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Description of key information

KOH is a corrosive substance at concentrations of about 2% and higher. Between about 0.5% and 2.0%, it is irritating. Case reports on human accidents or intentional exposure confirm that the risk posed by KOH for human health originates from its corrosive properties.
According to the CLP Regulation Annex VI, Table 3.1 and to the 25th Adaptation of Annex I of Council Directive (67/548/EEC), the concentration limit for corrosivity to the skin of potassium hydroxide is considered to be 2% (as stated in the OECD SIDS on potassium hydroxide, 2002). Up to the most recent ATP (29th), this has not been changed. Therefore, 2% is taken to the risk assessment as concentration limit for corrosivity.

Key value for chemical safety assessment

Skin irritation / corrosion

Endpoint conclusion
Endpoint conclusion:
adverse effect observed (corrosive)

Eye irritation

Endpoint conclusion
Endpoint conclusion:
adverse effect observed (irritating)

Additional information

Potassium hydroxide is a corrosive substance at concentrations of about 2% and higher. Between about 0.5% and 2.0%, it is irritating to the skin. Vernot (1977) also stated that KOH 2% is corrosive. However this study recieved a Klimisch 3 score for reliability. KOH (5%) is severe irritant/corrosive for the skin. This is based on in vitro (human epidermal model, rat biobarrier membrane, EpiSkin model) and in vivo (rabbit, guinea pig) data. KOH (1%) is irritating to the eyes (5 min exposure). KOH (5%) is extremely irritating and corrosive for the eyes (5 min exposure). This is based on an in vivo study in rabbits.

Skin irritation

In a classical rabbit Draize test with gauze covering, application of 0.5 ml of KOH 5% during 4 hours gave a PDII (primary dermal irritation indices) result of 4.8 (moderately irritating). A 10% solution was severely irritating (Nixon et al., 1990). With 19 mm diameter Hill Top Chamber pad covering during 1 or 4 hours and 0.2 ml applied, the 5 and 10% KOH solutions were qualified as severely irritating. A rabbit Draize test with gauze covering and application of 0.1 ml during 24 hours qualified a 5% KOH solution as mildly irritating on intact skin and highly irritating on abraded skin (Johnson et al., 1975). A 10% KOH solution was qualified as corrosive on both intact and abraded skin as the result of a Draize occlusive test on rabbits with 4 hours exposure to 0.5 ml of the solution. The results with guinea pigs were similar (Nixon et al., 1975). In a comparison study (reliability 2) of 4 “in vitro” methods, TER, Corrositex, Episkin and Skin²ZK1350, the 4 methods discriminated KOH 10% as highly corrosive or corrosive, while only the 3 first methods discriminated KOH 5% as highly corrosive or corrosive (Fentem et al., 1998). In a Draize rabbit test (reliability 3) with gauze covering and application of 0.5 ml of KOH solutions during 4 hours, the 1% solution was not corrosive, whereas the 2% solution was corrosive. There was no post-exposure assessment of the lesion (Vernot et al., 1977). In “in vitro” tests (reliability 3) with reconstructed human skin cultures Skin²ZS1301 and EpiDerm, and MTT vital dye metabolism, a 10% KOH solution was scored as corrosive (Perkins et al., 1996).

Eye irritation

Several concentrations of KOH were tested by a Draize test on rabbits by instilling 0.1 ml, rinsing after 5 minutes or 24 hours of exposure and examining with the aid of fluorescein at 1, 24, 48 and 72 hours, 7 days, and eventually 14-21 days. The results were as follows: 5% / 5 min.: extremely irritant and corrosive.1% / 5 min.: irritant; 1% / 24 hr.: irritant.0.5% / 24 hr.: marginal. 0.1% / 24 hr.: negative (Johnson, 1975). In an “in vitro” test (reliability 3) with human corneal endothelial cell cultures and cell viability quantification by a 51Cr-release assay, the ED50 result (50% maximal toxicity) of 0.073% was said to correlate with “severe irritating” in the Draize test (Douglas and Spilman, 1983).



The irritation potential of an unspecified dose of undiluted methanol in rabbits was examined under occlusive conditions after exposure intervals of 1, 5, and 15 minutes and 20 hours. According to Draize scoring, no signs of skin irritation were observed 24 hours and 8 days after treatment for any of the exposure time periods (BASF, 1975).


One hour after instillation of 0.05 mL undiluted methanol into the eyes of two rabbits, slight erythemas and corneal opacity as well as moderate edemas associated with secretion were observed. After 24 hours, the effects were assessed as mild, and after 8 days the animals had no symptoms (BASF, 1975).

In another study, mild to moderate conjunctivitis and edemas as well as mild iritis were produced in six rabbits after instillation of 0.1 mL undiluted methanol into the eyes. Average scores after 24, 48, and 72 hours were approximately 2 for conjunctivae and < 1 for other effects. Primary irritation subsided after 72 hours, although redness of the conjunctivae persisted at that time. Information on effects after 8 and 14 days was not available (Jacobs, 1990).

Exposure of rats to an atmosphere saturated by methanol vapours at 20 °C produced severe irritation of mucous membranes and milky corneal opacity (time not specified) and eventually led to mortality of all animals within 8 hours (BASF, 1975). This was an additional observation and is not applicable for evaluation of irritant effects. Therefore, it is not relevant for classification.

Overall, the registered susbtance is considered as corrosive.

Effects on skin irritation/corrosion: corrosive

Effects on eye irritation: corrosive

Justification for classification or non-classification

Due to the typical concentration of potassium hydroxide in the registered substance, it is classified as corrosive.

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