Classification of mixtures when toxicity data are not available for the complete mixture: bridging
principles

Where the mixture itself has not been tested to determine its aquatic environmental hazard, but
there are sufficient data on the individual ingredients and similar tested mixtures to adequately
characterise the hazards of the mixture, these data shall be used in accordance with the following
agreed bridging rules. This ensures that the classification process uses the available data to the
greatest extent possible in characterising the hazards of the mixture without the necessity for
additional testing in animals.

Dilution

Where a new mixture is formed by diluting a tested mixture or a substance with a diluent which
has an equivalent or lower aquatic hazard classification than the least toxic original ingredient and
which is not expected to affect the aquatic hazards of other ingredients, then the resulting mixture
shall be classified as equivalent to the original tested mixture or substance. Alternatively, the
method explained in 2.2.9.1.10.4.5 may be applied.

Batching

The aquatic hazard classification of a tested production batch of a mixture shall be assumed to be
substantially equivalent to that of another untested production batch of the same commercial
product when produced by or under the control of the same manufacturer, unless there is reason to
believe there is significant variation such that the aquatic hazard classification of the untested
batch has changed. If the latter occurs, new classification is necessary.

Concentration of mixtures which are classified with the most severe classification categories (Chronic 1 and Acute 1)

If a tested mixture is classified as Chronic 1 and/or Acute 1, and the ingredients of the mixture
which are classified as Chronic 1 and/or Acute 1 are further concentrated, the more concentrated
untested mixture shall be classified with the same classification category as the original tested
mixture without additional testing.

Interpolation within one toxicity category

For three mixtures (A, B and C) with identical ingredients, where mixtures A and B have been
tested and are in the same toxicity category, and where untested mixture C has the same
toxicologically active ingredients as mixtures A and B but has concentrations of toxicologically
active ingredients intermediate to the concentrations in mixtures A and B, then mixture C is
assumed to be in the same category as A and B.

Substantially similar mixtures

Given the following:
(a) Two mixtures:
(i) A + B;
(ii) C + B;
(b) The concentration of ingredient B is essentially the same in both mixtures;
(c) The concentration of ingredient A in mixture (i) equals that of ingredient C in mixture (ii);
(d) Data on aquatic hazards for A and C are available and are substantially equivalent, i.e. they
are in the same hazard category and are not expected to affect the aquatic toxicity of B.
If mixture (i) or (ii) is already classified based on test data, then the other mixture can be assigned
the same hazard category.

Classification of mixtures when toxicity data are available for all ingredients or only for some ingredients of the mixture

The classification of a mixture shall be based on summation of the concentrations of its classified
ingredients. The percentage of ingredients classified as "Acute" or "Chronic" will feed straight into the
summation method. Details of the summation method are described in 2.2.9.1.10.4.6.1 to
2.2.9.1.10.4.6.4.

Mixtures may be made of a combination of both ingredients that are classified (as Acute 1 and/or
Chronic 1, 2) and those for which adequate toxicity test data are available. When adequate toxicity
data are available for more than one ingredient in the mixture, the combined toxicity of those
ingredients shall be calculated using the following additivity formulas (a) or (b), depending on the
nature of the toxicity data:
(a) Based on acute aquatic toxicity:

where:
Ci = concentration of ingredient i (mass percentage);
L(E)C50i = LC50 or EC50 for ingredient i (mg/l);
n = number of ingredients, and i is running from 1 to n;
L(E)C50m = L(E)C50 of the part of the mixture with test data;
The calculated toxicity shall be used to assign that portion of the mixture an acute hazard
category which is then subsequently used in applying the summation method;
(b) Based on chronic aquatic toxicity:
where:
Ci = concentration of ingredient i (mass percentage) covering the rapidly degradable
ingredients;
Cj = concentration of ingredient j (mass percentage) covering the non rapidly
degradable ingredients;
NOECi = NOEC (or other recognized measures for chronic toxicity) for ingredient i
covering the rapidly degradable ingredients, in mg/l;
NOECj = NOEC (or other recognized measures for chronic toxicity) for ingredient j
covering the non-rapidly degradable ingredients, in mg/l;
n = number of ingredients, and i and j are running from 1 to n;
EqNOECm = equivalent NOEC of the part of the mixture with test data;
The equivalent toxicity thus reflects the fact that non-rapidly degrading substances are
classified one hazard category level more "severe" than rapidly degrading substances.
The calculated equivalent toxicity shall be used to assign that portion of the mixture a longterm
hazard category, in accordance with the criteria for rapidly degradable substances (Table
2.2.9.1.10.3.1 (b) (ii)), which is then subsequently used in applying the summation method.

 

When applying the additivity formula for part of the mixture, it is preferable to calculate the toxicity
of this part of the mixture using for each ingredient toxicity values that relate to the same taxonomic
group (i.e. fish, crustacea or algae) and then to use the highest toxicity (lowest value) obtained (i.e.
use the most sensitive of the three groups). However, when toxicity data for each ingredient are not
available in the same taxonomic group, the toxicity value of each ingredient shall be selected in the
same manner that toxicity values are selected for the classification of substances, i.e. the higher
toxicity (from the most sensitive test organism) is used. The calculated acute and chronic toxicity shall
then be used to classify this part of the mixture as Acute 1 and/or Chronic 1 or 2 using the same
criteria described for substances.

If a mixture is classified in more than one way, the method yielding the more conservative result shall

be used.

Summation method

Classification procedure

In general a more severe classification for mixtures overrides a less severe classification, e.g. a
classification with Chronic 1 overrides a classification with Chronic 2. As a consequence the
classification procedure is already completed if the results of the classification is Chronic 1. A more
severe classification than Chronic 1 is not possible; therefore, it is not necessary to pursue the
classification procedure further.

Classification for category Acute 1

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