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EC number: 231-867-5
CAS number: 7772-98-7
Derivation of freshwater PNEC
value using assessment factor methods
The general principle of these methods
is that the result from a laboratory test is divided by an appropriate
assessment factor (ECHA, 2008). The
sparser the available data, the higher is the assessment factor which is
are estimated by division of the lowest value for the toxicity with the
relevant assessment factor. Results
of long-term tests (expressed as EC10 or
NOEC for a sublethal parameter) are preferred to those of short-term
tests (E(L)C50), because such results give a more realistic
picture of effects on the organisms during their entire life cycle.
In establishing the size of these
assessment factors, a number of uncertainties have been addressed to
extrapolate from single-species laboratory data to a multi-species
• intra- and inter-laboratory variation
of toxicity data;
• intra- and inter-species variations
• short-term to long-term toxicity
• laboratory data to field impact
The assessment factors recommended for
the determination of the PNEC for the freshwater aquatic are shown in
Table: Assessment factors to derive a
At least one short-term L(E)C50from each of three trophic levels (fish, invertebrates (preferred Daphnia) and algae)
One long-term EC10or NOEC (either fish or Daphnia)
Two long-term results (EC10, NOECs) from species representing two trophic levels (fish and/or Daphnia and/or algae)
Long-term results (EC10, NOECs) from at least three species (e.g., fish, Daphnia and algae) representing three trophic levels
Species sensitivity distribution (SSD) method
5-1(to be fully justified case by case)
Field data or model ecosystems
Reviewed on a case by case basis)
When only short-term toxicity data are
available, an assessment factor of 1000 will be applied on the lowest
the relevant available toxicity data, irrespective of whether or not the
species tested is a standard test organism. A
lower assessment factor will be applied on the lowest EC10 or
NOEC derived in long-term tests with a relevant test organism.
For some substances, a large number of
validated short-term E(L)C50 values may be available. Therefore,
it is proposed to calculate the geometric mean if more than one E(L)C50 value
is available for the same species and endpoint. Prior
to calculating the geometric mean an analysis of test conditions must be
carried out in order to find out why differences in response were
The algal growth inhibition test of the
base-set is, in principle, a multi-generation test. However,
for the purposes of applying the appropriate assessment factors, the EC50 is
treated as a short-term toxicity value. The
NOEC from this test may be used as an additional long term result when
other long-term data are available. In
general, an algal EC10 or NOEC should not be used unsupported
by long-term EC10 or
NOECs of species of other trophic levels.
Micro-organisms representing a further
trophic level may only be used if non-adapted pure cultures were tested. The
investigations with bacteria (e.g., growth tests) are regarded as
short-term tests. Additionally,
blue-green algae should be counted among the primary producers due to
their autotrophic nutrition.
The assessment factors should be
considered as general factors that under certain circumstances may be
general, justification for changing the assessment factor could include
one or more of the following:
from structurally similar substances (evidence established by read
across from closely related substances may demonstrate that a higher or
lower factor may be appropriate);
of the mode of action including endocrine disrupting effects (Some
substances, by virtue of their structure, may be known to act in a
availability of test data from a wide selection of species covering
additional taxonomic groups other than those represented by the base-set
availability of test data from a variety of species covering the
taxonomic groups of the base- set species across at least three trophic
levels. In such a case the assessment factors may only be lowered if
these multiple data points are available for the most sensitive
Since no large dataset from long-term
tests for different taxonomic groups is available for sodium dithionite,
no Species Sensitivity Distribution (SSD) can be developed and
statistical extrapolation methods can thus not be used to derive the PNECaquatic. Instead,
The PNECaquatic calculation
will be conducted using assessment factors method.
An overview of the species-specific
data is given below. All
relevant effects data are expressed as mg S2O32-/L
and mg S/L.
Table: Overview of most sensitive
species-specific EC10/NOEC-values for thiosulfate substances
in the freshwater environment
(1): Sodium sulfite data
translated to sodium thiosulfate, assuming that all S is converted to
sulfite when thiosulfate oxidizes
In this scenario an assessment factor
(AF) of 10 should be used to calculate the PNECaquaticfrom
the lowest value for the toxicity. This
factor can be applied since three long-term results (e.g. NOECs) from
species representing three trophic levels (algae, invertebrates, fish)
are available. The
lowest value for chronic toxicity was and unbounded NOEC of 5.90 mg S2O32-/L.
Applying the AF of 10 results in a PNECaquatic of
0.59 mg S2O32-/L. Translating
this value to Na2S2O3gives a PNECaquaticof
0.80 mg test substance/L.
