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Description of key information

Key value for chemical safety assessment

Additional information

No key study on repeated dose toxicity is available for hydroxycitronellal. However, information from the most relevant repeated dose toxicity studies , performed with another terpene aldehyde with similar structural features as hydroxycitronellal, i.e. citral (CAS 5392-40-5), was taken for assessment of this endpoint via read across. The respective unsaturated aldehyde differs due to the presence of double bonds and lack of an additional hydroxyl residue but contain a terpenoid structure of comparable chain length such as hydroxycitronellal. Physicochemical parameters relevant for oral uptake show comparability in molecular weight and water solubility between hydroxycitronellal and citral (molecular weight of 172.3 and 152.2 ; log Pow at 1.68 and 2.76; water solubility of 35 and 0.420 g/l respectively). Although very low, a higher vapour pressure of citral vs hydroxycitronellal (4.6 vs 0.54 Pa respectively) indicates that citral data represent a worst case concerning adverse effects by vapour inhalation when compared to hydroxycitronellal. Furthermore, due to the additional hydroxyl residue, hydroxycitronellal is likely to be faster and more efficienty conjugated and eliminated than citral. Therefore citral is considered to represent a worst case also in this respect.


Oral exposure

In a chronic oral (feeding) repeated dose toxicity study, reported in literature with limited documentation, hydroxycitronellal was administered to rats at concentrations of 0.1% and 0.5% (approx. 50 and 250 mg/kg bw/d) for 2 years (Bär, 1967). The animals were checked in intervals for their appearance, behaviour and body weight. After 2 years the animals were necropsied and liver, kidney, adrenals, heart, spleen, pancreas and brain were examined histopathologically. It has been reported that both dietary levels are well tolerated in rats for the duration of their life-span. However, the available data are from an old feeding study that was only reported in a short summary table and that lacks the rigor, diversity and numbers of animals, multiplicity of dose levels and width of observation of modern studies.

Groups of male and female F344/N rats or B6C3F1 mice were exposed to diets containing a microencapsulated preparation with a load of 31.3% citral to minimize loss by volatilisation (Kuhn 1991). The test protocol of the 14-week toxicity study used for dose selection of the chronic toxicity study was similar to OECD Guideline 408 (except for partly different investigated organ weights and clinical chemistry parameters). The protocol of the combined chronic toxicity and carcinogenicity study was similar to OECD Guideline 453 except that a complete histopathology was performed for animals from all groups. Clinical chemistry, haematological or urinalyses parameters were not analysed in the chronic study, however, from the 14-week study there are no indications that there are direct effects related to citral exposure (NTP 2003 a-d).

   In the subchronic toxicity study with rats (similar to OECD TG 408, according to GLP), diet concentrations were 0 (vehicle control), 3900, 7800, 15600 and 31300 ppm citral yielding daily dosages of 0, 345, 820, 1785, and 1585 mg citral/kg bw/d in males and 335, 675, 1330, and 2125 mg/kg bw/d in females, based on the actual food consumption (NTP 2003a).


At 31300 ppm, all animals were killed moribund in the second study week. The LOAEL was 3900 ppm or 345 mg/kg bw/d in male rats and 335 mg/kg bw/d in female rats with significant decreases of final body weight and of body weight change compared to controls as adverse effects. Further effects observed were transient changes of hematological (erythrocytes, platelets) and clinical chemistry parameters (from 7800 and 3900 ppm onward, respectively), which appeared to be secondary to the decreased food consumption. Histopathological findings in bone marrow (athrophy and hemorrhage from 15600 ppm onward), minimal to mild nephropathy not mediated by alpha2µ globulin (significant from 7800 ppm onward) and forestomach (epithelial hyperplasia and hyperkeratosis only at 31300 ppm) seemed to be associated with treatment. A NOAEL could not be derived due to dose selection. 


In the study in mice (similar to OECD TG 408, according to GLP), subchronic feeding of diets with 0, 3900, 7800, 15600 and 31300 ppm citral resulted in daily dosages of 0, 745, 1840, 3915, and 8110 mg citral/kg bw/d in males and 790, 1820, 3870, and 7550 mg/kg bw/d in females, based on the actual food consumption (NTP 2003b). The LOAEL corresponded to the lowest doses of 745 mg/kg bw/d in male mice and 790 mg/kg bw/d in female mice with significantly reduced body weight gain and final body weights below controls (body weight loss during study in the 31300 ppm dose group) as adverse findings. Changes of hematological parameters (lymphopenia) as well as the histopathological finding of hypoplasia of ovaries (from 15600 ppm onward) appeared to be secondary to an overt general toxicity, i.e. decreased body weights and poor general condition of the test animals at high dose levels, and not an organ specific toxicity of citral. A NOAEL could not be derived.


Data on the long-term toxicity of citral are available from a combined toxicity and carcinogenicity study in Fischer 344 rats and B6C3F1 mice (NTP 2003). Diet concentrations were 0, 1000, 2000, and 4000 ppm yielding daily dosages of 0, 50, 100, and 210 mg citral/kg bw/d for rats. Mice were exposed to diets with 0, 500, 1000, and 2000 ppm citral yielding daily dosages of 0, 60, 120, and 260 mg citral/kg bw/d.


