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Administrative data

Description of key information

CaO is not acutely toxic.

Key value for chemical safety assessment

Acute toxicity: via oral route

Link to relevant study records
Reference
Endpoint:
acute toxicity: oral
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2007-03-22 to 2007-04-26
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP guideline study Minor deviations without an effect on the study results: - A rationale for choosing the limit dose was not given.
Qualifier:
according to guideline
Guideline:
OECD Guideline 425 (Acute Oral Toxicity: Up-and-Down Procedure)
Version / remarks:
, adopted 2006-03-23
Deviations:
yes
Remarks:
see "rationale for reliability"
GLP compliance:
yes (incl. QA statement)
Remarks:
signed November 2005
Test type:
up-and-down procedure
Limit test:
yes
Species:
rat
Strain:
Wistar
Sex:
female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Source: RCC Ltd, Laboratory Animal Services, CH-4414 Füllinsdorf/Switzerland
- Age at study initiation: 11 weeks
- Weight at study initiation: 179.9 to 192.3 g
- Fasting period before study: Approximately 16 to 19 hours; Food was provided again approximately 3 hours after dosing
- Housing: Individually in Makrolon type-3 cages with standard softwood bedding ("Lignocel", Schill AG, CH-4132 Muttenz) during treatment and observations
- Diet (ad libitum): Pelleted standard Provimi Kliba 3433 rat/mouse maintenance diet, batch no. 80/06 or 89/06 (Provimi Kliba AG, CH-4303 Kaiseraugst/Switzerland)
- Water (ad libitum): Community tap water from Füllinsdorf
- Acclimation period: Seven days under laboratory conditions

ENVIRONMENTAL CONDITIONS
- Temperature: 22 +/- 3°C
- Humidity: 30-70 %
- Air changes: 10-15 air changes per hour
- Photoperiod (hrs dark / hrs light): 12/12
No further information on the test animals was stated.
Route of administration:
oral: gavage
Vehicle:
polyethylene glycol
Details on oral exposure:
VEHICLE: Polyethylene glycol 300 (PEG 300)
- Description: Colourless viscous liquid
- Source: FLUKA Chemie GmbH, CH-9471 Buchs
- Stability of vehicle: Stable under storage conditions
- Expiration date: October 2007
- Storage conditions: At room temperature (range of 20 +/- 5°C), light protected
- Justification for choice of vehicle: The vehicle was chosen after a non-GLP solubility trial which was performed before the study initiation date. Polyethylene glycol 300 was the most suitable vehicel selected for the test item.
- Lot no.: 1300225
- Concentration in vehicle: 0.2 g/mL

MAXIMUM DOSE VOLUME APPLIED: The application volume was 10 mL/kg body weight.

