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Diss Factsheets

Administrative data

Hazard for aquatic organisms


Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.83 mg/L
Assessment factor:

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.083 mg/L
Assessment factor:


Hazard assessment conclusion:
PNEC value:
106.5 mg/L
Assessment factor:

Sediment (freshwater)

Hazard assessment conclusion:
no exposure of sediment expected

Sediment (marine water)

Hazard assessment conclusion:
no exposure of sediment expected

Hazard for air


Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms


Hazard assessment conclusion:
no exposure of soil expected

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential for bioaccumulation

Additional information

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 applied. PNECs 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 ecosystem. These areas comprise:


• intra- and inter-laboratory variation of toxicity data;

• intra- and inter-species variations (biological variance);

• short-term to long-term toxicity extrapolation;

• laboratory data to field impact extrapolation.


The assessment factors recommended for the determination of the PNEC for the freshwater aquatic are shown in Table below.


Table: Assessment factors to derive a PNECaquatic

Available data          

Assessment factor

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 E(L)C50 of 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 present. 


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 EC10 or NOEC from this test may be used as an additional long term result when other long-term data are available. In general, an algal EC10or 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 changed. In general, justification for changing the assessment factor could include one or more of the following:

-      evidence from structurally similar substances (evidence established by read across from closely related substances may demonstrate that a higher or lower factor may be appropriate);

-      knowledge of the mode of action including endocrine disrupting effects (Some substances, by virtue of their structure, may be known to act in a non-specific manner);

-      the availability of test data from a wide selection of species covering additional taxonomic groups other than those represented by the base-set species;

-      the 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 taxonomic group.



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


Trophic level


(mg S2O32-/L)


Pseudokirchneriella subcapitata



ECT, 2010

Daphnia magna

Crustacea (invert.)


BASF, 1994

Danio rerio



ECT, 2010

(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 CaS2O3gives a PNECaquatic of 0.83 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:

-      due to the natural oxidation of thiosulfates by microbial activity, no relevant test design and toxicity data can be generated

-      due to the absence of a relevant adsorption coefficient for thiosulfates to sediment, the equilibrium partitioning method for deriving a PNECsediment is not applicable

-      taking 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 thiosulfate substances.



 Taking 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-compounds (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 level


Based on this information it it concluded that the derivation of a PNEC for the terrestrial compartment can be waived:

-      taking into account the industrial use, exposure pathways and environmental fate of thiosulfates (long-term instability of thiosulfates under environmental conditions and rapid decomposition to sulfite and sulfate), unintentional exposure of the terrestrial compartment to thiosulfates can be excluded;

-      intentional exposure (use as a fertilizer) is not a continuous process, i.e., administration of thiosulfates only accurs at some fixed points in time, and the non-bioavailable thiosulfates will be tranformed to bioavailable sulfate within 1-2 weeks;

-      the derivation of a PNEC for agricultural soils using the available data on phytotoxicity and nitrification inhibition would lead to a PNEC that is counter-productive from an agricultural point of view.






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 evidence.


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.



A 3h-NOEC of ≥757 mg S2O32-/L (endpoint: respiration rate of activated sludge) has been put forward for the derivation of a PNECmicro-organism for thiosulfate. Application of an assessment factor of 10 results in a PNECmicro-organism of 75.7 mg S2O32-/L.

Yranslating this value to CaS2O3gives a PNECmicro-organism of 106.5 mg test substance/L.

Conclusion on classification

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. subspicatus (72h-EC50), and was ≥75.7 mg S2O32-/L. Translating this value to CaS2O3 results in an acute TRV of ≥100 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 calcium thiosulfate for the environment.