Definitions and data requirements

The basic elements for classification of environmentally hazardous substances (aquatic environment)
(a) Acute aquatic toxicity;
(b) Chronic aquatic toxicity;
(c) Potential for or actual bioaccumulation; and
(d) Degradation (biotic or abiotic) for organic chemicals.

While data from internationally harmonised test methods are preferred, in practice, data from national
methods may also be used where they are considered as equivalent. In general, it has been agreed that
freshwater and marine species toxicity data can be considered as equivalent data and are preferably to
be derived using OECD Test Guidelines or equivalent according to the principles of Good Laboratory
Practices (GLP). Where such data are not available, classification shall be based on the best available

Acute aquatic toxicity means the intrinsic property of a substance to be injurious to an organism in a
short-term aquatic exposure to that substance.
Acute (short-term) hazard, for classification purposes, means the hazard of a chemical caused by its
acute toxicity to an organism during short-term aquatic exposure to that chemical.
Acute aquatic toxicity shall normally be determined using a fish 96 hour LC50 (OECD Test Guideline
203 or equivalent), a crustacea species 48 hour EC50 (OECD Test Guideline 202 or equivalent) and/or
an algal species 72 or 96 hour EC50 (OECD Test Guideline 201 or equivalent). These species are
considered as surrogate for all aquatic organisms and data on other species such as Lemna may also be
considered if the test methodology is suitable.

Chronic aquatic toxicity means the intrinsic property of a substance to cause adverse effects to aquatic
organisms during aquatic exposures which are determined in relation to the life-cycle of the organism.
Long-term hazard, for classification purposes, means the hazard of a chemical caused by its chronic
toxicity following long-term exposure in the aquatic environment.
Chronic toxicity data are less available than acute data and the range of testing procedures less
standardised. Data generated according to the OECD Test Guidelines 210 (Fish Early Life Stage) or
211 (Daphnia Reproduction) and 201 (Algal Growth Inhibition) may be accepted. Other validated and
internationally accepted tests may also be used. The NOECs or other equivalent ECx shall be used.

Bioaccumulation means net result of uptake, transformation and elimination of a substance in an
organism due to all routes of exposure (i.e. air, water, sediment/soil and food).
The potential for bioaccumulation shall normally be determined by using the octanol/water partition
coefficient, usually reported as a log Kow determined according to OECD Test Guideline 107 or 117.
While this represents a potential to bioaccumulate, an experimentally determined Bioconcentration
Factor (BCF) provides a better measure and shall be used in preference when available. A BCF shall
be determined according to OECD Test Guideline 107, 117 or 123.

Degradation means the decomposition of organic molecules to smaller molecules and eventually to
carbon dioxide, water and salts.
Environmental degradation may be biotic or abiotic (e.g. hydrolysis) and the criteria used reflect this
fact. Ready biodegradation is most easily defined using the biodegradability tests (A-F) of OECD Test
Guideline 301. A pass level in these tests may be considered as indicative of rapid degradation in most
environments. These are freshwater tests and thus the use of the results from OECD Test Guideline
306, which is more suitable for marine environments, has also been included. Where such data are not
available, a BOD(5 days)/COD ratio ≥ 0.5 is considered as indicative of rapid degradation.
Abiotic degradation such as hydrolysis, primary degradation, both abiotic and biotic, degradation in
non-aquatic media and proven rapid degradation in the environment may all be considered in defining
rapid degradability11.
Substances are considered rapidly degradable in the environment if the following criteria are met:
(a) In 28-day ready biodegradation studies, the following levels of degradation are achieved:
(i) Tests based on dissolved organic carbon: 70%;
(ii) Tests based on oxygen depletion or carbon dioxide generation: 60% of theoretical
These levels of biodegradation shall be achieved within 10 days of the start of degradation
which point is taken as the time when 10% of the substance has been degraded ", unless the
substance is identified as a complex, multi-component substance with structurally similar
constituents. In this case, and where there is sufficient justification, the 10-day window
condition may be waived and the pass level applied at 28 days12; or
(b) In those cases where only BOD and COD data are available, when the ratio of BOD5/COD is ≥
0.5; or
(c) If other convincing scientific evidence is available to demonstrate that the substance can be
degraded (biotically and/or abiotically) in the aquatic environment to a level above 70% within
a 28 day period.