As the lowest NOEC-value is an
unbounded value (i.e., no effect was noted at the highest test
concentration), this value can be considered as a worst-case estimate.
Further refinement of the NOEC-value for daphnids could increase the PNECaquaticup
to a maximum value of 7.6 mg S2O32-/L
(i.e., an assessment factor of 10 on the algal 72h-EC10value).
Due to the physicochemical properties
which make adsorption to sediments unlikely, and the microbial oxidation
of thiosulfates under environmental conditions, the derivation of a PNEC
for the sediment compartment is not feasible/appropriate:
to the natural oxidation of thiosulfates by microbial activity, no
relevant test design and toxicity data can be generated
to the absence of a relevant adsorption
coefficient for thiosulfates to sediment, the equilibrium partitioning
method for deriving a PNECsediment is
into account the industrial use, exposure pathways and environmental
fate of thiosulfates, long-term exposure of sediment organisms to this
substance can be excluded.
Consequently, there is no need to
derive a PNECsediment for
into account its physicochemial properties, the industrial use, exposure
pathways, and the long-term instability of thiosulfates under
environmental conditions (decomposition to sulfite and sulfate), a
continuous, long-term exposure of terrestrial organisms to thiosulfates
can be excluded.
However, an important use of ammonium
thiosulftate (ATS) - and thiosulfates in general – is the application in
the agricultural industry as S fertilizer to crops. Consequently, there
is a direct pathway for thiosulfates to enter the terrestrial
compartment. Upon application to soil ATS is rapidly transformed to
plant available SO4, thereby effectively alleviating S
deficiency in crops (Graziano, 1990). Indeed, thiosulfate is not
generally available for plant uptake until it is converted to sulfate.
The counter-cation of S2O32-(NH4,
K, Ca) also contributes to the fertility of the treated soil.
Depletion of thiosulfate in soils
occurs rapidly: depending on the intitial concentration (5-100 mg S as
thiosulfate/kg dw soil), the conversion occurs within 4-15 days in
aerobic conditions, and within 10-20 days under anaerobic conditions
(Saad et al, 1996). With regard to toxic effects of thiosulfates for
plants, McCarty et a l(1990) observed
that at excessive rates of 2500 or 5000 mg ATS/kg soil, germination of
corn or wheat seeds in soil was significantly decreased . Early seedling
growth in soil decreased dramatically when ATS was applied at the rate
of 1000 or more mg/kg soil; At these rates, ATS inhibited root and shoot
growth. The observation that high levels of ATS can induce phytotoxic
effects, was in harmony with previous findings from Goos (1985) and
Mahler and Lutcher (1989) that high levels of ATS can cause severe crop
damage. ATS is also applied as
blossom thinning product to regulate crop load in fruit trees (Janoudi
and Flore, 2005; Fallahi and
Willemsen, 2002; Greene et al, 2001; Holb, 2008), as it is considered as
a safe product for both the consumer and the environmùent (Wertheim,
2000). ATS at concentrations of 1% and 2%, for instance, resulted in
damage ranging from 40% to 86% of all flowers that were open at the time
of the treatment (damage to pistils, petals and stamens due to its
action as a caustic agent).
Publications on the effect of
thiosulfates on soil invertebrates have not been identified.
Beside the fact that thiosulfates
substances are a source of plant nutrients, they can also improve the
nitrogen efficiency in agricultural processes. Inhibition of both
nitrification and/or hydroysis of N-substances (urease activity
inhibition) upon addition of ATS has been suggested in literature: in
combination with UAN (urea ammonium nitrate), ATS administraton
effectively inhibits the hydrolysis of urea, thus preventing
volitilization of ammonia from the soil. Administered separately, Goos
et al (1986a,b) have been unable to demonstrate significant inhibition
of nitrification by ATS, and significant retardation of urea hydrolysis
in soils was only observed when applied at rates as high as 2500 or 5000
mg/kg soil (Bremner et al, 1990). Under laboratory conditions,
nitrifiation inhibition in soil by ATS has been observed at rates ≥ 250
mg/kg soil, thereby causing accumulation of potentially toxic amount of
nitrite (Bremner et al, 1986; Goos and Fairlie, 1988). However, under
field conditions, this nitrification inhibition could not be demonstrated Goos
et al, 1986a,b). It is noteworthy that not ATS, but its oxidation
product tetrathionate (S4O62-) is most
likely the actual urease inhibitor (Sullivan and Havlin, 1992)
Effects on nitrification are generally
considered as a negative property for a substance, and NOEC/EC10for
the endpoint nitrification are considered relevant for assessing risks
to the terrestrial environment. However, in the specific case of
thiosulfates, the only relevant exposure of the terrestrial environment
is an intentional exposure (use as fertilizer), and the inhibition of
nitrification is a “desired” effect, as it reduces leaching of NO3and
keeps N for a longer period in a form under which it can be taken up by
plants. Consequently, the derivation of a PNEC which is based on a
nitrification inhibition NOEC (+ asessment factor) would lead to a
maximum allowed concentration in agrucultural soils that reduces the
beneficial effects of thiosulfates fro man agricultural point of view.
