Risk assessment of leather dyestuff
Risk assessment of leather dyestuffs
by
Dr. Alois Püntener
Abstract
Public interest is increasingly focused on leather goods and the potential risks to our
health and the environment arising from them. Although these hazards are not
substantiated by any established findings, the media seize on them and often paint
a very alarmist picture in their stories.
Fortunately, the real picture is not all black, there are also some positive aspects.
The leather industry is making considerable progress in improving its environmental
performance. This paper will look at risk and life-cycle assessment of leather dyes.
The identification of human health and environmental hazards are important
prerequisites for risk and life cycle assessment. Good quality information on
exposure is needed to address the risk objectively, and possible options for risk
reduction. Risk management should indeed start with a careful selection of colorants
that exhibit the required performance with regard to substrate affinity, fastness and
other boundary conditions.
The environmental risk posed by a colorant can be defined in both its inherent
ecotoxicity and the concentrations attained in the environmental compartments.
There is an increasing body of circumstantial evidence indicating that the portion of
colorants entering the environment is ultimately degradable either by biological or
photochemical pathways.
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Introduction
There appears to be growing concern regarding the impact of leather on the
environment and the health of consumers. The general public and the authorities
are paying increasing attention to these areas, and additional regulations are being
developed accordingly. All aspects from origination right through to disposal have to
be taken into account. Life Cycle and Risk Assessments are important tools to
investigate the ecological and toxicological impact of processes and products. The
results of Life Cycle or Risk Assessments are only meaningful in the context of the
goals, scope and limitation of the study.
Life Cycle and Risk Assessments address different but similar issues. Life Cycle
Assessments 1 (LCA) examine the ecological consequences, or what is known as
the ”Environmental Burden" of producing and using goods: ISO 14 040 (1996 Draft).
Any systematic approach to shifting the life cycle of leather towards a lower
environmental impact should cover all the relevant aspects including, the air, soil,
water, energy and resources involved for all parties.
The cycle starts with the farmer and slaughterhouses, and moves on to the tanners,
and dye and chemical manufacturers, then on to the leather goods manufacturers
and retailers, and it finally ends with disposal. All relevant emissions and resource
consumption should be studied. However the study can only be completed if all the
participants along the process chain including tanners, leather goods producers,
retailers and consumers contribute the necessary ecological data.
Risk assessment 2 (RA) studies, like ”Risk Assessment of Leather Dyes”, focus on
toxicity and the impact on human beings and the environment: European Union
Council Directive of June 27, 1997 (67/548/EEC).
A leather dye manufacturer’s responsibility does not start with the production of
dyes and end with the delivery to the tanners. The integrated approach of such a
study needs to consider the starting chemicals, the method and control of synthesis
including purification, the packaging, the means of transport and storage,
performance in dyeing, impact on the consumer and finally behaviour on ultimate
disposal.
Two different issues that are often not clearly differentiated have to be addressed:
Workplace Risk and Consumer Risk.
Leather Dyes in the Environment
Workplace risk:
Today, there is a great deal of data available on the toxicological aspects of dyes.
Human exposure to leather dyes occurs primarily at dye manufacturing plants and
within tannery dyehouses.
Dye production is akin to a balancing act between the needs of a broad public on
the one hand, and the demand for responsible treatment of the environment on the
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other. Irrespective of whether ”synthetic” or ”natural” dyes are being produced, they
are usually accompanied by unwanted by-products. In these respects, dye
manufacture is no different from other industrial scale production such as in the
manufacture of pharmaceuticals, detergents, ”natural” or ”synthetic” chemicals, or
even leather. Not surprisingly, the authorities, scientists, and general public are
taking an increasingly active role in the debate about the risk potential of the
production of dyes, and the risks of exposure to dyes.
In common with other chemicals and natural substances, dyes can trigger allergies
in some individuals. Finding an universal solution to this problem is as difficult as the
allergy is unpleasant to the person affected. Every replacement substance, whether
”natural” or ”synthetic”, may affect yet other individuals. The OECD Guideline No.
406 regulates a test procedure to investigate sensitisation, in order to determine
possible hazards. It has been reported that the sensitisation effects of some
commercial grades are due to the presence of soluble impurities.
Under a recently established occupational skin surveillance scheme in the United
Kingdom (UK), out of a total 2811 case reports of occupational skin disease
received, only 19 cases (less then 1%) were related to employees who could
possibly have been exposed to dyes 3.
Consumer risk:
Direct contact between dyed leather and the skin of the consumer represents a very
small part of leather usage. It occurs in some watchband and sandal leathers, but
the bulk of leather articles do not normally involve direct skin contact. In contrast,
most textile articles are designed for direct skin contact.