DOSAGE PREPARATION:
The dose formulations were made shortly (i.e. less than 30 minutes) before each dosing occasion using a magnetic stirrer and a spatula. The test item was weighed into tared glass beakers on a suitable precision balance and the vehicle added (weight:volume). The pH of the freshly prepared dose formulations was mesaured at each occasion following preparation and was found to be between pH 6-7 for the test item. Temperature of the freshly prepared dose formulations were generally between 20-23 °C. Homogeneity of the test item in the vehicle was maintained during administration using a magnetic stirrer. The stability of the test item in the vehicle was not determined in this test. This is not considerd to be required because of the short period between dose preparation and administration to the animals.
No further information on oral exposure was stated.
Doses:
2000 mg/kg body weight
No. of animals per sex per dose:
5 female rats
Control animals:
no
Details on study design:
- Duration of observation period following administration: 14 days
- Frequency of observations and weighing: Observations of mortality/viability was made daily during the acclimatization period, during the first 30 minutes and at approximately 1, 2, 3 and 5 hours after administration on the day of dosing (test day 1; with the clinical signs) and twice daily during days 2 - 15. Observations of body weights were made on the day prior to the administration of the test item, on the the day of administration of the test item (prior to administration of the test item), and on days 8 and 15. Observations on clinical signs were made daily during the acclimatization period, during the first 30 minutes and at approximately 1, 2, 3 and 5 hours after the administration on test day 1. Once daily during days 2-15.
- Necropsy of survivors performed: Yes
All animals were killed by carbon dioxide asphyxiation and discarded after macroscopic examinations have been performed.
- Other examinations performed: For clinical signs all abnormalities were recorded.
No further information on the study design was stated.
Statistics:
No statistical analysis was performed.
Sex:
female
Dose descriptor:
LD50
Effect level:
> 2 000 mg/kg bw
Mortality:
No deaths occured during the study.
Clinical signs:
Slightly ruffled fur was noted at the reading performed within 30 minutes or 1 hour after administration up to the 5-hour observation, day 2, 3 or 6 in all animals. Hunched posture was observed at the 30-minutes reading up to the 3- or 5-hour reading or up to day 3 in three of these animals and at the 30-minutes or 1-hour reading in the remaining two animals. In addition, slight sedation at the 1-hour reading up to day 3 as well as deep respiration and rales at day 2 were noted in one animal.
Clinical signs were shown to be reversible until scheduled sacrifice on day 15.
Body weight:
The body weight of the female animals used was within the range commonly recorded for this strain and age.
Gross pathology:
No macroscopic findings were observed at necropsy.
Interpretation of results:
not classified
Remarks:
Migrated information Criteria used for interpretation of results: EU
Conclusions:
The median lethal dose for the test item Precal 30S (Calcium oxide (burnt lime)) after single oral administration to female rats, observed over a period of 14 days is: LD50 (rat): greater than 2000 mg/kg body weight.
According to the criteria specified by Directive 67/548/EEC and subsequent regulations, the test item is not classified.
According to the EC Regulation No. 1272/2008 and subsequent regulations, the test item is not classified.
Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
discriminating dose
Value:
2 000 mg/kg bw
Quality of whole database:
One k=1 study available.

Acute toxicity: via inhalation route

Link to relevant study records

Referenceopen allclose all

Endpoint:
acute toxicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
22 march 2010 to 30 July 2010
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 436 (Acute Inhalation Toxicity: Acute Toxic Class Method)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Test type:
fixed concentration procedure
Limit test:
yes
Species:
rat
Strain:
Wistar
Sex:
male/female
Details on test animals or test system and environmental conditions:
Adult, male and female Wistar outbred (Crl:WI[WU]) rats were obtained from a colony maintained under SPF-conditions by Charles River Laboratories. Three male and three female animals arrived on 7 April 2010 at an age of 7 weeks. They were taken in their unopened shipping containers to animal room 5.2.10, were checked for overt signs of ill health and anomalies, and were kept in quarantine. After approval of the lot (negative titers to micro-organisms tested in a few animals), quarantine was raised on 9 April 2010 and the animals were moved to 6.0.04, a similar animal room. The rats were separated by sex and uniquely identified by ear tattoo. Just before the start of the study on 21 April 2010, the animals were weighed. The average body weights of the rats on day 0 before exposure were 277.7 g and 190.3 g for the males and females, respectively. The duration of the acclimatization period was 12 days.

The animals were housed under conventional conditions in macrolon cages with bedding of wood shavings (Lignocel, type ¾, Rettenmaier, Rosenberg, Germany) and strips of paper (Enviro-dri, Lillico, Betchworth, England) as environmental enrichment. The number of air changes was about 10 per hour. The animals were housed three males or three females to a cage. During the exposure, the animals had no access to feed or water and were housed individually in the holders. After exposure, the animals returned to their living cages and were held for an observation period of 15 days before sacrifice and necropsy.

The temperature in the animal room was within the range of 20 – 24°C, except on 14 April 2010 when temperature was above 24°C (25.3°C at maximum) for a maximum period of 20 minutes, most probably due to cleaning activities with hot water. Relative humidity occasionally exceeded the range of 45 – 65% for short periods of time, probably due to cleaning activities or meteorological circumstances. On 24 April 2010, relative humidity in the animal room was below 45% (43.4% at minimum) for a maximum period of 20 minutes. A 12-hour light and 12-hour dark cycle was maintained.
Route of administration:
inhalation: dust
Type of inhalation exposure:
nose only
Vehicle:
air
Details on inhalation exposure:
The animals were exposed to the test atmosphere in a nose-only inhalation chamber, a modification of the design of the chamber manufactured by ADG Developments Ltd., Codicote, Hitchin, Herts, SG4 8UB, United Kingdom (see Figure 1). The inhalation chamber consisted of a cylindrical stainless steel column, surrounded by a transparent cylinder. The column had a volume of ca. 50 liters and consisted of a top assembly with the entrance of the unit, a rodent tube section and at the bottom the base assembly with the exhaust port. The rodent tube section had 20 ports for animal exposure. Several empty ports were used for test atmosphere sampling, particle size analysis, measurement of oxygen concentration, temperature and relative humidity. The animals were secured in plastic animal holders (Battelle), positioned radially through the outer cylinder around the central column. Male and female rats were placed in alternating order. The remaining ports were closed. Only the nose of the rats protruded into the interior of the column.

