Registration Dossier

Administrative data

Description of key information

The acute toxicity tests in rats resulted in

LD50= 1531 mg/kg bw for the oral toxicity of the solid substance

LC50 = 0.168 mg/L for the respirable fraction of the substance

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
LD50
1 531 mg/kg bw
Quality of whole database:
Test sufficiently described to assess the quality

Acute toxicity: via inhalation route

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
LC50
168 mg/m³

Acute toxicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

Acute toxicity: oral

Aminoiminomethanesulphinic acid was studied for oral toxicity in male and female rats in an acute oral toxicity test at doses 1024, 1280, 1600 and 2000 mg/kg bw in study conducted according to OECD 401 and under GLP conditions (Ogata, 1990). Deaths occurred in males of at concentrations ≥ 1600 mg/kg bw and in females at concentrations ≥ 1280 mg/kg bw. Based on the general observation, soft faeces were observed in all animals of administered groups in both sexes at day 1. After day 3, locomotor activity was suppressed and bradypnea was observed in males at 1600 mg/kg bw and above and in females at 1280 mg/kg bw and above. Dead animals showed soiled anus region, soiled perineal region due to urine, abdominal position, cyanosis, skin pallor, pale eyeball, decreased body temperature or emaciation. Surviving animals recovered from soft feces at day 4, from bradypnea at Day 8 and from decreased locomotor activity at Day 10. Decrease of body weight and/or depression of body weight gain were observed in all treated groups. In dead animals, dark red coloration and collapse in the lung, retention of frothy fluid in the trachea, retention of watery fluid in the thoracic cavity, and discoloration of the kidneys were noted at necropsy. Discoloration of kidney was found in one female in 1280 mg/kg bw group, one male in 1600 mg/kg bw group, one female in 2000 mg/kg bw group. White spots on the mucosa in the forestomach were found in one female in 1600 mg/kg bw group and one male in 2000 mg/kg bw group. White spot in the glandular stomach was found in one female of the 1600 and 2000 mg/kg bw group. Two females of 1600 mg/kg bw and one male of 2000 mg/kg bw group, which had abnormalities in stomach, showed retention of black-brown contents in the intestine as well. Atrophy in the thymus and the spleen were found in almost all the dead animals. 

In the dead animals, histopathological examination showed edema of the alveoli and congestion in the lung, necrosis and mineralization of the tubules and basophilic tubules in the kidney, ulceration in the forestomach and erosion in the glandular stomach, mineralization in the cortex of thymus and atrophy in the thymus and the spleen.

Survived animals showed basophilic tubules, tubular dilatation, fibrosis and cellular infiltration of lymphocytes in the kidneys, hyperplasia of squamous epithelial cells in the forestomach, atrophy in the thymus, atrophy in the testis and debris of germ cell in lumen.

Thus on the basis of the above exposed LD50 was determined to be 1565 mg/kg bw for males and 1496 mg/kg for females.

A good conducted acute oral toxicity test was performed with aminoiminomethanesulphinic acid (CAS 1758-73-2). The test was performed before guidelines and GLP regulations were developed. Groups of 5 male and 5 female Sprague Dawley rats each received of test substance at 5 different doses once orally by gavage. Doses used are not indicated. Signs of intoxication were sedation, ataxia, lying on belly or flank, hypoventilation and reduced reflexes beginning after 6 to 8 hours. Death occurred within 18 to72 h. After necropsy there was possibly lung edema. The LD50 was calculated according to Litchfield and Wilcoxon at 1120 mg/kg bw. "Toxicity threshold" is reported to be approximately at 900 mg/kg bw.

Acute toxicity: inhalation

The LC50 determined for a respirable aerosol of the substance will be used to derive an LC50 for the substance as manufactured using the EU split-entry concept (see justification for classification or non-classification)

Acute toxicity: dermal

For acute dermal toxicity no study is available for aminoiminomethanesulphinic acid. However, for the test substance only a low dermal absorption potential of not more than 10% is estimated based on chemical characteristics like the molecular weight, logKow and water solubility. Specifically, a permeability constant of 3.49708 cm/h was estimated resulting in a flux of 0.00005 µg/cm²/h as calculated with DERMWIN (v.2.01, 2011, modified considering the Fick´s first law). Thus, dermal absorption of aminoiminomethanesulphinic acid can be considered as nearly negligible. Therefore, testing via the dermal route is not necessary according to Column 2 of Annex VIII especially as appropriate data are available for acute toxicity after oral exposure or inhalation.