Dyes can also be inhaled in the form of dust but this happens, if at all, during dye
manufacturing and application processing. Possible uptake through the skin or via
ingestion is extremely slight, but may occur if the dye is not properly fixed.
Moreover, leather products for medical use or for children have to conform to stricter
restrictions than standard articles.
But if leather dyes have a high level of fastness and they are properly fixed, there is
little risk for the consumer of dye uptake via skin or mouth, and the risk of allergies
is very small.
However, the German Consumer Act 4, which includes leather as a consumer
material, has intensified the discussion about the potential risk of dyed leather
goods. A common misconception is that these regulations apply to all azo dyes and
that every azo dye is dangerous. This is far from true.
Azo dyes are the major dye class of commercial synthetic dyes for leather, textile,
paper and food. Today, only a few dyes are produced that fall under the German
decree and alternatives are available for those dyes.
We have to remember that leather dye producers such as TFL (previously Ciba) do
not manufacture or market azo dyes for leather or other materials that, when
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analysed according to DIN 53316, could through cleavage of one or more azo
groups form the specified amines listed in the German Consumer Goods Act. This
applies to most members of the Ecological and Toxicological Association of Dyes
and Organic Pigments Manufacturers (ETAD).
Toxicological aspects
Some of the raw materials and intermediates required for the synthesis of dyes are
more toxic than the final dyes produced. Inevitably, therefore, these raw materials
are potentially hazardous, and that must be minimised by appropriate precautions
during production. These risks do not apply to the user in the tanneries and the
consumer. A prerequisite is that the dye manufacturer ensures the absence of
possibly hazardous raw material.
Before being released to the trade, new leather dyes have to be tested for
ecological and toxicological properties, as dyes are chemicals foreign to the human
body. The test criteria are specified in official regulations, and followed by the
manufacturer as part of his responsibility towards the user.
The principles of risk assessment of new substances, like leather dyes, are laid
down in European Union Commission Directive 93/67/EEC and include: hazard
identification, dose-response assessment, exposure assessment for the
environmental compartments, and risk characterisation. Cost of registration of a new
leather dye lies between 100’000 and 250’000 US$. A testing program for existing
products is also under discussion. Analysis of the available data on these dyes
provides adequate evidence that no major hazard is to be feared. An examination of
more than 4400 organic dyes by analysing the Material Safety Data Sheets has
confirmed that most of them have an LD 50 value (lethal dose by 50% of the tested
rats) that is greater than 2000 milligrams/kg 5. Only a minority of commercially
available dyes must be classified as ”harmful” according to EC Guidelines.
Pathways of Leather Dyes into the Environment
Despite the negligible acute toxicity of dyes and the numerous efforts undertaken to
avoid or reduce risks, dye manufacturers and tanneries are confronted with
effluents, wastes and contaminated containers or packaging material that require
carefully thought out disposal. TFL and other dye suppliers use reusable containers
for bulk products, or packaging material which can be emptied without leaving any
residues and does not generate any special waste.
The primary route by which dye enters the environment from dye manufacturers and
tanneries is through the production of wastewater, and also through the disposal of
sludge containing dyes precipitated from the effluent by flocculation.
A prerequisite for a colorant to enter the environment is its solubility. Colorants that
enter the wastewater streams (estimates are 1 to 5% of the world production of
leather dyes) normally pass through a wastewater treatment plant where they are
eliminated to a large degree by adsorption on the sludge. The extent to which
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residual amounts reach the surface waters depends on the efficiency of treatment
processes. The fate of dyes adsorbed to sludge is generally incineration or disposal
in a controlled landfill.
Dye manufacturers:
The following mass flow can be estimated for the production building of an average
dye manufacturing plant. For the production of 10’000 tonnes of dyes per year 1-2%
of waste and 5-10% of carbon-based chemicals in the waste water have to be
expected. These materials are generated as by-products during synthesis, and are
removed from the dyes to maintain quality. These by-products could also interfere
with the application of the dye. For purification, special processes are often
necessary which yield waste, which needs special disposal.
Tanneries:
In tanneries the exhaustion of a good dye is 96 to 99% (or higher) and therefore the
problem is less relevant than with other chemicals. In tanneries, which in many
cases produce large quantities of waste, only approximately 1% of waste arises
from the dyeing process.
It has to be noted that dyes are typically applied for a limited time in a number of
different drums (point sources). Detailed data for the weight of processed dye are
difficult to generate because of the use of different shades and production
strategies, so it is not normally possible to predict usage for individual customers.
