampler
for workplace air measurements. A stainless steel cyclone that achieves these characteristics (PM
10
) at a flow rate of 5.34 L min
1
was
fabricated. 49 measurements were carried out in 21 companies where there is a potential exposure to sulfuric acid. At concentrations
well below the IOELV, there is little difference between the thoracic and inhalable particle concentrations. At higher concentrations
(>0.1 mg m
3
inhalable aerosol), larger droplets have a marked effect on the measured values and the thoracic fraction accounts for
32.1 12.5% of the inhalable fraction.
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acid although they have no significant toxicity. The second
footnote now names the thoracic particle fraction as the relevant
particle fraction for sulfuric acid. This marks a new departure for
aerosol sampling. Although the thoracic particle fraction has
long been in existence and defined in the field of occupational
safety and health, no limit values have so far been established for
this particle fraction in Germany and in Europe. However, there
is already a limit value of 0.2 mg m
3
sulfuric acid for the thoracic
particle fraction in Canada and a recommendation to this effect
of the American Conference of Governmental Industrial
Hygienists (ACGIH).
4
Otherwise, air limit values only refer to
the inhalable fraction, like the recommended value of the Senate
Commission for the Investigation of Health Hazards of
Chemical Compounds in the Work Area (MAK Commission) of
0.1 mg m
3
(ref. 5 and 6).
The sampling of thoracic particles
To satisfy the definition of the thoracic fraction 50% of particles
with a 10 mm aerodynamic diameter have to be collected.
7
Much
more common is the thoracic fraction as particle matter PM
10
in
the environmental sector.
The sampling of thoracic particles at workplaces is not very
usual, and suitable sampling systems are therefore rare and
experience with them is limited. Fortunately, the sampling
characteristics of cyclones designed for the sampling of respirable
particles can be modified to make them suitable for the collection
of thoracic particles.
Cyclones are centrifugal separators and have to be fabricated
with great precision in order to cleanly separate the particle
fraction in question. This and the problems related to sulfuric
acid make choosing an appropriate sampling system difficult.
Sulfuric acid attacks numerous materials, so the range of possible
sampler materials is very limited. Plastics such as Teflon and
polypropylene that would have sufficient resistance to sulfuric
acid are very difficult to work with precision and do not have the
necessary shape stability and electrical conductivity. Sulfuric acid
attacks most metals, although certain stainless steels are suffi-
ciently resistant.
The limit value of 0.05 mg m
3
is fairly low and the sensitivity
for ion-chromatographic analysis is largely exhausted. The
required limit of quantification of a tenth of the limit value in
accordance with EN 482 (ref. 8) would then be 0.005 mg m
3
.
This is precisely the value given for the limit of quantification for
sulfuric acid in the ISO 21438-1 measuring method
9
for a sample
air flow of a cubic metre and an extraction volume of four mil-
lilitres. To avoid problems with the limit of quantification, the
sampling system should have quite a high sampling rate.
A cyclone available in the USA (GK2.69—BGI Incorporated,
Waltham, MA), which is resistant to sulfuric acid, is designed for
a flow rate of 1.6 l min
1
. For the required air volume of 1 m
3
,
a sampling time of 10 h is necessary.
A cyclone designed for sampling respirable aerosols with
a flow rate of 10 l min
1
can be modified for the sampling of
thoracic aerosols. In its aluminium version, this cyclone is used at
the Institute for Occupational Safety and Health of the German
Social Accident Insurances (IFA) for the sampling of respirable
dusts at a flow rate of 10 l min
1
. The cyclone is suitable for the
thoracic particle fraction at a flow rate of 5.34 l min
1
.
10
The
selected material is a V4A steel, type 1.4404 (stainless steel
X2CrNiMo17-12-2, AISI 316L), which has the required resis-
tance to sulfuric acid (Fig. 1). It should be mentioned that
fabrication was extremely elaborate. The cyclone is therefore
unlikely to become commercially available within the foreseeable
future and if it did, it would be unduly expensive.
The aim was to compare the measurement results for the
inhalable and thoracic fractions. The most important question
was undoubtedly the ratio between the two fractions.
Is the ratio of thoracic to inhalable particles constant?
