Ashes (residues), rice husk

Endpoint summary

Description of key information

No information on the acute toxicity of rice husk ashes via oral, dermal or inhalative route is available. The crystalline polymorph of silica is the toxicologically relevant component of rice husk ashes. Limited or not useful data are available on lethal doses of crystalline silica in experimental animals. In humans, acute oral and dermal exposure to crystalline silica may not lead to systemic toxic effects (probable oral LD50 > 15000 mg/kg bw) (WHO, 1974), based on its physicochemical and toxicokinetic properties. Acute silicosis occurs after acute exposures to high concentrations of respirable crystalline silica particles.
A single dose of ca. 277-333 mg rice husk ashes/kg bw  intratracheally applied to rats was not lethal, but caused caused diffuse interstitial fibrosis and nodules after 12 months.

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion

Dose descriptor:

LD50

Value:

3 160 mg/kg bw

Additional information

Oral  

There are no data available on the acute toxicity of rice husk ashes upon oral exposure. There are limited animal data available on lethal oral doses of crystalline silica, the main toxicologically relevant component of rice husk ashes. Due to its physicochemical properties and based on the available data on toxicokinetics of crystalline silica (see Chapter 7.1), no relevant absorption via the gastrointestinal tract leading to acute systemic effects is expected. The oral LD50 in rats suggested to be 3160 mg/kg bw, and the probable lethal dose of silica (oral) for man is over 15 g/kg bw (JEFCA, 1974). However, it is not clear whether these values regard to crystalline silica. For amorphous silica, a more soluble polymorph, the oral LD50 is greater than 5000 mg/kg bw, and is therefore regarded as practically non-toxic (Lewinson, 1994).

 

Dermal

There are no data available on the acute toxicity of rice husk ashes upon dermal exposure. There are no animal data available on lethal dermal doses of crystalline silica, the main toxicologically relevant component of rice husk ashes. Due to its physicochemical properties and based on the available data on toxicokinetics of crystalline silica (see Chapter 7.1), no relevant absorption through the epithelium of the gastrointestinal tract is expected. Hence, both crystalline silica particles and rice husk ashes are unlikely to be absorbed by the skin causing acute systemic effects. For amorphous silica, the dermal LD50 is greater than 5000 mg/kg bw, and is therefore regarded as practically non-toxic (OECD SIDS, 2004).

 

Inhalation

There are no data available on the acute toxicity of rice husk ashes upon inhalative exposure. There are no useful animal data available on lethal inhalative doses of crystalline silica, the main toxicologically relevant component of rice husk ashes. In humans, Acute exposures to high concentrations cause cough, shortness of breath, and acute silicosis. Acute silicosis develops after exposure to high concentrations of respirable crystalline silica and results in symptoms within a period ranging from a few weeks to 5 years after the initial exposure. Acute silicosis is typically associated with a history of high exposures from tasks that produce small particles of airborne dust with a high silica content, such as sandblasting, rockdrilling, or quartz milling. The pathologic characteristics of acute silicosis (sometimes referred to as silicoproteinosis) resemble those of alveolar proteinosis. Pulmonary fibrosis may not be present in acute silicosis (NIOSH 2002).

For amorphous silica, the results of an acute inhalation toxicity study indicated that fumed hydrophobic amorphous silica is virtually acutely nontoxic at the maximum attainable aerosol concentration of 0.477 mg/L over 4 h (Lewinson, 1994). Further studies indicate that no lethal effects occur following inhalation exposure of rats to the highest technically feasible concentrations of 140 to ca. 2000 mg/m³ (OECD SIDS, 2004).

Other routes

The induction of pulmonary fibrosis by rice husk ashes incinerated at 350°C, 650°C, and 1300°C was investigated in male rats intratracheally given 50 mg/animal (ca. 277-333 mg/kg bw) of the experimental dust once. Quartz dust served as positive control. Groups rats in each exposure group were killed after the 1, 3, 6 and 12 months of the experiment for pathological examination. The results of pathological studies showed that the RHA heated at 1300°C could cause diffuse interstitial fibrosis and silicotic nodules.

Justification for classification or non-classification

Taking into account the whole body of evidence on the acute toxicity of crystalline silica, rice husk ashes are not considered to be toxic after acute exposure via the oral, dermal or inhalation route. Therefore, rice husk ashes need not be classified for acute toxicity according to DSD and CLP criteria for classification and labelling.