Initial Environmental Concentration may be calculated for a day’s operation of a
tannery dyehouse using the following formula 6 :
E=
W1 x W2 100-F x A
100
100
where
E
W1
W2
F
A
=
=
=
=
=
(1)
emission per day (kg/d)
mass of dyed goods per day (t/d)
mass of dye used per mass of raw hides (kg/t)
degree of fixation of dyes (%)
participation factor (50 %)
It should be taken into consideration that many tanneries are only processing wet
blue, wet end or finishing operations. W2 refers to the mass of raw hides which is 4
times greater than the mass of dye goods per day. Instead of using this somewhat
unclear definition, we propose the following equations
E=
where
D x 100-F x A
100
100
D
=
(2)
mass of dyes used (kg)
A participation factor on production per day (50 %) may be used by dyestuffs, since
in practice the dyeing step with a specific dye does not take a full day. We
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recommend a formula similar to that of ETAD7
E=
where
D x (100-F) x (100-P)
100
100
P
=
(3)
degree of elimination (%)
Elimination of dyes may occur through adsorption on sediments and suspended
particles with subsequent removal from the waste water by settling or filtration. The
results can be quite significant depending on the nature of the dye molecule.
The above equations are only applicable to exhausted drum dyeing processes.
Major dye-classes used in the leather industry are direct and acid dyes (this
accounts for about 85%), metal complex dyes (around 10%) and, to a lesser extent,
sulphur (around 2%), cationic and other dyes. A straightforward calculation of
environmental releases is possible by using equation (3).
Realistic parameters for a tannery producing dyed grain leather with an output of
approximately 400 - 600 hides per day are as follows 8 :
Dyes
% fixation
% dyeing
dyed grain leather
dyes used
% elimination
emission
sulphur
92
2
10 t
200 kg
70
4.8 kg/d
acid
96
2
10 t
200 kg
70
2.4 kg/d
direct
98
2
10 t
200 kg
95
0.2 kg/d
metal complex
99
2
10 t
200 kg
90
0.20 kg/d
Realistic parameters for a tannery dyeing suede leather with an output of 400 -600
hides per day are as follows. In this case we have to consider that exhaustion of
suede is normally higher than grain leather, however more dye is often needed and
therefore fixation levels are somewhat lower:
Dyes
% fixation
% dyeing
dyed suede leather
dyes used
% elimination
emission
sulphur
90
4
10 t
400 kg
70
12 kg
acid
94
4
10 t
400 kg
70
7.2 kg/d
direct
96
4
10 t
400 kg
95
0.8 kg/d
metal complex
98
4
10 t
400 kg
90
0.8 kg/d
The volumes of effluent and the concentration of ingredients varies to a large
degree depending on plant capacity, tanning technology, dyeing and waste water
management. To produce 10t of leather, approximately 40t of raw hides are used
and up to an estimated 2000 m3 water 9. Today, modern tannery have reduced this
amount dramatically down to around 1000 m3 10.
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Applying the above equation results to an Initial Environmental Concentration gives
a concentration of 10 to 0.1 ppm dyes in the waste water for an average tannery.
Such a concentration is well below any level for concern for ecotoxicity and would
hardly be visible in a well-mixed effluent.
However, it should not be forgotten that low concentrations, that is more than 10
ppm of colorant in receiving waters, can cause visible coloration and may raise
public concern, although the low concentrations involved do not normally pose any
significant environmental hazard. Dyes can be removed easily in the effluent
treatment plant. Before passing through a mechanical-biological treatment, dyes are
part-purified by decoloring, pH neutralisation and precipitation and through
adsorption with flocculation agents. The sludge can then either be incinerated or
dumped.
In both cases, in a landfill or in water, biodegradability of the dye is important and
ecotoxicity must be considered. In order to study the effects on aquatic organisms,
aquatic toxicity is tested using Zebra fish (Brachydanio rerio) and Daphnia magna.
The growth inhibition on alga (Scenedesmus subspicatus) is also measured. There
is a considerable body of evidence that synthetic colorants in general, and watersoluble dyes in particular, are unlikely to be bio-accumulative. On this basis it may
be predicted that long-term chronic effects on aquatic organisms are very unlikely to
result from continuous exposure.
Where sewage sludge is concerned, and in particular when it is used as an
agricultural fertiliser, the possible effects of sludge contaminants on the soil must be
considered. The Zahn-Wellen Test (OECD Guideline 302B) was adopted to
determine inherent biodegradability. According to different studies, it seems unlikely
that a calculation of the theoretical concentration of a dye in sewage sludge will lead
to any concern about use as an agricultural fertiliser.
Disposal of Leather Goods
Leather is used to make consumer goods which sooner or later end up as
household refuse. The consumer has a responsibility not to use the simplest method
of disposal - unfortunately still in common practice - that is, just throwing refuse
away. Campaigns to collect worn and unwanted articles and sporadic recycling
initiatives do somewhat improve the situation. However the best solution would be to
make new material or products from recovered leather. The second best way to
dispose of used leather is by incineration, because leather provides considerable
amounts of energy.
