Diarsenic trioxide

Endpoint summary

Additional information

Reliable chronic toxicity data are available for the long-term effect of arsenic on 19 terrestrial species or microbial endpoints covering the 3 trophic levels (13 terrestrial plants, 4 invertebrates and 2 microbial endpoints). A total of 101 reliable EC₁₀ or NOEC values, ranging between 6.9 and 704 mg added As /kg dry weight (dw), were selected for derivation of a PNEC value. All results were derived for soluble pentavalent arsenic substances (Na₂HAsO₄ and Na₃AsO₄).

Arsenic is naturally present in all environmental compartments. The median ambient background concentrations in the topsoil of agricultural and grazing land are 5.5 and 5.6 mg As/kg dw, respectively (http://gemas.geolba.ac.at). Background As concentrations in the soils used for the terrestrial ecotoxicity tests ranged between 0.6 and 240 mg As/kg dw, with a median of 4.4 mg As/kg dw. These background concentrations are significant compared to the lowest reliable NOEC and EC₁₀ values for effects of inorganic arsenic substances to terrestrial organisms. Therefore, an added approach was selected for the risk assessment of arsenic in soil. All NOEC and EC₁₀values are based on added arsenic concentrations, without taking into account the natural background in the soil. In essence this added risk assessment approach assumes that species are fully adapted to the natural background concentration and therefore that only the anthropogenic added fraction should be regulated or controlled (Appendix R.7.13-2 of the REACH guidance on “Environmental risk assessment for metals and metal compounds”).

The bioavailability and toxicity of arsenic to most soil organisms was significantly affected by the properties of the soils tested. Toxicity to terrestrial invertebrates and most plants decreased with higher clay content of the soil. Data for one plant (Cucumis sativus) showed decreasing toxicity with higher pH. No significant relationships between toxicity and soil properties were observed for As toxicity to microbial endpoints. The slope of a simple linear regression between log-transformed EC₅₀values for the various organisms and the log-transformed soil property of the soils resulting in the largest adjusted R² values were selected for normalization of the toxicity data to standard soil conditions, as shown in the table below.

 

Test organism	Endpoint	Regression equation	Adj. R^2	N	P
Eisenia fetida	reproduction	log EC50 = 1.060 +0.989*log clay	0.59	6	0.045
Folsomia candida	reproduction	log EC50 = 0.847 +1.074*log clay	0.89	6	0.003
Avena sativa	shoot yield	log EC50 = 0.697 +0.981*log clay	0.83	6	0.007
Cucumis sativus	shoot yield	log EC50 = 3.587 –0.273*pH	0.80	7	0.004
Hordeum vulgare	root elongation	log EC50 = 0.711 +1.034*log clay	0.43	19	0.001
Solanum lycopersicon	shoot yield	log EC50 = 1.225 +0.712*log clay	0.68	5	0.054

 

Wherever possible (i.e., when data on the soil properties of the test soil and a proper normalization model were available), EC₁₀ and NOEC values were normalized to reasonable worst-case soil properties (10% clay, pH 7). Species-specific geometric mean values were derived for the most sensitive endpoint per species in case multiple data were available for one species. These species-specific geometric mean values vary between 5.0 mg As/kg for reproduction of the invertebrate Enchytraeus albidus and 142.8 mg As/kg for root elongation of Triticum aestivum.

 

The table below presents an overview of the chronic toxicity data selected for the PNEC derivation for toxicity of inorganic arsenic to terrestrial organisms.

Test organism	Taxonomic group	Endpoint	Range (and amount) of NOEC or EC10values (not normalized) mg As/kg dw	Species mean EC10(normalized to pH 7 and 10% clay) mg As/kg dw
Eisenia andrei	Lumbricidae (annelida)	reproduction	15.0 – 300.0 (n=5)	31.5
Eisenia fetida	Lumbricidae (annelida)	reproduction	10.0 – 413.1 (n=7)	45.3
Enchytraeus albidus	Enchytraeidae (annelida)	reproduction	10.0 (n=1)	5.0
Folsomia candida	Isotomidae (arthropoda)	reproduction	20.0 – 320.0 (n=15)	36.1
Arthrobacter globiformis	Bacteria	dehydrogenase activity	30.0 – 112.6 (n=5)	53.3
Natural soil microbial community	Bacteria	microbial N transformation	58.9 (n=1)	58.9
Avena sativa	Poaceae (monocotyledon)	shoot yield	11.6 – 134.2 (n=6)	18.6
Cucumis sativus	Cucurbitaceae (eudicotyledon)	shoot yield	14.8 – 115.0 (n=5)	23.3
Helianthus annuus	Asteraceae (eudicotyledon)	yield	16.3 – 43.0 (n=2)	15.0
Hordeum vulgare	Poaceae (monocotyledon)	root elongation	16.3 – 704.0 (n=18)	33.2
Lactuca sativa	Asteraceae (eudicotyledon)	root elongation	40.0 – 156.4 (n=4)	77.4
Medicago sativa	Fabaceae (eudicotyledon)	shoot yield	25.0 (n=1)	25.0
Oryza sativa	Poaceae (monocotyledon)	grain yield	8.9 – 101.6 (n=10)	10.0
Phaseolus vulgaris	Fabaceae (eudicotyledon)	yield	8.2 – 15.5 (n=2)	5.3
Raphanus sativus	Brassicaceae (eudicotyledon)	yield	15.8 – 67.7 (n=2)	18.5
Solanum lycopersicon	Solanaceae (eudicotyledon)	shoot yield	6.9 – 227.6 (n=5)	21.5
Sorghum bicolor	Poaceae (monocotyledon)	yield	8.4 – 94.7 (n=3)	10.3
Triticum aestivum	Poaceae (monocotyledon)	root elongation	78.9 – 270.0 (n=6)	142.8
Zea mays	Poaceae (monocotyledon)	yield	18.3 – 68.2 (n=3)	20.0

 

In addition, several studies are available on the chronic effect of arsenic to mammals and birds, resulting in lowest NOECoral values of 30 and 41.6 mg As/kg diet for birds and mammals, respectively.