The inhalation equipment was designed to expose rats to a continuous supply of fresh test atmosphere. To generate the test atmosphere, Flue Dust T –fine (REACH) was aerosolized using a dust feeder (Hethon Nederland BV, Hengelo, The Netherlands), a venturi and a jet mill (Institute’s design). The latter two were supplied with humidified compressed air. The resulting test atmosphere was led to the top inlet of the exposure chamber and from there to the noses of the animals. At the bottom of the unit, the test atmosphere was exhausted .

During the generation of the test atmosphere, the settings of the dust feeder and the air pressure on the jet mill were recorded at regular intervals (approximately each half hour). The airflow through the exposure chamber at the pressure settings of the jet mill and the venturi was determined in a preliminary experiment and was established to be 89.25 L/min. The animals were placed in the exposure unit after stabilization of the test atmosphere. The period between the start of the generation of the test atmosphere and the start of exposure of the animals was 27 minutes. The concentration C in a perfectly stirred test atmosphere in a chamber with volume V (L) and flow F (L/min) increases according to C = C ∞* (1 – e -(F*T/V) ), in which T (min) is the time and C ∞is the steady state concentration. Hence T 95 , the time it takes to reach 95% of the steady state concentration is given by e -(F*T95/V) = 0.05, from which it follows that T 95 was approximately 1.7 minutes. In practice, after the start of the generation the aerosol will spread from the top to the bottom and T 95 will be shorter than in a perfectly stirred chamber.


Analytical verification of test atmosphere concentrations:
yes
Duration of exposure:
ca. 4 h
Concentrations:
Actual concentration
The actual concentration (± standard deviation, number of measurements) of Flue Dust T –fine (REACH) in the test atmosphere during exposure was 6.04 g/m
3 (± 0.54, n=14; indicating that the concentration in the test atmosphere was amply above the target limit concentration of 5 g/m3.

Nominal concentration
The nominal concentration, calculated from the total amount of test material used (by weighing) and the air flow was 14.51 g/m3. This indicates a generation efficiency of 42%, which is within the range expected for aerosol generation.

No. of animals per sex per dose:
3
Control animals:
no
Details on study design:
Behaviour, clinical signs, and mortality
The rats were visually inspected just before exposure, for reactions to treatment during the exposure, shortly after exposure, and at least once daily during the observation period.

Body weights
Body weights of the animals were recorded just before exposure (day 0), on days 1, 3 and 7, and on day 15 prior to necropsy. In addition, animals were weighed on day 2, because of their ill health.

Pathology
At the end of the 15-day observation period, animals were killed by exsanguination from the abdominal aorta under pentobarbital anaesthesia (intraperitoneal injection of sodium pentobarbital) and examined for gross pathological changes.
Key result
Sex:
male/female
Dose descriptor:
LC50
Effect level:
> 6.04 mg/L air (nominal)
Based on:
test mat.
Exp. duration:
4 h
Mortality:
One female animal (No. 3) was found dead after approximately three hours of exposure.
Clinical signs:
other: All animals demonstrated a decreased breathing rate during exposure, which became more severe with time. Clinical signs observed shortly after exposure in surviving animals included slight to moderate laboured breathing, slight to moderate rales, slight
Body weight:
All animals showed substantial body weight loss during the first few days after exposure (Table 3). Although a small decrease in body weight gain is expected due to the constraint of the animals during exposure, effects of this magnitude are considered treatment-related. Body weights recovered during the second week of the 15-day observation period.
Gross pathology:
The nose of the female animal that died during exposure seemed blocked, with brown powder on the exterior of the nose. In addition, the animals’ lungs were red discoloured with several petechiae .
Necropsy of the surviving rats at the end of the observation period revealed petechiae in the medial lung lobe of one male animal, which was not considered to be related to the exposure. No other macroscopic abnormalities were observed at necropsy.
Interpretation of results:
Category 5 based on GHS criteria
Conclusions:
One female animal died during exposure. All other animals survived the 14-day observation period. It is therefore concluded that the 4-hour LC50 of Flue Dust T –fine (REACH) is above 6.04 g/m3 for male and female rats. According to the OECD Guideline for Testing of Chemicals 436, the test material should be classified as Category 5 of the Globally Harmonized Classification System (GHS); according to the EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) regulation (EC) No. 1272/2008, the test material does not need to be classified.
Executive summary:

