Ashes (residues), rice husk

Endpoint summary

Description of key information

Several epidemiologic studies demonstrate that workers have a significant risk of developing chronic silicosis when they are exposed to respirable crystalline silica over a working lifetime at a mean concentration of 0.05 mg/m³ (NIOSH, 2002). Therefore, this concentration is regarded as the LOAEC for chronic silicosis in humans.

Additional information

There is limited information available on effects of rice husk ashes in humans, while more reliable data are available for crystalline silica.

 

In one study, a group of 16 male workers of a ceramic factory was investigated for pneumosilicosis. The subjects had been engaged in packing rice husk ashes into a saggar and in screening rice husk ashes. Occupational history and dust exposure history were obtained through interviews. The average age of the subjects was 54.7 years (maximum 56 years). The subjects received a health examination, chest radiographs were taken and previous chest radiographs were obtained. The previous and recent radiographs were evaluated according to the "Roentgen Diagnostic Criteria and Principles of Management of Pneumoconiosis" applied in the P.R. of China. Seven cases in pneumoconiosis category I (3 in category I+), 3 cases in category II (2 in II+) and 6 cases in category 0+ were found. The average length of employment up to the occurrence of pneumoconiosis was 21.3 years, but in one case only 4 years. Roentgenographic abnormalities were mainly small round shadows in the early stage. Then the small shadows increased, and became big and widespread, and developed into clusters. The radiographs in each case showed typical pneumosilicosis changes. During the cleaning of the saggars, screening and blending of the rice husk ashes, the mean concentrations (geometric mean) of dust were 4.9 mg/m³, 19.3 mg/m³ and 12.8 mg/m³, respectively. The diameter of 80% of the particles was less than 5 mm. The percentage of free silica in the sedimentary dust was 72.3% (Liu et al., 1996).

 

Silicosis, which most commonly occurs as a diffuse nodular pulmonary fibrosis, is the disease most associated with exposure to respirable crystalline silica. Probably the most important factor in the development of silicosis is the "dose" of respirable silica containing dust in the workplace setting; that is, the product of the concentration of dust containing respirable silica in workplace air and the percentage of respirable silica in the total dust.

Other important factors are (1) the particle size, (2) the crystalline or noncrystalline nature of the silica, (3) the duration of the dust exposure, and (4) the varying time period from first exposure to diagnosis (from several months to more than 30 years) (NIOSH, 2002).

 

The International Programme on Chemical Safety (IPCS) and the US National Institute for Occupational Safety and Health (NIOSH) have identified some critical epidemiologic studies providing evidence of an exposure-response relationship for crystalline silica and silicosis using cumulative exposure data (IPCS, 2000; NIOSH 2002). Exposure-response models based on cumulative exposure data can predict silicosis risk for a particular silica dust exposure over a period of time.

 

Epidemiological studies of silicosis usually define the profusion of small opacities present in the disease according to a standard system used by trained readers and developed by the

International Labour Organization for classification of chest radiographs of pneumoconioses (ILO, 1980). Each reader assesses the profusion according to a 12-point scale of severity. Categories 0/- and 0/0 are the first and second points on the scale and represent a normal chest radiograph. The third point, category 0/1, represents the borderline between normality and abnormality, and category 1/0, the fourth point, represents definite, but slight, abnormality (Love et al., 1994). The shape (rounded or irregular) and size of the opacities can also be described by the readers.

 

A retrospective cohort study was conducted with 1,809 workers in the diatomaceous earth industry. In this setting, exposures to crystalline silica are primarily to the cristobalite form. Based on the median of three independent readings, 81 (4.5%) workers were judged to have opacities on chest radiographs (small opacities, profusion≥1/0, and/or large opacities). Age-adjusted relative risk of opacities increased significantly with cumulative exposure to crystalline silica. The concentration of respirable crystalline silica to which workers were exposed, which highly correlated with period of hire, was an important determinant of risk after accounting for cumulative exposure. For workers with an average exposure to≤0.50 mg/m³ of crystalline silica (or hired≥1950), the cumulative risk of opacities for a cumulative exposure to crystalline silica of 2.0 mg/m³-yr was approximately 1.1%; for an average exposure >0.50 mg/m³ (or hired<1950), the corresponding cumulative risk was 3.7%. The results of the study indicated an exposure-response relationship between cumulative exposure to crystalline silica and radiographic opacities (Hughes et al., 1998).