Substance classification categories and criteria

Substances shall be classified as "environmentally hazardous substances (aquatic environment)", if
they satisfy the criteria for Acute 1, Chronic 1 or Chronic 2, according to Table These
criteria describe in detail the classification categories. They are diagrammatically summarized in
11 Special guidance on data interpretation is provided in Chapter 4.1 and Annex 9 of the GHS.
12 See Chapter 4.1 and Annex 9, paragraph A9. of the GHS.
Table Categories for substances hazardous to the aquatic environment (see Note 1)
(a) Acute (short-term) aquatic hazard

NOTE 1: The organisms fish, crustacea and algae are tested as surrogate species covering a range of
trophic levels and taxa, and the test methods are highly standardized. Data on other organisms may
also be considered, however, provided they represent equivalent species and test endpoints.
NOTE 2: When classifying substances as Acute 1 and/or Chronic 1 it is necessary at the same time to
indicate an appropriate M factor (see ) to apply the summation method.
NOTE 3: Where the algal toxicity ErC50 (= EC50 (growth rate)) falls more than 100 times below the
next most sensitive species and results in a classification based solely on this effect, consideration
shall be given to whether this toxicity is representative of the toxicity to aquatic plants. Where it can
be shown that this is not the case, professional judgment shall be used in deciding if classification
shall be applied. Classification shall be based on the ErC50. In circumstances where the basis of the
EC50 is not specified and no ErC50 is recorded, classification shall be based on the lowest EC50
NOTE 4: Lack of rapid degradability is based on either a lack of ready biodegradability or other
evidence of lack of rapid degradation. When no useful data on degradability are available, either
experimentally determined or estimated data, the substance shall be regarded as not rapidly
NOTE 5: Potential to bioaccumulate, based on an experimentally derived BCF ≥ 500 or, if absent,
a log Kow ≥ 4 provided log Kow is an appropriate descriptor for the bioaccumulation potential of the
substance. Measured log Kow values take precedence over estimated values and measured BCF values
take precedence over log Kow values.
Figure Categories for substances long-term hazardous to the aquatic environment


The classification scheme in Table below summarizes the classification criteria for
Table Classification scheme for substances hazardous to the aquatic environment
NOTE 1: Acute toxicity band based on L(E)C50 values in mg/l for fish, crustacea and/or algae or
other aquatic plants (or Quantitative Structure Activity Relationships (QSAR) estimation if no
experimental data13).
NOTE 2: Substances are classified in the various chronic categories unless there are adequate
chronic toxicity data available for all three trophic levels above the water solubility or above 1 mg/l.
("Adequate" means that the data sufficiently cover the endpoint of concern. Generally this would
mean measured test data, but in order to avoid unnecessary testing it can on a case by case basis also
be estimated data, e.g. (Q)SAR, or for obvious cases expert judgment).
NOTE 3: Chronic toxicity band based on NOEC or equivalent ECx values in mg/l for fish or
crustacea or other recognized measures for chronic toxicity.


Mixtures classification categories and criteria

The classification system for mixtures covers the classification categories which are used for
substances, meaning categories Acute 1 and Chronic 1 and 2. In order to make use of all available
data for purposes of classifying the aquatic environmental hazards of the mixture, the following
assumption is made and is applied where appropriate:
The "relevant ingredients" of a mixture are those which are present in a concentration equal to or
greater than 0.1% (by mass) for ingredients classified as Acute and/or Chronic 1 and equal to or
greater than 1% for other ingredients, unless there is a presumption (e.g. in the case of highly toxic
ingredients) that an ingredient present at less than 0.1% can still be relevant for classifying the mixture
for aquatic environmental hazards.

The approach for classification of aquatic environmental hazards is tiered, and is dependent upon the
type of information available for the mixture itself and for its ingredients. Elements of the tiered
approach include:
(a) Classification based on tested mixtures;
(b) Classification based on bridging principles;
(c) The use of "summation of classified ingredients" and/or an "additivity formula".
Figure below outlines the process to be followed.
13 Special guidance is provided in Chapter 4.1, paragraph and Annex 9, Section A9.6 of the GHS.
Figure Tiered approach to classification of mixtures

Classification of mixtures when toxicity data are available for the complete mixture

When the mixture as a whole has been tested to determine its aquatic toxicity, this information
shall be used for classifying the mixture according to the criteria that have been agreed for
substances. The classification is normally based on the data for fish, crustacea and algae/plants
(see and When adequate acute or chronic data for the mixture as a
whole are lacking, "bridging principles" or "summation method" shall be applied (see to
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