Justification for selection of acute toxicity – oral endpoint

Highest Klimisch score

Justification for selection of acute toxicity – inhalation endpoint

Only available study

Justification for classification or non-classification

According to the results of the oral and of the dermal toxicity study in rats the classification of the substance as Xn; R22-Harmful if swallowed (DSD) and Acute Tox 4; H302 (CLP) are clearly demonstrated.

However, even though the available study on acute inhalation toxicity was conducted according to GLP and a standard method, the resulting LC50 is not appropriate for classification of the substance to be registered.

The DSD and CLP classifications T+; R26 and Acute Tox.2; H331, respectively, which would formally follow the 4-h LC50 of 0.168 mg/L, are not justifiable because of following reasoning:

The substance was tested as aerosol using water as vehicle. The aerosol had a high respirability, as stipulated by the relevant OECD test guideline 403. The MMAD of the droplets in this test was 2.2-3.3 µm. 67-75% of the aerosol mass had a diameter < 5 µm and thus represented the alveolar fraction.

In contrast, the substance is manufactured, transported, handled, stored and used as coarse grained, crystalline solid with a very low dustiness. Several independent measurements of particle size distribution have been conducted with aminoiminomethanesulphinic acid from two different manufacturers by two different laboratories. The MMAD range determined in this series of measurement was 168-302 µm (see IUCLID Section 4.5). The inhalable fraction (<100 µm) makes up 7.8-33.4%, whereas the thoracic fraction (<10 µm) makes up 0.0-5.6%. These measurements have been conducted applying up to 4 bar of suspending pressure, simulating a high-energy manipulation of the substance. According to the ECHA Guidance Document R.8, inhalation testing is not required for solids with an MMAD of >100 µm. Particles of this size settle so rapidly that they are unlikely to reach the breathing zone of the worker. A spray application is not foreseen in any stage of the life cycle of aminoiminomethanesulphinic acid.

As consequence, the acute inhalation test with the liquid aminoiminomethanesulphinic acid aerosol is neither representative nor relevant for TDO in the physical form that is subject of this registration dossier. This means only a very small amount will be inhalable during of manufacturing and using the substance.

Inhalation of aminoiminomethanesulphinic acid vapour is negligible because of the low vapour pressure (VP) of only 1.61E-04 Pa at 25°C. This VP gives rise to a saturated vapour concentration (SVC) of 0.0071 mg/m³. The SVC is the maximum possible workplace exposure to aminoiminomethanesulphinic acid vapours and is 23,000-fold below the LC50 in the rat (168 mg/m³).

The clinical signs and necropsy results of rats that had died after aminoiminomethanesulphinic acid inhalation exposure clearly demonstrated that aminoiminomethanesulphinic acid has a strong irritating effect on the lungs. This is in line with the strong eye irritating properties of aminoiminomethanesulphinic acid. Severe irritation of the airways undoubtedly was the cause of death. It is obvious that such local effects depend on the ability of the test substance to deposit at the site of damage. The classification with H331 (also EUH071: ‘corrosive to the respiratory tract’) would only pertain to the substance fraction that is able to reach at least the thoracic region of the lung. Conventionally, this applies to particle sizes < 10 µm.

The so-called “split-entry” concept allows classifying substances in a differentiated manner if their inhalation toxicity is only mediated by respiratory tract irritation (reviewed by Pauluhn, Exp. Toxicol. Pathol. 60(2008), 111-24).

The concept applies to substances which have a very coarse particle size distribution (MMAD >100 µm) and a low vapour pressure (SVC ≤ 1% of the LC50). TDO clearly fulfills both prerequisites.

Following this concept, technical aminoiminomethanesulphinic acid can be viewed as a mixture of fine particles (<10 µm) and coarse particles that cannot reach the thorax, let alone the alveolar region. The fine particles are classified as Acute Tox.2; H331. The coarse particles are not classified for acute inhalation toxicity. For TDO samples with the highest proportion of the thoracic fraction (5.6%), the LC50 of the total TDO can be calculated according to CLP mixture rules:

100%/LC50(total) = 5.6%/LC50(thoracic) = 5.6% / 0.168 mg/L

LC50(total) = 100%/5.6% × 0.168 mg/L = 3.0 mg/L

With an estimated LC50 of 3.4 mg/L for total TDO (worst case), classification as Acute Inhal Cat.4, H332 - Harmful if inhaled (Xn, R20 – Harmful by inhalation) is warranted. No mixture rules exist in the CLP regulation for classifying mixtures containing an ingredient classified as the EUH071: ‘corrosive to the respiratory tract’ (which the thoracic fraction would carry). With a maximum 5% content of thoracic fraction, a classification of total TDO as STOT SE3, H335 - May cause respiratory irritation (Xi, R37 - Irritating to respiratory system) is adequate, based on expert judgement.