Is it possible to derive a conversion factor?
Measurement programme
Sampling and analysis
To quantify the thoracic fraction, the above-mentioned cyclone
was used and the GSP sampling head
11
(conical inhalable aerosol
sampler) was simultaneously used for quantification of the
inhalable fraction. Measurements were all carried out simulta-
neously and in each case as repeat determinations and as
stationary measurements (Fig. 2).
In each work area, up to three samples were taken indepen-
dently of each other. In some work areas, a GK2.69 cyclone was
used simultaneously for single measurements. A total of 49
measurements were conducted in 21 companies.
For all sampling systems, 37 mm quartz fibre filters MN QF-10
(Macherey Nagel Ltd., D€
uren, Germany), or T 293 filters
(Munktell Ltd., Falun, Sweden) were employed for intercepting
the sulfuric acid droplets. The quartz fibre filters were placed
immediately after sampling in 4 mL of alkaline eluent solution
(Na
2
CO
3
3.1 mmol L
1
, NaHCO
3
0.35 mmol L
1
in ultrapure
water). The plastic bottles were closed and shaken. After trans-
portation to the laboratory the extracts were filtered and ana-
lysed by ion chromatography.
12
Work areas
Since sulfuric acid is by far the most important industrial acid,
it is used in numerous, totally different branches of industry.
Fig. 1 Stainless steel cyclone with a filter holder.
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The use of sulfuric acid is extremely widespread in lead-acid
batteries. Sulfuric acid aerosols can be released both in produc-
tion and when charging the battery at its place of use. Leaving
aside electrically powered cars, all cars have a lead-acid battery
installed as the starter battery. In electrically powered forklift
trucks, large lead-acid batteries are used. Large banks of lead-
acid batteries are also used as emergency power supply units.
For surface finishing, aluminium is anodised in baths con-
taining sulfuric acid.
Many metals are electrochemically cleaned in sulfuric acid
baths. The largest systems of this kind are used in copper
refining.
In galvanic processes, sulfuric acid is often used as the acid in
the baths.
The so-called sulfuric acid process is a technique commonly
used in Germany for the production of titanium dioxide.
Although sulfuric acid production is a contained process,
sulfuric acid can nevertheless escape in small quantities in certain
places.
Measurement results
Manufacture of lead-acid batteries
The critical production step for sulfuric acid emissions is the
formation when lead plates are electrochemically charged for the
first time in a sulfuric acid bath. There are two basic principles for
this: block formation in the finished battery, and tank formation
in which the lead plates are charged in large tanks and then
installed in the battery case.
(Wet) tank formation is clearly more critical for the release of
sulfuric acid, although it is in many cases preferred for produc-
tion reasons. During the charging of lead-acid batteries, gas
bubbles form and burst, causing the creation of droplets that
may be released into the surroundings. Obviously, attempts are
made to minimise the release of sulfuric acid, and the tanks are
covered or a surfactant is added that forms a layer of foam on the
surface (Fig. 3). However the emission of sulfuric acid droplets
cannot be entirely prevented.
During (dry) formation in the finished battery, the droplets are
largely contained in the battery, which is closed except for a small
vent hole. As one might expect, larger droplets cannot escape.
The sulfuric acid concentration is much lower here than during
wet formation (Table 1). In the overview of the results, one can
clearly see that tank formation is by far the more critical process.
Additionally listed are the results of measurements conducted
during battery recycling. At this workplace, empty used batteries
are shredded in order to recover the battery lead. The batteries
are of course emptied of acid before shredding, but residues of
sulfuric acid were detected in the vicinity of the shredder.
Battery recharging
To investigate the recharging of commercially used lead-acid
batteries, measurements were carried out at charge stations for
forklift trucks and in battery rooms for emergency power
supplies.
In emergency power supply units, as used, for instance, in
telecommunications, larger banks of lead-acid batteries have to
be able to provide a power supply for several hours. In Germany,
for example, precautions have been taken that the terrestrial
telephone network can be operated for at least six hours without
connection to the external electricity grid. Such systems are
usually trickle-charged, i.e. they are kept fully charged by off-
setting continuous discharge with a continuous low level of
charging current. For our measurements, the systems were
partially discharged beforehand and the full charging current
was applied.
