Ashes (residues), rice husk

Epidemiological data

Administrative data

Endpoint:

epidemiological data

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable, well documented publication which meets basic scientific principles

Data source

Reference

Reference Type:

publication

Title:

Radiographic Evidence of Silicosis Risk in the Diatomaceous Earth Industry

Author:

Hughes, J.M. et al.

Year:

1998

Bibliographic source:

Am J Respir Crit Care Med 158:807–814

Materials and methods

Study type:

cohort study (retrospective)

Endpoint addressed:

repeated dose toxicity: inhalation

Principles of method if other than guideline:

Epidemiologic Exposure-Response Model of Silicosis

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

Cristobalite

EC Number:

238-455-4

EC Name:

Cristobalite

Cas Number:

14464-46-1

IUPAC Name:

14464-46-1

Details on test material:

- Name of test material (as cited in study report): crystalline silica (mainly cristobalite)
- Analytical purity: no data
- Other: The material studied was crystalline silica from one diatomaceous earth mining and processing facility in Lompoc, California; operations at the facility included extraction of the mineral from open-pit mines, crushing of the ore, and heating of the crushed ore at high temperatures (calcination). When extracted, the mineral exists primarily as amorphous (noncrystalline) silica; after heating, the product typically consists of 10 to 60% crystalline silica, primarily in the form of cristobalite.

Method

Type of population:

occupational

Ethical approval:

not specified

Details on study design:

HYPOTHESIS TESTED (if cohort or case control study): The study was undertaken to assess radiographic evidence of silicosis among workers in the diatomaceous earth industry, who are exposed primarily to the cristobalite form of crystalline silica. The main goals of the study were to determine whether there is an exposure–response relationship with the risk of opacities on chest X-ray, and to estimate risk for workers exposed at levels similar to current and recent levels prevailing in the workplace.


METHOD OF DATA COLLECTION
- Type: Record review / Work history
- Details: chest X-ray films from a health-surveillance program


STUDY PERIOD: 1942 - 1987


STUDY POPULATION
- Total population (Total no. of persons in cohort from which the subjects were drawn): 2342
- Selection criteria: Of the 2,342 workers, 1,983 (84.7%) had X-ray films available. Availability of films varied by decade of hire, and was 77%, 79%,
86%, and 91% for workers hired < 1940, during the 1940s, during the 1950s, and 1960 or later, respectively. The quality of the films was generally
good; only 0.3% (five of 1,983) were judged by median reading to be unreadable, and 13.5% (268 of 1,983) were judged to be of poor quality but readable.
Of the 1,978 workers with a readable film, 169 had only one film made at the time of hire (at or before hire or up to 1 mo after hire). These films were included in the readings and provided information about background rates of X-ray changes, but not information about the development of opacities after hire. Therefore, exposure–response analyses included only the 1,809 men with a film taken more than 1 mo after hire.
- Total number of subjects participating in study: 1809
- Sex/age/race: male/age categories 20 to 34, 35 to 44, and >= 45 yr/Caucasian
- Smoker/nonsmoker: Smoking information, collected systematically beginning in the 1960s, was available for approximately half the cohort members. This information, when available, only permitted distinctions of ever and never cigarette smokers.
- Total number of subjects at end of study: 1809


HEALTH EFFECTS STUDIED
- Disease(s): Silicosis

Exposure assessment:

estimated

Details on exposure:

TYPE OF EXPOSURE: inhalation


TYPE OF EXPOSURE MEASUREMENT: Area air sampling. Estimates of exposure to respirable dust for individual workers were derived through the use of quantitative air-monitoring data, available since 1948, and company work histories.


EXPOSURE LEVELS: Mean respirable dust concentrations by year for the study cohort have been reported previously (Seixas et al., 1997. Ann. Occup. Hyg. 41:591–6044). Mean crystalline silica concentrations exhibited the same patterns over time as did respirable dust levels, with four distinct time periods. For 1932 to 1943, 1944 to 1953, 1954 to 1973, and 1974 to 1994, the mean crystalline silica concentrations for workers with a chest radiograph were approximately 0.90 mg/m3, 0.40 mg/m3, 0.15 mg/m3, and 0.10 mg/m3, respectively.


EXPOSURE PERIOD: 1942 - 1987

Results and discussion

Results:

On the basis of 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 (highly correlated with period of hire) was an important determinant of risk after accounting for cumulative exposure. For workers with an average exposure to crystalline silica of ≤ 0.50 mg/m³ (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%.

Strengths and weaknesses:

82 workers had radiographs taken after retirement—development of opacities was not recorded for other workers after they left employment.
Quantitative air-monitoring data were available after 1948; respirable dust concentrations before 1948 were estimated. Cumulative risk estimates for radiographic opacities were lower for workers who were hired after 1950 and who had lower average exposures to crystalline silica dust (mainly cristobalite). Estimated percentages of respirable crystalline silica reported by Checkoway et al. (1997 Am. J. Epidemiol. 145:680–688) in mortality study of same cohort: 10% for calcined diatomaceous earth, and 20% for flux-calcined diatomaceous earth.

Any other information on results incl. tables

For details on results refer to attached background material.

Applicant's summary and conclusion

Conclusions:

These findings of this study indicate an exposure–response relationship between cumulative exposure to crystalline silica and radiographic opacities; moreover, the relationship was substantially steeper among workers exposed at the highest average concentrations of crystalline silica.