In the study in rats (similar to OECD TG 453, according to GLP), application of citral for 2 yearsresulted in no treatment-related non-neoplastic or neoplastic effects (NTP 2003c). In high-dosed rats, mean body weights were generally reduced compared to vehicle controls from week 49 onward in males and from week 25 onward in females while food consumption was comparable in all groups. Based on this decrease of mean body weight the LOAEL in male and female rats was 4000 ppm or 210 mg/kg bw/d. The NOAEL was 2000 ppm or 100 mg/kg bw/d.


In the study in mice (similar to OECD TG 453, according to GLP), application of citral for 2 years resulted in reduced body weights compared to controls in males from 1000 ppm onward and in females in all dosed groups, while food consumption was not affected (NTP 2003d). No treatment-related non-neoplastic lesions with clear toxicological significance were observed. The LOAEL, based on a persistent reduction of body weight, was 500 ppm or 60 mg/kg bw/d in female mice and 1000 ppm or 120 mg/kg bw/d in male mice.


Inhalative exposure

In a subacute inhalation study (acc. to OECD TG 412 and GLP), Crl:CD(SD) rats (10 per sex and dose) were treated to aerosolized hydroxycitronellal by nose-only inhalation 6 hours per day on a 5-day per week basis for a period of 2 weeks (minimum of 10 exposures; WIL2013). Target exposure concentrations were 0.7, 7.0 and 70 mg/m3(analytical overall mean exposure concentrations: 0.84, 6.4 and 73 mg/m3). Animals were examined for mortality, clinical signs, body weights, food consumption, hematology/ coagulation, clinical chemistry. Bronchoalveolar lavages were performed in 5/10 animals per sex and dose and BALF was assessed for lactate dehydrogenase, total protein, alkaline phosphatase, cytology and cytokines. All animals were sacrificed, subjected to necropsy and selected organs were weighed and examined microscopically in 5/10 animals per sex and dose.

All animals survived to scheduled necropsy. Yellow material was found on various body surfaces in the females at dose level of 70 mg/m3. No other test material related clinical observations at this or lower dose levels were observed. Body weights, food consumption, clinical pathology parameters and organ weights were unaffected by test material exposure and no alterations in BALF clinical chemistry, BALF cytology or BALF cytokine parameters that were associated with test material exposure was found. Furthermore, no test material related macroscopic or microscopic findings were observed in all dose groups.

Overall, hydroxycitronellal-related effects were limited to non-adverse clinical observations in the 70 mg/m3group females. Therefore, the NOAEC was set at 70 mg/m3(equivalent to 10 ppm).


In support, repeated dose inhalation studies with citral are available, where exposure atmospheres contained citral concentrations of 1, 3, 10 and 34 ppm (both as vapour), or 68 ppm (aerosol/vapour mixture) corresponding to 6, 19, 63, 215, 430 mg/m3.The exposure condition at 68 ppm comprised an aerosol/vapour mixture with the intention to produce signs of toxicity(Gaworski 1992). Groups of F344 rats were exposed for 6 hrs/day for either 21 consecutive days at concentration of 10 and 34 ppm (both as vapour), or 68 ppm (aerosol/vapour mixture), or for 13 w, 5 days per week, at concentrations of 1, 3, or 10 ppm (all vapour atmospheres).

During the 21 day study, no mortalities occurred. Rats in the 68 ppm-dose group displayed signs of severe ocular, oral and nasal irritation, and significantly reduced body weights. In the nasal respiratory epithelium treatment-related lesions consisted of a dose-related chronic-active inflammation, hyperplasia, squamous metaplasia and goblet cell atrophy of the nasal respiratory epithelium. High-dose rats developed changes indicative of irritation in the tracheas and lungs, as well as corneal inflammation and ulceration. In the subchronic inhalation toxicity study there was no mortality and no treatment related signs of toxicity. Body weight gains, clinical pathology indices and organ weights were not adversely affected by exposure. Rats exposed to 10 ppm citral developed minimal hyperplasia and squamous metaplasia of the laryngeal epithelium; however, these changes were completely reversed during a 5-week recovery period. No significant lesions were observed in rats exposed to 1 or 3 ppm. Taken together, there was no change for the worse as exposure duration increased from 21 days to 13 weeks (Gaworski 1993).


During a developmental toxicity study, maternal toxicity was indicated at 68 ppm (430 mg/m3) by decreased body weights and by clinical signs as ocular opacity, breathing difficulty, nasal discharge and salivation. These signs of maternal toxicity were secondary to the stress produced by severe respiratory tract irritation, as recovery of body weight and clinical signs of toxicity occurred after completion of the exposure period. At 10 and 34 ppm, findings were incidental and not siginficantly different from control animals (Gaworski 1992).

Overall, the NOAEC has been set to 34 ppm or 215 mg/m3 based on evident local irritation and systemic effects, i.e. body weight changes, observed at 68 ppm or 430 mg/m3.


Dermal exposure

No key data on repeated dermal exposure of hydroxycitronellal or the structurally comparable citral are available. However, data from literature indicate an induction benign and atypic prostrate hyperplasia (BPH and APH) in adolescent male rats after topical administration of citral. The postulated mode of action, i.e. putative interactions with testosterone levels or estrogen-like effects of citral were discussed in chapter “Toxicity to reproduction”.

Justification for classification or non-classification

The present data on repeated dose toxicity do not fulfill the criteria laid down in 67/548/EEC and regulation (EU) 1272/2008, and therefore, a non-classification is warranted.