The aim of the present study was to investigate the acute inhalation toxicity of Flue Dust T –fine (REACH) in rats for REACH registration and classification purposes. Therefore, three male and three female animals were exposed to a target limit concentration of 5 g/m 3 during a single period of four hours. Animals were kept for an observation period of 15 days before sacrifice. To characterize the toxicity, the animals were observed during exposure, shortly after exposure and daily thereafter, body weight was measured before exposure and 1, 2, 3, 7 and 15 days after exposure and the animals were examined for gross pathological changes at necropsy.

The actual concentration during exposure was 6.04 ± 0.54 g/m 3 . The mass median aerodynamic diameter was 3.5 and 3.8 µm (duplicate measurements) and the distribution of particle sizes had a geometric standard deviation (gsd) of 2.2 and 2.1, respectively.

Animals demonstrated a decreased breathing rate during exposure, which became more severe with time. One female was found dead after 3 hours of exposure. At necropsy, the nose of this animal seemed blocked and the animals’ lungs were red discoloured with several petechiae. Clinical signs observed after exposure in surviving animals included breathing abnormalities, soiled eyes, nose and fur, blepharospasm, encrustations around the eyes, nose and mouth, dark eyes, and sluggishness. Female animals were slightly more affected than males. Generally, abnormalities were no longer seen after 5 to 6 days. Substantial body weight loss was observed during the first few days after exposure, which recovered in the second week of the 15-day recovery period.

Macroscopic examination at necropsy revealed red discoloured lungs with several petechiae in the animal that died during exposure, and the animals’ nose seemed blocked. No treatment-related gross abnormalities were found in the surviving animals at the end of the recovery period.

Physical obstruction of the nose may well have played a role in the observed respiratory abnormalities and mortality. Since rats are obligatory nose breathers, the relevance of this model for human exposure may be questioned for these effects.

Endpoint:
acute toxicity: inhalation
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
1. HYPOTHESIS FOR THE ANALOGUE APPROACH
Flue dust, Portland cement (Source substance) contains calcium oxide (Target substance), which hydrolyses to calcium dihydroxide in aqueous systems, and calcium carbonate, as well as salts, such as tricalcium silicate and dicalcium silicate which also hydrolyse to form calcium dihydroxide as one of the products in aqueous solution. Both the Source and the Target will have a common mode of action upon inhalation, particularly for short term exposure, i.e. local effects due to irritation caused by alkalinity.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Source: Flue dust, Portland cement (EC 270-659-9). UVCB inorganic substance. Contains approximately 26% calcium oxide, 5.5% calcium carbonate, 9% tricalcium silicate and 24% dicalcium silicate (Full composition provided in TMI).
Target: Calcium oxide (EC 215-138-9). Purity/impurity profile as described in Section 1.2

3. ANALOGUE APPROACH JUSTIFICATION
For Flue dust, Portland cement, the pH is described as >11.5, due to hydration reaction (See RSS for acute inhalation; TNO 2010). This compares to the pH of 12.3 determined for calcium oxide (See RSS for water solubility; Fox 2010). Both substances are classified as Skin Irritant 2 (H315: Causes skin irritation) and STOT SE3 (H335: May cause respiratory irritation). Based the similar pH, the hydrolysis of the main constituents of Flue dust, Portland cement and of calcium oxide itself to calcium dihydroxide and similar skin and respiratory irritation potentials, it is concluded that it is acceptable to read-across from Flue dust, Portland cement (Source) to the calcium oxide (Target) for acute inhalation toxicity.