At the charge stations for forklift trucks, there are two basic
procedures: the replacement of forklift batteries and forklifts that
are plugged straight into the charge station. At charge stations
for replaceable batteries, the battery units are lifted out of the
forklift by a crane and a charged new unit is inserted into the
forklift. Battery-changing takes just a few minutes and the
forklift is ready for use again. At the same time, the discharged
battery unit is connected to the charge station and a new
charging cycle is initiated.
Like during block formation, these battery recharging systems
are also enclosed and only small quantities of acid can escape via
the vent holes. We extended the measuring time here to up to 6 h.
Fig. 2 Experimental arrangement for work area sampling.
Fig. 3 (Wet) tank formation of lead plates.
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The measured concentrations are very low and the droplets
mostly belong to the thoracic fraction (Table 2).
Copper refining
Copper is of crucial importance for the electrical industry. Since
resources of copper are limited, the share of recycled copper has
steadily grown and in Germany now amounts to well over 50%
(2009: 56.7%).
13
The crude copper is refined in large baths to yield
99.99% electrolyte copper at the end of the recycling process. For
the refining of copper, two types of baths are employed.
In baths in which both the anode and cathode are made of
copper, copper is released from the anode and deposited on the
cathode. No gas bubbles are generated in these baths. In the
second type of bath, the anodes are made of lead, and gas
bubbles (oxygen) may rise during the process. Baths with lead
anodes have to be integrated in the production process to prevent
the bath solutions being enriched with copper and thus to ensure
that the optimal bath composition for the process is maintained.
In the proximity of these baths, sulfuric acid mist can arise in
larger concentrations. This electrochemical process also
produces gas bubbles and sulfuric acid droplets can be released.
Again, quite high sulfuric acid concentrations are measured here
with a low share of fine droplets (Table 3).
Anodising of aluminium
This process also makes use of sulfuric acid baths in which
a dense film of aluminium oxide is oxidised anodically onto
aluminium profiles that give long-term weather protection to
such items as window profiles, garden chairs and football goal
posts. The share of the thoracic fraction here is also only 40 to
50% (Table 4).
Electroplating
Sulfuric acid is also used in the galvanising industry as acid
electrolyte for pickling or electroplating. The relevant measure-
ments were conducted at a pickling bath and an electroplating
bath with a sulfuric acid/copper sulfate solution. The low figures
for sulfuric acid confirmed the observation that visible droplet
formation did not occur during measurement (Table 5).
Titanium dioxide
Titanium dioxide is the most widely used white pigment. In
Germany the sulfate process is commonly used in TiO
2
production. One important step in this process is the filtration of
a TiO
2
suspension in diluted sulfuric acid. The measurements are
performed at the Moore filtration station where the pigment is
purified and at the filtration station where the acid is separated
from TiO
2
residues for acid recycling (Table 6).
Further measurements
The production of sulfuric acid is an enclosed chemical process
and sulfuric acid droplets can only arise a significant concen-
tration when the process is disrupted. During our measurements
without any disruptions, we found only very low concentrations
of sulfuric acid (Table 7).
Table 7 also lists the results of another process where sulfuric
acid is used. The production process cannot be described for
Table 1 Results—manufacture of lead-acid batteries
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/mg
m
3
Thoracic
fraction
(%)
Tank
formation—
company 1
420 I 1.02 37
640 T 0.37
Tank
formation—
company 2
420 I 0.071 42
640 T 0.030
Block formation 420 I 0.008 100
640 T 0.008
Battery
recycling
630 I 0.024 42
960 T 0.011
a
I: inhalable and T: thoracic.
Table 2 Result—battery recharging
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/
mg m
3
Thoracic
fraction
(%)
Back-up power
supply—company 1
1050 I 0.0034 91
1600 T 0.0031
Back-up power
supply—company 2
1260 I 0.0024 85
1920 T 0.0020
Back-up power
supply—company 3
840 I <0.001 —
1280 T <0.001
Back-up power
supply—company 4
1050 I <0.001 —
1600 T <0.001
Forklift batteries—
company 1*
840 I 0.021 74
1280 T 0.016
Forklift batteries—
company 2
840 I 0.0052 87
1280 T 0.0047
Forklift batteries—
company 3
840 I 0.0044 84
1280 T 0.0036
a
I: inhalable and T: thoracic. * Recharging outside the forklift.
