Silver

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.04 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.86 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.025 mg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

438.13 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

438.13 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

1.41 mg/kg soil dw

Assessment factor:

3

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

See CSR Annex 4 - PNEC Summary Report

Conclusion on classification

In the below table, the ERV values, TDp data and derived classifications (incl. M-factors) for Ag metal in massive form (≥1 mm), powder form (>100 nm - <1 mm) and nanoform (≥1 nm - ≤100 nm) are summarised.

Table 1a: Ag metal in massive and powder form are assessed aspoorly soluble substance.Basis for classification: CLP Guidance Fig IV.1 for acute endpoint, CLP Guidance Fig IV.2 for chronic endpoint (ECHA 2017)
(all values in µg Ag/L)	Acute ERV	TDp, 7d	Acute classification	M-factor	Chronic ERV	TDp, 28d			Rapid environmental transformation?	chronic classification	M-factor
		1 mg/L				0.01 mg/L*	0.1 mg/L*	1 mg/L
Ag massive	0.22	<0.02	not classified	/	0.1	0.000004	0.00004	<0.02	Not considered for classification	not classified
Ag powder	0.22	1.3 - 2.6**	Acute 1	10	0.1	0.03	0.41	3.55 - 5.7**	Not considered for classification	Chronic 1	10
* extrapolated value based on average dissolution rate for the 3 replicates
** range of values obtained for different samples and pHs
Table 1b: Ag metal in nanoform is assessed asreadily soluble substance.Basis for classification: CLP Guidance Fig IV.4 for acute endpoint, CLP Guidance Fig IV.5 for chronic endpoint (ECHA 2017)
	Acute ERV		Acute classification	M-factor	Chronic ERV				Rapid environmental transformation?	chronic classification	M-factor
Ag nanoform	0.22	***	Acute 1	1000	0.1	***	***	***	Not considered for classification	Chronic 1	1000

*** assuming 100% solubility

1.    Acute and chronic ERV values for silver 

Acute and chronic aquatic toxicity data are available for a range of different freshwater and marine species. For silver metal and silver compounds, the acute ecotoxicity reference value (ERV) is 0.22 µg Ag/L and the chronic ERV is 0.1 µg Ag/L.

In line with the risk assessment/classification approach adopted for other metals and inorganic metal compounds (ECHA 2017), ecotoxicity data are reported in terms of the concentration of dissolved silver ions from soluble inorganic silver species. Predominantly, these are studies that used silver nitrate (AgNO₃) as the source of dissolved silver ions. Silver nitrate is considered as the silver substance with the greatest toxicity as it dissociates rapidly and completely in aqueous media. Where data for silver nitrate was not available, data derived from other inorganic silver salts (e.g. silver chloride) were used, but only after the exposure conditions were determined to be acceptable (e.g. testing was conducted within the limits of solubility and the Ag+ ion was likely to be the dominant dissolved species).

A complete base set of acute ecotoxicity studies is available for soluble inorganic silver species, comprising numerous studies for fish, invertebrates and algae. The lowest reliable acute value is an EC50 of 0.22 µg Ag/L for the invertebrateDaphnia magna(pH 8.2, total organic carbon <0.1 mg/L; Bianchini et al. 2002).

A complete chronic data set is also available for soluble inorganic silver species, with chronic ecotoxicity data available for various species of fish, invertebrates and algae. The lowest reliable chronic value is an EC10 of 0.1 µg Ag/L for the algaePseudokirchneriella subcapitata(pH 7.2-8.5, DOC 0.38-0.88 mg/L; Fraunhofer 2017). Additional chronic toxicity data of similar sensitivity are also available forIsonychia bicolor(14 day NOEC of 0.16 µg Ag/L; Diamond et al, 1990),Salmo trutta(217 day EC10 of 0.23 µg Ag/L; Davies et al, 1998) andBrachionus calyciflorus(EC10 of 0.31 µg Ag/L; Arijs et al. 2021).

CONCLUSION: Acute ERV = 0.22 µg Ag/L, Chronic ERV = 0.1 µg Ag/L

 

2.    Silver metal - Transformation/Dissolution tests

The massive and powder forms of silver metal are poorly/sparingly soluble. The environmental hazard of poorly/sparingly soluble forms of metals is associated with their potential to release metals ions (Ag+ ions in this case) as most relevant toxic moiety (ECHA 2013). The potential for aqueous transformation or dissolution of elemental or sparingly soluble forms of metals is measured using standardised Transformation/Dissolution (T/D) tests (as described in OECD TG29).

Transformation/Dissolution tests have been performed on silver metal in both massive and powder forms (cfr details below).

Table 2: TDp data for silver metal powder, Ag metal flake and silver metal massive generated according to OECD TG29 (Ag release, expressed as average value of 3 replicate vessels in µg Ag/L).

	pH6				pH8
	1mg/L		10mg/L	100mg/L	1mg/L		10mg/L	100mg/L
	7d	28d	7d	7d	7d	28d	7d	7d
Ag metal powder	1.25	3.55	8.46	37.36	2.55	5.71	12.14	26.03	CIMM, 2009
Ag metal flake	1.79	3.60	7.92	38.1	nd	nd	nd	nd	ECTX, 2010
					3 mg/L		9mg/L	27mg/L
	7d	28d	7d	7d	7d	28d	7d	7d
Ag metal massive	nd	nd	nd	nd	<0.02	<0.02	<0.02	0.03	ECTX, 2013

 nd = no data

 

3.    Environmental classification of silver metal

The classification strategy for poorly soluble silver substances has been based on CLP guidance (ECHA 2017). Annex IV in this guidance describes in detail the environmental classification of metals and inorganic metal compounds and how to determine the appropriate M factors for poorly soluble substances like silver metal in powder and massive form. Key figures in the CLP guidance document used in the below environmental hazard assessment are figures IV.1 and IV.2, and considers the acute and chronic ERVs and the results of T/D testing. Note that the (experimental) evidence for environmental transformation (‘rapid removal from the water column’) of silver has not been considered for classification in the below assessment.

