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Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Link to relevant study record(s)

Description of key information

An high quality OECD 201 study has recently been made available which gives a ErC50-73h = 0.0499 mg available chlorine/L In this study, the NOECr = 0.0171 mg available chlorine/L. In a laboratory multispecies microcosm the chronic effects of chlorine to naturally derived periphytic communities exposed for 7 days to sodium hypochlorite in a flow-through system were examined. A NOEC of 3 μg TRC/L equivalent to 2.1 µg FAC/L was derived as an indication of long-term toxicity to algae.
Marine water:
The effects of chlorine to three phytoplanktonic algae species were tested in static tests with filtered sea water. The most sensitive species was Dunaliella primolecta, an initial dose of 0.4 mg TRO/L caused 50% mortality in 24 hours. The degradation of the hypochlorite was monitored by measuring the concentration with the colorimetric DPD method. Based on the experimental results, the authors extrapolated that in a natural medium, having a population of 1 cell/mL, mortality would reach 65% after 24 h exposure at 0.2 mg TRO/L.

Key value for chemical safety assessment

EC50 for freshwater algae:
0.05 mg/L
EC10 or NOEC for freshwater algae:
0.002 mg/L

Additional information

Carrying out toxicity test of sodium hypochlorite on microalgae under standard conditions is really challenging for the following reasons. Tests are based on evaluation of growth inhibition for unicellular algae culture placed under intense illumination. Hypochlorite is readily photodegradable: the photolysis half-life of aqueous chlorine exposed to summer noon sunlit with clear sky (47°N) at a pH 8 is 12 min when measured at the surface (Nowell and Hoigné, 1992). As it is not possible to use flow through conditions with algae, it is not possible to maintain exposure conditions given the very rapid decay of the test substance.

For these reasons, during the preparation of this registration dossier, in accordance with column 2 of REACH Annex VII, no attempt to carry out a new study on algae growth inhibition (required in section 9.1.1) was made.

Nevertheless, during preparation of the biocide registration dossier, it had been required for sake of dossier completion to carry out a study according to OECD 201. The results of this study have been made available recently to the REACH registration dossier, which is updated to take account of this information. This study (Liedtke, 2013) has been carried out with the best possible care and is consequently considered as the key study for this endpoint.

The available non standard studies from public literature sources considered as scientifically robust are nevertheless taken into account as supporting information for the risk assessment.


Short-term toxicity:

The study on the freshwater algae Pseudokirchneriella subcapitata (Lietdke, 2013) was carried out according to OECD 201 under GLP conditions. Nominal concentrations were set from 0.017 to 0.27 mg available chlorine/L. Measured concentrations at the start of the test were between below the limit of quantification (LOQ = 0.0108 mg available chlorine/L) for the low nominal concentrations and 0.256 mg available chlorine/L corresponding to 93% of the nominal values at the start of the test. During the test period of 72 hours, a decrease of the available chlorine in the test media occurred in the same order for samples with and without algae. After 24 hours the measured values were between below the limit of quantification and 10% of the nominal values. After 48 hours the measured values were between below the limit of quantification and 5% of the nominal values. At the end of the test, the measured values were below the limit of quantification. This confirms the difficulty to keep constant the exposure to hypochlorite in a static ecotoxicity test, under light intensity as needed for algae. The report provides results according to initial measured concentrations and according to nominal concentrations.The test item is known to be a potent oxidizing agent in aqueous solution. This is confirmed by the strong toxic effects observed within the first 24 hours of exposure. In the two highest test concentrations the damage of the algal cells observed during the first 24 hours was complete; consequently, no algal biomass could be determined after 24 hours of exposure. In the two lowest test concentrations, the decrease of available chlorine was accompanied by a slight recovery of the algae during the last 48 hours of exposure. Due to these observations, the biological results were considered to be mainly referring to the concentration of test item measured at the start of the test (initial measured). On the other hand, according the OECD Guidance on Aquatic Toxicity Testing of Difficult Substances and Mixtures, section 5 - Calculation and expression of test results, it is recommended: "for tests with chemicals that cannot be quantified by analytical methods at the concentrations causing effects, the effect concentration can be expressed based on the nominal concentrations".

Based on nominal available chlorine concentrations, ErC50-72h = 0.0499 mg available chlorine/L, ErC10-72h = 0.0299 mg available chlorine/L and NOECr = 0.0171 mg available chlorine/L.