Table 3 Results—copper refining
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/
mg m
3
Thoracic
fraction
(%)
Electrolysis in a copper
electrode bath
420 I 0.018 26
640 T 0.004
Electrolysis in a lead
electrode bath
630 I 0.22 13
960 T 0.029
a
I: inhalable and T: thoracic.
Table 4 Results—anodising of aluminium
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/
mg m
3
Thoracic
fraction
(%)
Anodising of
aluminium—company 1
420 I 0.025 62
640 T 0.016
Anodising of
aluminium—company 2
840 I 0.021 36
1280 T 0.0073
a
I: inhalable and T: thoracic.
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reasons of trade secrecy. Parallel to this measurement a 10-stage
low-pressure cascade impactor (AERAS LPI 25/0.018/2.0—flow
rate 25 l min
1
, the cascade impactor has 10 stages with the
following mean aerodynamic particle diameters: 0.0253/0.0498/
0.0983/0.194/0.382/0.754/1.49/2.93/5.78/11.4 mm, Hauke Ltd.,
Gmunden, Austria) was used to determine the particle size
distribution. The total sulfuric acid concentration was 0.032 mg
m
3
and only 0.0012 mg m
3
of droplets larger than 11.4 mm
aerodynamic diameter were found. This is in good agreement
with the results listed in Table 7 for the supplementary process.
Furthermore in three additional measurements the compar-
ison between the cyclone and the cascade impactor was per-
formed. The calculated thoracic sulfuric acid concentration for
the cascade impactor and the measured average concentration
(N¼4) for the cyclone were in good agreement with an average
deviation of +9% for the cyclone (Table 8). This is an excellent
consistency for workplace air measurements.
As already mentioned, the GK2.69 cyclone was also used
simultaneously during some of the measurements. Owing to the
low throughput of this cyclone, the results for low sulfuric acid
concentrations were often below the quantification limit. For
higher concentrations, the concurrence with the IFA cyclone was
satisfactory.
Discussion and conclusion
All the individual obtained values are given in Fig. 4. It is clearly
evident that for concentrations >0.1 mg m
3
for the inhalable
fraction, the share of the thoracic fraction was below 50% in all
cases. If one considers this aspect of the results, the thoracic
fraction accounts for 32.1 12.5% of the inhalable aerosol.
At concentrations below <0.01 mg m
3
, only very low differ-
ences arise between thoracic and inhalable particles. The
processes involved are in most cases enclosed systems from which
larger droplets cannot escape. In all cases, the concentrations are
well below the limit values recommended by the EU and the
MAK Commission.
Higher sulfuric acid concentrations were measured in
processes where droplets are generated, for example by bursting
bubbles when these droplets were able to enter the breathing air.
At the magnitude of the recommended limit value or above there
is then a radical shift in the ratio of the thoracic to the inhalable
fraction. In the case of measured values on the scale of the
proposed limit values, larger droplets, which are only measured
as part of the inhalable fraction, always account for more than
half of the total sulfuric acid emission.
Table 5 Results—electroplating
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/
mg m
3
Thoracic
fraction
(%)
Pickling bath—
company 1
840 I 0.0087 64
1280 T 0.0056
Pickling bath—
company 2
840 I 0.030 31
1280 T 0.0092
Copper-plating 840 I 0.029 31
1280 T 0.0091
a
I: inhalable and T: thoracic.
Table 6 Results—manufacture of titanium dioxide
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/
mg m
3
Thoracic
fraction
(%)
Moore filtration—
company 1
630 I 0.019 68
960 T 0.013
Moore filtration—
company 2
630 I 0.076 32
960 T 0.024
Filtration for acid
recycling
630 I 0.041 63
960 T 0.026
a
I: inhalable and T: thoracic.