Note that the chronic classification for silver metal in massive and powder form has been derived using the chronic ERV and 28-day T/D data. The available 28-day T/D data for silver metal (powder and massive form) is restricted (for technical reasons) to a minimum loading rate of 1 mg/L. However, as chronic classification requires T/D data obtained with lower loading rates of 0.01 and 0.1 mg/L, data for these theoretical loading rates have been extrapolated, as detailed in the CLP guidance (footnote 93, page 585; ECHA, 2017), from tests conducted at higher loading rates using either empirically derived relationships (i.e. linear regression) or precautionary assumptions (i.e. based on analytical limits of detection).

 

Classification of silver in nanoform (≥1 nm - ≤100 nm)

A precautionary classification for silver metal nanoparticles has been read across from data for soluble silver and the appropriate M factors have been derived.

Transformation/Dissolution-data for Ag nanoparticles have been generated as part of the Silver Substance Evaluation under REACH (cfr. details in Mertens et al, 2019). On specific request of the Evaluating Member State, this T/D study was performed in a non-standard modified daphnia and algae medium with chloride salts replaced by nitrate salts to maximise the concentration of ionic Ag. These conditions are deviating from the standard medium described in OECD Test Guideline 29. The OECD 29 Test Guideline was developed for hazard classification purposes, and therefore any deviation from the guideline (like changes in the medium composition that may affect the solubility and bioavailability of the test compound) makes the results not useable for hazard assessment. An alternative publication by Wasmuth et al (2016) describes a TDp test, performed according to OECD TG29, with a commonly used reference material for Ag nanoforms (NM-300 K; 20 nm mean diameter) as test item. Although performed in a standard medium and in compliance with the test guideline, the authors conclude on a low Ag dissolution under all test conditions (nominal Ag loadings of 1, 10 and 100 mg/L, pH 6 and 8, sampling up to 7d for all loadings and up to 28d for 1 mg/L loading) with values for ionic silver varying between 23 and 71 µg Ag/L after 7d. No increase was observed in the next 21 days at 1 mg/L loading. This low concentration of dissolved Ag in solution was attributed to quick and extensive complexation and precipitation of silver as silver chloride, which was most pronounced at higher loadings. The general conclusion of the authors was that the observations were biased due to rapid aggregation of silver nanoparticles and precipitation of dissolved Ag ionic species.

In absence of reliable T/D data for Ag metal nanoparticles (generated according to OECD Testing Guideline 29), the environmental classification of uncoated and coated silver metal nanomaterials has therefore been directly read across from soluble forms of silver, as specified in Figures IV.4-5 in the CLP guidance (ECHA, 2017), as conservative approach.

Note that the conclusion of the Silver Substance Evaluation specifically mentions that environmental toxicity data, generated with soluble silver substances like silver nitrate, are a suitable and conservative approach for assessing the effects of silver nanoparticles: “Taking into account that silver nitrate can be seen as a ‘worst case’ in the risk assessment regarding toxicity of the nanoforms of silver that are covered by the REACH registration dossier(s) submitted for Silver (as shown in Section 7.8), the eMSCA can agree with the approach to use the harmonised classification for silver nitrate (and corresponding Mfactors) to self-classify the nanoforms of silver.” (https://echa.europa.eu/documents/10162/776ad739-c591-16fd-2e2c-62b9e50169ee).

Acute classification under CLP

The Acute ERV for silver is 0.22 µg Ag/L. This value is ≤1 mg/L and silver metal nanoparticles are therefore classified as Aquatic Acute category 1. The appropriate acute M factor is 1000 (0.0001 < acute ERV (in mg/L) ≤ 0.001 as inCLP Guidance Table IV.1; ECHA 2017).

Chronic classification under CLP

The chronic ERV for silver is 0.1 µg Ag/L. This value is below 0.1 mg/L and triggers a classification of silver metal nanoparticles as Aquatic Chronic category 1. The appropriate chronic M-factor is 1000 since rapid transformation to non-bioavailable forms is not considered for this substance (0.00001 < chronic ERV (in mg/L) ≤ 0.0001as in CLP Guidance Table IV.1; ECHA 2017).

CONCLUSION: Ag metal in nanoform is classified as Aquatic Acute 1 (Macute=1000) and Aquatic Chronic 1 (Mchronic=1000).

 

References cited

Arijs et al (2021) Setting a Protective Threshold Value for Silver Toward Freshwater Organisms. Environmental Toxicology and Chemistry, 40, 1678-1693.

ECHA (2017) Guidance on the Application of the CLP Criteria. Guidance to Regulation (EC) No 1272/2008 on classification, labelling and packaging (CLP) of substances and mixtures. Version 5.0, July 2017.

Mertens et al (2019) Effects of Silver Nitrate Are a Conservative Estimate for the Effects of Silver Nanoparticles on Algae Growth andDaphnia magnaReproduction. Environmental Toxicology and Chemistry, 38, 1701–1713.

Wasmuth et al (2016) Assessing the suitability of the OECD 29 guidance document to investigate the transformation and dissolution of silver nanoparticles in aqueous media. Chemosphere 144, 2018-2023.