This approach is supported by the Scientific Committee on Occupational Exposure Limits in their recommendation for calcium oxide and calcium hydroxide (SCOEL/SUM/137, February 2008) in which the alkaline effects of Portland cement are used as a surrogate for the alkaline effects of calcium oxide and hydroxide (and may be considered a worse-case due to the potential higher pH). Per the SCOEL, systemic effects of CaO and Ca(OH)2 are negligible at normal occupational exposure levels, since exposure by inhalation (7.1 and 5.4 mg Ca/day, respectively) adds a negligible body burden of calcium compared to the tolerable upper intake level of 2500 mg/day.
Reason / purpose for cross-reference:
read-across source
Key result
Sex:
male/female
Dose descriptor:
LC50
Effect level:
> 6.04 mg/L air (nominal)
Based on:
test mat.
Exp. duration:
4 h
Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
discriminating conc.
Value:
6 040 mg/m³
Quality of whole database:
The key study is GLP compliant and has a Klimisch score 1

Acute toxicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

Acute oral toxicity:

In an acute oral toxicity test, performed to OECD TG 425, five female rats (Wistar) were dosed by gavage with calcium oxide at 2000 mg/kg bw and observed for 14 days. No deaths occurred during the study. Clinical signs included ruffled fur in all animals up to day 6, hunched posture up to day 3 in three animals and up to the 1-hour observation in the remaining two animals. In addition, slight sedation up to day 3 as well as deep respiration and rales at day 2 were noted in one animal. The oral LD50 for rats was > 2000 mg/kg bw.

In a reliable GLP study performed to OECD TG 420 [Bradshaw 2008] calcium carbonate was administered by gavage at 2,000 mg/kg bw to 5 female rats (Sprague-Dawley). The animals were observed for clinical signs and mortality for 14 days. No mortalities were observed in the observation period and there were no clinical signs of systemic toxicity, adverse changes in bodyweight or macroscopic effects noted at necropsy. The oral LD50 for rats was > 2000 mg/kg bw.

Based on these studies it may be concluded that all grades of calcium oxide, including those with a calcium carbonate content of up to 35% are not acutely toxic via the oral route.

Acute inhalation toxicity:

No data are available for calcium oxide. However, data are available for the read-across substance Flue dust, Portland cement (EC 270-659-9). In a reliable GLP study performed to OECD TG 436 [TNO, 2010] three male and three female animals were exposed to a target limit concentration of 5 g/m3(6.04 g/m3measured) Flue dust, Portland cement for four hours. Animals were kept for an observation period of 15 days before sacrifice. Animals demonstrated a decreased breathing rate during exposure, which became more severe with time. One female was found dead after 3 hours of exposure. Clinical signs observed after exposure in surviving animals included breathing abnormalities, soiled eyes, nose and fur, blepharospasm, encrustations around the eyes, nose and mouth, dark eyes, and sluggishness. Female animals were slightly more affected than males. Generally, abnormalities were no longer seen after 5 to 6 days. Substantial body weight loss was observed during the first few days after exposure, which recovered in the second week of the 15-day recovery period. Macroscopic examination at necropsy revealed red discoloured lungs with several petechiae in the animal that died during exposure, and the animals’ nose seemed blocked. No treatment-related gross abnormalities were found in the surviving animals at the end of the recovery period. Physical obstruction of the nose may well have played a role in the observed respiratory abnormalities and mortality. Since rats are obligatory nose breathers, the relevance of this model for human exposure may be questioned for these effects. The 4-h LC50 in this study was > 6.04 g/m3air.

Based on the adopted Recommendation from the Scientific Committee on Occupational Exposure Limits (SCOEL) for Calcium oxide (CaO) and Calcium hydroxide (Ca(OH)2) the European Commission has established an IOELV of 4 mg/m³ respirable fraction (STEL) and 1 mg/m³ respirable fraction (8h-TWA) for both calcium dihydroxide and calcium oxide (Commission Directive (EU) 2017/164 of 31 January 2017) which is considered protective against long-term exposure to CaO and Ca(OH)2.