Deriving a PNEC based on the lowest
NOEC for plants (root growth for corn and wheat) according to ECHA
(2008) guidance would equally be counterproductive from an agricultural
point of view. As chronic effects are availabele for plants and
micro-organisms but not for soil invertebrates, an assessment factor of
50 should be applied in the NOEC of 75.7 mg thiosulfate/kg soil,
resulting in a PNEC of 1.51 mg thiosulfate/kg.
From an agricultural point of view,
such value makes no sense: it is more than factor of 100 below the
concentration that reportedly induces the (desired) nitrification
inhibition, and more than a factor of 200 below the lowest concnetration
that caused an effect on root and shoot growth of wheat and corn.
Secondly, the amount of added
S-fertilizer to the soil becomes almost negligible at such concentration
Based on this information it it
concluded that the derivation of a PNEC for the terrestrial compartment
can be waived:
The aim of the assessment for
micro-organisms is the protection of the degradation and nitrification
functions and process performance and efficiency of domestic and
industrial STPs. A PNECmicro-organism can be derived in different ways
based on the information at hand, and expert judgement of the weight of
For many substances, including a
data-poor substances like sufites/disulfites, there are insufficient
useful data for aquatic micro-organisms available for the application of
the statistical extrapolation method for PNEC-derivation. In that case,
the assessment factor methodology can be used on the extracted
relevant/reliable test results. Two types of tests are considered
relevant for deriving the PNECmicro-organism in
STPs: (1) tests with a mixed inoculum (e.g. activated sludge) for the
endpoint respiration, and (2) a test with ciliated protozoa (preferablyTetrahymena)
for the endpoint mortality.
In general, an AF of 10 is to be
applied to the NOEC/EC10of a sludge respiration test,
reflecting the lower sensitivity of this endpoint as compared to
nitrification, as well as the short duration of the test. The
corresponding AF is 100 when based on the EC50. The PNECmicro-organism is
set equal to a NOEC (AF = 1) for a test performed with specific
bacterial populations such as nitrifying bacteria,P. putida,
ciliated protozoa, the Shk1 Assay. An EC50from this test is
divided by an AF of 10 to derive the PNECmicro-organism. If
no standard microbial inhibition test data are available, the PNECmicro-organism can
also be derived from available ready biodegradation tests. An assessment
factor of 10 is applied to the test concentration at which no toxicity
to the inoculum is observed. This approach can also be used for inherent
biodegradability tests. From an activated sludge simulation study, aPNECmicro-organism can
be derived based on the PECmicro-organism or
PECinfluent, using an AF between 1 and 10 depending on the
parameters monitored. The AF of 1 can be used in case there is no impact
on nitrification and BOC/COD removal performance (NB: if sludge from an
industrial STP was used for the test, the PNECmicro-organism can
not be used for the extrapolation to a domestic STP). No AF is needed to
derive a PNECmicro-organism based
on good quality field data.
So, the rationale for the application
of a higher assessment factor for the heterotrophic micro-organisms
compared to the nitrifying bacteria, is that they are exposed to a
higher concentration which relates more to the influent concentration.
For the nitrifying bacteria the exposure concentration is more related
to the effluent concentration since nitrification is the last treatment
step in a STP.
Translating this value to Na2S2O3
gives a PNECmicro-organisms of 102.6 mg test substance/L.
Acute and chronic toxicity data were
available for the three main aquatic trophic levels that are considered
for classification purposes. Classification is based on the lowest acute
and chronic value, referred to as the acute and chronic toxicity
reference value (TRV).
The lowest acute effect concentration
was observed for the alga S.
and was ≥75.7 mg S2O32-/L.
Translating this value to Na2S2O3 results
in an acute TRV of ≥106.7 mg/L mg/L for this substance.
Substances for which the acute TRV is
situated above 100 mg/L, are not environmentally classified.
Consequently, there is no need to
classify sodium thiosulfate for the environment.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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