 

A community based random sample survey was conducted with 134 male subjects at least 40 years of age, living in a hard rock (i.e., molybdenum, lead, zinc, and gold) mining town in,. Of the 134 subjects, 100 were silica-exposed hard rock miners (including 32 silicosis cases) and 34 were community "controls" without occupational dust exposure. Ninety-seven percent of the dust-exposed subjects were 20 years since first exposure. The crystalline silica content of the total dust was estimated to be 12.3%. Exposure assessment was performed with information from occupational histories, gravimetric dust exposure data from 1974-1982, and a cumulative silica exposure index. Exposure estimates for the time before 1974 were based on job-specific gravimetric data collected after 1974. Exposures estimates were also done for mines for which no exposure data were available (17.1% of person-years of follow-up). Thirty-two per cent of the 100 dust-exposed subjects had silicosis. There was a relationship between prevalence of silicosis and average silica dust exposure. Among the 94 dust-exposed subjects with data on cumulative and average dust exposures, those subjects with average silica exposure <0.05 mg/m³ had 10% prevalence of silicosis; subjects with >0.05-0.10 mg/m³ had a prevalence of 22.5%; subjects with greater than 0.10 mg/m³ average silica exposure had a silicosis prevalence of 48.6%. (Kreiss and Zhen, 1996).

 

A cohort study was conducted with 2235 South African underground gold miners, being 45-54 years of age at the time of medical examination in 1968-1971. The subjects started working after 1938, worked ≥10 years, and were followed until 1991. Of the 2235 miners, 313 were followed to the time when radiological signs developed, 658 miners were followed up to death, and 1264 miners were followed to the year of the most recent radiograph. Mean respirable dust concentrations, after heat and acid treatment, in mg/m³ per shift were calculated for nine gold mining occupations. After heat and acid treatment, the respirable dust in South African gold mines was found to contain about 30% quartz. Cumulative dust exposure for the miners was calculated in mg/m³-yr by using data for mean mass respirable dust concentrations for the nine occupational categories, the average number of hours underground, and the number of dusty 8-h shifts. Of the 2235 subjects, 313 developed silicosis (rounded opacities with profusion of ILO category ≥1/1) during the follow-up period (1968-1971 to 1991). The onset of silicosis occurred after a mean of 27 years of net service, at a mean age of 56 years. For 178 miners (57%), the onset of silicosis occurred in average 7.4 years (standard deviation 5.5; range 0.1-25 years) after their employment at the mines, at 59 years of age (range 44-74 years). For the other 135 miners (43%), the onset occurred while they were still employed, at 51 years of age (range 39-61 years). These results showed that most of the cases occurred in workers who were no longer employed at the mine and who were at least 50 years old (Hnizdo and Sluis-Cremer, 1993).

 

A cohort study was conducted with 3330 male underground gold miners from. The subjects had been employed for at least 1 year between 1940 and 1965 and were followed through 1990. One hundred seventy cases of silicosis (128 cases identified on death certificates, 29 cases found during X-ray surveys of workers carried out in 1960 and 1976, and 13 cases identified on both X-ray and death certificate). Cases were defined as (1) an underlying or contributing cause of death of silicosis, silicotuberculosis, respiratory tuberculosis, or pneumoconiosis, and/or (2) ILO category ≥1/1 silicosis identified in the radiographic survey in 1976 or "small opacities" or "large opacities" identified in the radiographic survey in 1960. The subjects had been exposed to a median quartz level of 0.05 mg/m³ (0.15 mg/m³ for workers hired before 1930). The average length of follow up was 37 years, and the average length of employment underground was 9 years. Quartz exposure was estimated by converting dust particle counts to mg/m³, based on an estimate of 13% quartz content of total dust. A job-exposure matrix was created to estimate dust exposures for each job over time, then average dust exposures for the job categories were calculated using existing measurements for each year from 1937 to 1975. The estimated daily dust exposures (constant over each year) were weighted to account for daily time spent underground. Summation of the estimated daily dust levels over time provided an estimate of cumulative quartz exposure. The risk of silicosis was less than 1% for miners with a cumulative exposure less than 0.5 mg/m³-years. The risk increased to 68-84% for the highest cumulative exposure category (corresponding to 4 mg/m³-years) (Steenland & Brown, 1995).

 

Taken together, the epidemiologic studies described above showed that chronic silicosis may develop or progress even after occupational exposure to crystalline silica has been discontinued (Hnizdo and Sluis-Cremer 1993; Kreiss and Zhen 1996). Three of the studies have shown that the estimated risk of silicosis for a 45-year working lifetime is 47% to 90% for cumulative silica exposures at concentrations of 0.09- 0.1 mg/m³, and approximately 10% to 30% at concentrations of 0.05 mg/m³ (IPCS, 2000; NIOSH, 2002). Therefore, this concentration of respirable crystalline silica is regarded as the LOAEC for chronic silicosis in humans.