Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
17.1 µg/L
Assessment factor:
50
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
1.2 µg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
1.2 µg/L
Assessment factor:
50
Extrapolation method:
assessment factor

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
80.3 µg/L
Assessment factor:
100
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
171.1 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
12 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Hazard for air

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
0.7 mg/kg soil dw
Assessment factor:
10
Extrapolation method:
assessment factor

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
PNEC oral
PNEC value:
1.31 mg/kg food
Assessment factor:
30

Additional information

Read-across approach

A read-across approach based on data available for inorganic arsenic compounds is applied for the ecotoxicity assessment of diarsenic trioxide. This grouping of arsenic substances for estimating the aquatic hazard properties is based on the assumption that these properties are similar as the specific environmental conditions predominantly affect speciation and toxicity of arsenic substances and not the inorganic arsenic source. For most of the metal-containing substances, it is the metal ion that becomes available upon contact with and dissolution in water and that is predominantly of ecotoxicological concern. This assumption holds when i) differences in solubility of different As compounds do not affect the ecotoxicity, ii) there are not any important differences in the speciation of inorganic arsenic substances in the environment.

Arsenic is present naturally in the aquatic and terrestrial environments from weathering and erosion of rock and soil.Because of its reactivity and mobility, however, arsenic can cycle extensively through both biotic and abiotic components of local aquatic and terrestrial systems, where it can undergo a variety of chemical and biochemical transformations. Three major modes of biotransformation of arsenic species have been found to occur in the environment: redox transformation between arsenite and arsenate, the reduction and methylation of arsenic, and the biosynthesis of organoarsenic compounds.

When diarsenic trioxide is deposited directly into aerobic surface waters, it forms As(III) species, i.e. arsenite. Arsenite is thermodynamically unstable, and therefore tends to oxidize to dissolved As(V) species, i.e. arsenates. This oxidation can be accelerated by oxidizing agents such as manganese and iron oxyhydroxides which are fairly abundant in natural environments or by the action of certain bacteria. Some As(III) and As(V) species can interchange oxidation states depending on Eh, pH and biological processes. The ratio between oxidized and reduced species appears to be significantly influenced by the presence of iron and manganese oxides. However, the predominant arsenic species in oxidizing environments is the thermodynamically stable form, i.e. arsenate. Arsenite is present in amounts exceeding those of arsenate only in reduced, oxygen-free micro- and macro-environments.

Information about the redox speciation of arsenic compounds during the various tests was not available, but the reliable data for the various endpoints do not appear to differ significantly between the different arsenic substances tested. Thus, all reliable ecotoxicity data of arsenic were considered based on measured dissolved arsenic concentrations or the fact that the respective As substance is soluble. For the ecotoxicity assessment of metals in different environmental compartments (aquatic, soil and sediment), it is typically assumed that the toxicity is not controlled by the total concentration of a metal, but rather by the bioavailable form in the respective medium. Regarding metals, this bioavailable form is typically accepted to be the free metal-ion or the oxy-anion in solution. In the absence of speciation data and as conservative assessment, it was assumed that i) all dissolved arsenic is bioavailable when dissolved concentrations are provided, and that ii) in the absence of information about dissolved levels, all of the applied arsenic is dissolved and bioavailable.

Reliable ecotoxicity results selected for read-across from different arsenic substances are based on tri- and pentavalent As substances (diarsenic trioxide, diarsenic pentaoxide, sodium dioxoarsenate, monosodium arsenate, aluminium arsenate, calcium arsenate, disodium hydrogenarsenate, potassium dihydrogenarsenate). Respective counter-ions (Al3+, Ca2+, K+and Na+) are abundant in natural environments and are not expected to cause any toxic effect at the concentration tested. Thus, the hazard assessment based on dissolved arsenic levels is considered conservative.

Conclusion on classification

Acute and chronic reference values for environmental classification are based on standard test as laid down in Council Regulation (EC) No 440/2008 on “Test methods pursuant to Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)”.

Acute toxicity data of arsenic are available for three trophic levels. The lowest L(E)C50 for freshwater fish, crustacean or algae/plant growth is a 7-d LC50 of 0.43 mg As/L (based on dissolved As concentration), observed in a test with Hyalella azteca (Borgman et al, 2004). The lowest L(E)C50 for marine fish, crustacean or algae/plant growth is a 42-h EC50 in the range of 0.09 - 0.11 mg As/L for standardized reproduction-related endpoints, i.e. tube growth, of the macro-alga Macrocystis pyrifera based on a graphical display of test data by Garman et al, 1994. However, as the later value has not been derived using appropriate statistical tools and as this EC50 value may be above the cutoff value of 0.1 mg/L, this endpoint is equivocal for the purpose of classification and labelling according to Regulation (EC) No 1272/2008 and subsequent adaptations. Thus, considering also the 7-d LC50 of 0.43 mg As/L observed in a test with Hyalella azteca (Borgman et al, 2004), it is assumed that acute LC/EC50 values are above 0.1 mg As/L.  

Chronic toxicity data of arsenic are also available for three trophic levels.The lowest chronic NOEC/EC10 value for freshwater fish, invertebrates or algae is a 28-d NOEC of 0.633 mg As/L (based on dissolved As concentration) observed in a reproduction test with Daphnia magna (Lima et al, 1984). The lowest chronic NOEC/EC10 value for marine fish, crustacean or algae/plant growth, i.e. a 42-h NOEC value of 0.04 mg As/L, is reported for germ tube growth of a macro-alga (Brown alga, Macrocystis pyrifera), a standardised effect in this assay, by Garman et al. (1994). This value is taken forward as the critical threshold concentration.

Acute and chronic reference values of 0.43 mg As/L and 0.04 mg As/L are based on dissolved elemental As concentrations. Because arsenic is an inorganic element, there is no potential for degradation and there is not sufficient evidence for removal from the water column. For the classification of diarsenic trioxide, the acute and chronic reference values based on mg As/L must be corrected for the molecular weight of diarsenic trioxide (75.7% As) resulting in reference values of 0.57 mg As2O3/L and 0.05 mg As2O3/L, respectively. Based on these reference values, the classification for diarsenic trioxide is “Aquatic Acute Category 1” and “Aquatic Chronic Category 1” (acute reference value ≤ 1 mg/L and chronic reference value ≤ 0.1 mg/L).The resulting acute M-Factor is 1 as the L(E)C50 is in the range 0.1 – 1 mg/L. The resulting chronic M-Factor is also 1 since the NOEC value is in the range 0.01 – 0.1 mg/L andAs2O3 is non-rapidly degradable.