The simplest method would be to return the used natural product leather to nature,
i.e. to compost it. We are therefore studying this solution carefully. Some initial
progress has been achieved. We have ascertained that soluble dyes and some
organic pigments degrade under given biological conditions. This also applies to
chromium complex dyes. Only copper complex dyes in high concentrations can slow
down the degradation of organic substances to minerals and humic acids, since
copper salts have a fungicidal effect 11.
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Various studies have shown that dyes are degradable, and degradation of colorants
in the environment is likely to be a very natural process 12, particularly under
anaerobic conditions. Reductive cleavage to metabolites, which may be further
degradable to natural species, could occur. Dyes and leather can also degrade by
photochemical means as an ageing process, but this is also usual for dyes
dissolved in water or leather exposed to sunlight. The same natural species are
obtained as through composting where degradation is a biological process.
Degradation in the soil or air is comparable to the process which takes place in
water.
Role of natural dyes
The use of natural dyes is attracting growing attention. However their extraction and
processing cannot, in general, be regarded as environmentally friendly or even
environmentally benign. To dye 100 kg. leather with 2% dye requires approximately
100kg. dried leaves, equivalent to 500 to 1000kg. freshly picked leaves. Even if only
a small fraction of the leather produced were dyed with vegetable dyes, very large
plantations would be necessary. In addition, dye producing plants, with a few minor
exceptions, supply no foodstuffs or other useful by-products. The chromophore is
often present only in very small, often unknown quantities and is unevenly
distributed. The extraction process also produces a large amount of inert materials
which have to be removed from the goods or the dyebaths.
Take indigo, one of best known organic dyes:
From 300 kg of fresh indigo plants a farmer could produce after extraction roughly 1
kg of pure Indigo dyes with a lot of waste as a by-product. ”Isatis tinctoria” plants are
harvested before they bloom and fermented in water pools. After the intense carbon
dioxide output has finished, the indigo has to be oxidised, filtered and dried.
The farmer would fetch a price comparable to that of synthetic Indigo - about US-$
20/kg. This is in great contrast to the expense of growing, harvesting and isolating
the Indigo component.
Another well know natural dye is Cochenille:
For 1 kg dye 140’000 ”Coccus cacti lice” are needed. The insect grows on the
”Opuntia coccinellifera” cactus. Compare this with the two-step synthesis of 1 kg azo
dyestuff, which needs approximately:
1
1
1
0.25
0.50
50
kg raw material
kg acid
kg alkali
kg soda nitride
kg common salt
l water
A more realistic option in the long term could well be the production of natural dyes
with the aid of genetically engineered micro-organisms. Today it is possible to
achieve indigo yields of a few grams per litre per day by fermentation, however, this
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process is still too expensive. Whether more efficient yields and better costs will be
forthcoming with this or other dyes cannot be predicted at this time.
Conclusion
Colours and coloration in their various manifestation are a part of every culture and
heritage and our daily lives would be unthinkable without them. Dyed leather in the
form of shoes, clothing, upholstery and other consumer goods offer wearers and
users a sense of well being and comfort. There is an increasing body of
circumstantial evidence that the small portion of colorants entering the water or soil
does not significantly harm the environment.
Risk Assessment of leather dyes is a function of both the hazard characteristics and
the environmental exposure (concentration and duration). Effects on organisms in
the environment can be either short-term, e.g., acute toxicity to fish or daphnia,
reproductive effects, or long-term (chronic), e.g., effects on growth. There are
currently no data known that would indicate accumulation of dyes in terrestrial and
aquatic food chains.
If the characteristics of natural and synthetic dyes are compared, synthetic dyes are
clearly superior with better fastness properties and shade reproducibility. In addition
they generate less pollution during synthesis and application. High quality dyeing
includes, of course, not only features such as high fastness and good fixation, but
also safety in use, and no toxic hazard.
Dyed leather itself is not a hazard to human beings nor is it harmful to the
environment.
At the consumer level, the stability of leather and fastness properties of dyes ensure
a long functional life for leather goods. On balance this has the long term effect of
saving raw material and energy against that needed to produce cheap throw away
articles which pollute the environment.
Members of the public and the authorities have to recognise that the aggregate
environmental balance-sheet for dyed leather and synthetic dyes has been
substantially improved by the responsible use of natural resources and pollution
control in order to produce a long lasting quality product.
Acknowledgements
This paper is based on several in-house reports and application tests carried out by
TFL Colleagues. Grateful thanks also to the ETAD for discussions.
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9
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updated 5.Jan. 2004
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