Based on initial measured available chlorine concentrations, ErC50-72h = 0.0365 mg available chlorine/L, ErC10-72h = 0.0199 mg available chlorine/L and NOECr = 0.0054 mg available chlorine/L.

Before this OECD 201 GLP compliant study, toxicity of hypochlorite to freshwater algae had not been widely studied. Data are available on the aquatic toxicity of hypochlorite to Chlorella sorokiniana specifically (Kott and Edlis, 1969) and phytoplankton generally (Brooks and Liptak, 1979).

Kott and Edlis (1969) ran a short-term test with Chlorella to determine the concentration needed to inhibit its growth. One litre Chlorella solutions (containing 225 cells/mm3) were maintained at 28-32°C for a 20 hour exposure (in the dark) to two concentrations of hypochlorite The concentrations of chlorine were measured by amperometric titration and were initially 0.2 mg/l and 0.6 mg/l - these were checked and readjusted after 8 hours of contact (no information is given about the decay curve or concentration maintenance at the end of the test). The data were presented as % kill of algae after 20h: at 0.2 mg/l the % kill was 26.8%, whereas at 0.6 mg/l the % kill was 43.0%. The test report does not provide sufficient information and the test methodology does not meet the requirement for a valid test (algicidal effect is not an endpoint equivalent to growth inhibition).

The study by Brooks and Liptak (1979) reports the results of a 30 minutes static test. The endpoint measured was chlorophyll a depletion. The experimental conditions are not sufficiently described (not valid).

Long-term toxicity:

Data on the long-term effect of sodium hypochlorite to algae can be drawn from laboratory microcosm and field mesocosm studies or the above mentioned recent OECD 201 study and its EC10/NOECs. The study of Cairns et al. (1990) on the peryphytic community indicated a 7d NOEC=3 μgTRC/l equivalent to 2.1µg FAC/L for the measure of algal biomass can be kept as a worst case.

Marine water:


Short-term toxicity

Data on short-term toxicity to algae were retrieved from the literature but they were not considered adequate for the effects assessment of sodium hypochlorite.

The effects of chlorine to three phytoplanktonic algae species were tested by Videau et al. (1979) in static tests with filtered sea water, aimed to evaluate a number of variables. The most sensitive species was Dunaliella primolecta, for which a initial dose of 400 μg/l TRO caused 50% mortality in 24 hours. After 3 hours from dosing, free chlorine had disappeared in chlorinated water below 500 μg/l. Based on their experimental results, the authors extrapolated that in a natural medium, having a population of 1 cell/ml, mortality would reach 65% after 24 h exposure at 200 μg/l.

Gentile et al. (1976) report the Cl2 concentrations causing 50% growth reduction in a series of 24 h static tests on 11 phytoplanctonic species; the LC50 ranged from 75 to 330 μg/l. Only the highly concentrated stock solution was measured. Other tests performed on the diatom Thalassiosira pseudonana, using different exposure times up to 20 minutes showed that, 48h after exposure to 200 μg/l chlorine, growth was reduced by about 60%.

These effect concentrations very likely underestimate the toxicity following a continous exposure but none of these studies can be used for risk assessment as they address algicidal efficiency, not growth inhibition.

Long- term toxicity

Long-term toxicity data for algae were not found. Sanders et al (1981) studied the effects of prolonged chlorination on natural marine phytoplankton communities cultivated in large tanks under flow through conditions (semi-field test). To achieve measurable concentration in the exposure tanks, HOCl was added by single daily additions directly to the tanks, where it degraded within 2 hours (an intermittent exposure was therefore resulting). A 50% reduction in cell density (the most sensitive endpoint) was observed in the 21 day test at concentrations as low as 1-10 μg/l TRC. These data provide evidence of the severe impact of free chlorine on phytoplankton at very low concentrations, even at intermittent exposure (data used as supportive information).


Long- term toxicity

Erickson and Foulk (1980) used outdoor and indoor flow-through systems to evaluate the effects of continuous chlorination (1 year) on entrained estuarine plankton communities consisting of eggs, larvae, algae and juveniles (not better specified). NaOCl was continuously applied at dose levels of 125 to 1441 μg/l, which resulted in concentrations of residual chlorine in the systems below the detection limits of the amperometric analyzer used (10 μg/l). In all treatments a reduction of ATP measured as indication of biomass, was observed (from 13% to 58%). This result is used as supportive information.


For the risk assessment a NOEC of 0.0021 mg FAC/L will be used to calculate the PNEC(aquatic) both for fresh and salt water derived from a laboratory microcosm study (Cairns, 1990).