Two ethically approved human volunteer studies [Cain, 2004, 2008, section 7.10.1] also considered by SCOEL indicate chemosensory feel and local irritation of mucous membranes in the respiratory tract being the primary effect upon inhalation exposure of alkaline mineral materials (due to a pH shift).

A voluntary proposal for the classification of calcium hydroxide/calcium oxide is made as R37 / STOT SE 3 (H335 - May cause respiratory irritation), based on the findings of sensory irritation in humans.

In a reliable GLP study performed to OECD TG 403 [Schuler, 2010] calcium carbonate was administered by inhalation at a measured concentration of 3 mg/L air to 5 male and 5 female rats (Wistar) for 4 hours. The animals were observed for clinical signs and mortality for 14 days and on test day 15, all animals were sacrificed and necropsied. All animals survived the scheduled observation period. Ruffled fur was observed in all animals from the end of the exposure up to test day 4. Thereafter, all animals were free of clinical signs. Transient body weight loss was noted in most animals from test day 1 to test day 2 but normal body weight development was observed in general thereafter. No macroscopic findings were present at necropsy. The inhalation LC50 for rats was > 3 mg/L air. This was the highest technically achievable concentration.

The study on calcium carbonate demonstrates that it is not acutely toxic to rats up to the limit of the highest technically achievable concentration that could be tested. On this basis, calcium carbonate itself is not classified for acute inhalation toxicity. However, it may be assumed that grades of calcium oxide containing up to 35% calcium carbonate will still exhibit the chemosensory feel and local irritation of mucous membranes in the respiratory tract attributed to calcium oxide itself, and hence the STOT SE 3 classification will be relevant equally for these grades.

Acute dermal toxicity:

An acute dermal toxicity study with CaO is considered scientifically unjustified and no study is available.

A study is available for calcium hydroxide, which will be formed when calcium oxide is in the presence of moisture. In a reliable study performed to OECD TG 402 [Kietzmann, 1994] white lime paste (a mixture of calcium dihydroxide with water) was applied to the sheared skin of 5 male and 5 female rabbits (New Zealand White) under semi-occlusive conditions at a dose of 2500 mg/kg bw. After 24 hours, the treated area was washed with water. There were no deaths of clinical signs of systemic toxicity during the 14-day observation period. Irritation-related effects were seen, including redness and scabbing. No abnormalities were noted at necropsy. The LD50 was > 2500 mg/kg bw.

In a reliable GLP study performed to OECD TG 402 [Bradshaw 2010] calcium carbonate (nano) was applied as a paste in arachis oil to the skin of 5 male and 5 female rats (Wistar) at a dose of 2000 mg/kg bw and covered with a semi-occlusive dressing for 24 hours. After completion of the exposure period the skin was wiped with cotton wool moistened with arachis oil to remove any residual test material. There were no deaths or clinical signs of systemic toxicity during the 14 -day observation period. All animals showed expected bodyweight gains over the study period. No abnormalities were noted at necropsy. The LD50 was > 2000 mg/kg bw.

Based on these studies it may be concluded that all grades of calcium oxide, including those with a calcium carbonate content of up to 35% are not acutely toxic via the dermal route.

Justification for classification or non-classification

Acute oral toxicity: LD50 > 2000 mg/kg bw

Acute dermal toxicity: No data available. Data for calcium dihydroxide indicates LD50 > 2500 mg/kg bw

Based on these results, calcium oxide is not classified for acute oral or dermal toxicity according to CLP.

Acute inhalation toxicity:

Based on read-across to Flue dust, Portland cement (EC 270-659 -9) the 4 -h LC50 in rats is > 6.05 mg/L. Based on this result, calcium oxide is not classified for acute inhalation toxicity.

Two ethically approved human volunteer studies [Cain, 2004, 2008, section 7.10.1] also considered by SCOEL indicate chemosensory feel and local irritation of mucous membranes in the respiratory tract being the primary effect upon inhalation exposure of alkaline mineral materials (due to a pH shift). On this basis, calcium oxide is classified according to CLP as STOT SE 3 (H335 - May cause respiratory irritation).