Ashes (residues), rice husk

Toxicological Summary

Workers - Hazard via inhalation route

Systemic effects

Acute/short term exposure

DNEL related information

Local effects

Long term exposure

Hazard assessment conclusion:

DNEL (Derived No Effect Level)

Value:

0.017 mg/m³

Most sensitive endpoint:

repeated dose toxicity

DNEL related information

Overall assessment factor (AF):

3

Dose descriptor:

LOAEC

Acute/short term exposure

DNEL related information

Workers - Hazard via dermal route

Systemic effects

Acute/short term exposure

DNEL related information

Additional information - workers

For the alveolar fraction (aerodynamic diameter <5 µm) of crystalline silica, epidemiological data indicate that the LOAEC (human) for silicosis is 0.05 mg/m³. Applying an assessment factor 3 to account for extrapolation from a LOAEC to a NAEC, results in a DNEL for the alveolar crystalline silica fraction of 0.017 mg/m³ [(0.05 mg/m³)/3]. No further assessment factors for workers are required as the LOAEC was determined from data of subjects occupationally exposed (8-10 h/day for a working lifetime) to respirable silica particles.

 

Comparable results are obtained from animal data. The lowest published toxic concentration for chronic inhalation exposure in rats is 0.74 mg/m³ (Muhle et al., 1991; NTP, 2009). Rats were exposed to crystalline silica particles (MMAD 1.4 µm) at that analytic concentration for 6 h/day, 5 days/week for 24 months.

 

A DNEL value for the alveolar fraction of crystalline silica is derived by applying the following corrections and assessment factors:

 

Step 1) Relevant dose descriptor	LOAECrat= 0.74 mg/m³
Step 2) Modification of starting point Correction differences between experimental and human exposure conditions Correction for differences in 8-h inhalative volumes between workers in rest and workers under light activity	(6 h/day)/(8 h/day)   6.7 m³/10 m³
Step 3) Assessment factors Interspecies Intraspecies Exposure duration Dose response Quality of data base	2.5 5 1 3 1
DNEL value for the alveolar crystalline silica fraction	0.01 mg/m³ (0.74 x [6/8] x [6.7/10] x [1/(2.5 x 5 x 3)])

 

For further calculations, the DNEL value of 0.017 mg/m³ obtained from human data is preferred.

 

As the above mentioned DNEL values applies for the alveolar fraction of crystalline silica particles, DNEL values for rice husk ashes as a whole are derived taking into account the content of respirable particles (≤5 µm) as well as the total silica and crystalline silica contents.

 

Chemical composition, granulometry and crystallinity data are available for 3 rice husk ash samples:

 

Sample	Test substance 1	Test substance 2	Test substance 3
Silica content	>94%	>99%	>95%
Thereof, crystalline silica content	1%	73%	57%
Particles ≤5 µm	99%	11%	10%
Overall alveolar crystalline silica fraction (assuming homogeneous distribution of crystalline material throughout all particle size fractions)	0.009	0.08	0.054

 

This results in the following DNELs derived for whole rice husk ashes:

 

Sample	Test substance 1	Test substance 2	Test substance 3
NAEC for alveolar crystalline silica (mg/m³)	0.017	0.017	0.017
Alveolar crystalline silica fraction in rice husk ashes	0.009	0.08	0.054
DNEL for rice husk ashes (mg/m³)	1.89	0.213	0.315

 

The most critical DNEL value of 0.213 mg/m³(test substance 2) is used for the CSA.

General Population - Hazard via inhalation route

Systemic effects

Acute/short term exposure

DNEL related information

Local effects

Acute/short term exposure

DNEL related information

General Population - Hazard via dermal route

Systemic effects

Acute/short term exposure

DNEL related information

General Population - Hazard via oral route

Systemic effects

Acute/short term exposure

DNEL related information

Additional information - General Population

Since exposure for the general public is precluded, DNELs for the general population are not relevant and thus not derived.