Table 7 Results—sulfuric acid production
a
Work area
Air
volume/l
Aerosol
fraction
Sulfuric
acid/
mg m
3
Thoracic
fraction
(%)
Sulfuric acid
production
840 I 0.0074 84
1280 T 0.0054
Supplementary
process
b
630 I 0.031 90
960 T 0.028
a
I: inhalable and T: thoracic.
b
The production process cannot be
described for trade secrecy reasons.
Table 8 Comparison between the cyclone and cascade impactor
Work area
Cyclone Impactor Ratio
Sulfuric
acid
a
/mg m
3
Sulfuric
acid
a
/mg m
3
Cyclone/impactor
Manufacture of
lead-acid batteries
0.017 0.018 0,93
Copper refining 0.032 0.028 1,14
Anodising of aluminium 0.044 0.031 1,41
Supplementary process
b
0.027 0.032 0,85
a
Thoracic aerosol.
b
The production process cannot be described for
trade secrecy reasons.
Fig. 4 Thoracic fraction as a share of the inhalable fraction.
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If one now compares the proposed limit values of the EU and
the MAK Commission, it can be concluded that these two limit
values represent a comparable level of protection. In fact, the
numerically higher value of 0.1 mg m
3
for the inhalable fraction
may even be more demanding in terms of its implementation for
regulatory purposes.
Notes and references
1Report: Sulfuric Acid: 2010 World Market Outlook,
MarketPublishers, Birmingham, UK, 11/2009, p. 288.
2 International Agency for Research on Cancer (IARC) monograph
summary, vol. 54, 1992, http://monographs.iarc.fr/ENG/Monographs/
vol54/volume54.pdf.
3 Commission Directive 2009/161/EU of 17 December 2009
Establishing a Third List of Indicative Occupational Exposure
Limit Values in Implementation of Council Directive 98/24/EC and
Amending Commission Directive 2000/39/EC, 2009.
4 Carex, Canada School of Environmental Health, Strong Inorganic
Mists Containing Sulfuric Acid, March 2009, updated March 2010,
www.carexcanada.ca/en/sulfuric_acid_mists.pdf.
5List of MAK and BAT Values 2010: Maximum Concentrations and
Biological Tolerance Values at the Workplace, ed. Deutsche
Forschungsgemeinschaft (D F G), Wiley, Report 46, August 2010.
6Database Gestis International Limit Values, ed. Institute for
Occupational Safety and Health of the German Social
Accident Insurances, http://www.dguv.de/ifa/en/gestis/limit_values/
index.jsp.
7 EN 481, Workplace atmospheres. Size fraction definitions for
measurement of airborne particles, 1993.
8 EN 482: Workplace atmospheres. General requirements for the
performance of procedures for the measurement of chemical agents,
2006.
9 ISO 21438-1: Workplace air—Determination of inorganic acids by
ion chromatography—Part 1: Non volatile acids (sulfuric acid and
phosphoric acid), 2007–12.
10 J. R. Cossey and N. P. Vaughan, Ann. Occup. Hyg., 1987, 31,
39–52.
11 Ger€
ate zur Probenahme der einatembaren Staubfraktion (E-Staub),
in IFA-Arbeitsmappe Messung von Gefahrstoffen, ed. Institute for
Occupational Safety and Health of the German Social Accident
Insurances, Erich Schmidt Verlag, Bielefeld, 2000, vol. 27, Sheet
No. 3010.
12 D. Breuer and K. Gusbeth, Anorganische S€
auren, partikular:
Phosphors€
aure, Schwefels€
aure. in IFA-Arbeitsmappe, Messung von
Gefahrstoffen, ed. Institute for Occupational Safety and Health of
the German Social Accident Insurances, Erich Schmidt Verlag,
Bielefeld, 2007, vol. 38, Sheet No. 6173.
13 Metallstatistik, Berlin, Hrsg.: Wirtschaftsvereinigung Metalle, 2009,
www.wvmetalle.de/wvmprofi/medien/doc_6693_201072815934.pdf.
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Authors Queries
Journal: EM
Paper: c2em10659k
Title: Sulfuric acid at workplaces---applicability of the new Indicative Occupational Exposure Limit Value (IOELV) to
thoracic particles
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ART  C2EM10659K
This journal is ªThe Royal Society of Chemistry 2012 J. Environ. Monit., 2012, xx, 1–7 | 7
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