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Based on a retrospective cohort study with 2342 workers of a diatomite mine and processing plant Checkoway et al  and coworkers found for lung cancer, morbidity from silicosis and for death from lung diseases other than cancer a clear correlation with exposure to calcinated diatomaceous earth. For all three adverse effects the excess lifetime risk is far above the acceptance level of 1 in 1000 if a exposure at the recommended exposure limit (REL) of 0.05 mg/m³ (NIOSH) is assumed for a work time of 45 years.

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EXPOSURE RELATED OBSERVATIONS IN HUMANS

 

The following studies have been analyzed and are summarized here:

7.10.1 Health surveillance data

Okawa 1977 / supporting / increased mortality among diatomite workers

Beskow 1978 / supporting / silicosis in diatomaceous workers

Jorna 1994 / supporting / no pneumononiosis from raw diatomaceous earth

Cooper 1984 / supporting / radiographic examination of diatomite wrkers

Ebina 1952 / supporting / diatomite plant health surveillance

7.10.2 Epidemiological data

Checkovay 1997 / key / retrospective cohort study on diatomaceous earth workers

Checkovay 1993 / supporting / retrospective cohort study on diatomaceous earth workers

Seixas 1997 / supporting data / Quantification of historical dust exposures in the diatomaceous earth industry

Harber 1998 / supporting study / radiographic and spirometric analysis of exposed workers

Rice 2001 / supporting / quantitative risk assessment of lung cancer after crystalline silica exposure

Park 2002 / supporting / quantitative risk assessment of lung disease other than lung cancer after crystalline silica exposure

Rafnsson Journal TBD-Date TSD workers

Rafnsson 1997/ supporting / cancer incidence in icelandic cohort of diatomaceous earth exposed workers

Motley 1955 / supporting / pulmonary function study on workers

Motley 1950 / supporting / diatomite worker survey

7.10.3 Direct observations: clinical cases, poisoning incidents and other

Dutra 1955 / supporting / clinical case report

Vigliani 1948 / supporting / clinical case reports + crystallographic analysis of calcinated keiselgur

Smith 1953 / supporting / description of general features of diatomaceous earth pneumoconiosis

Please note that several publications are summarized in the entries of Checkoway 1993 and 1997

 

Adverse effects by exposure to dusts of calcinated diatomaceous earth

The occupational exposure to diatomaceous earth has already been correlated to respiratory diseases in the 1920’s and 1930’s of the recent century. Legge 1932 reported slight to moderately advanced pneumoconiosis for most examined workers after 5 or more years of employment at a diatomaceous earth mine and processing plant in Lopoc. Exposure levels of that time are not reported but are expected to be in the range of 1-7 mg/m³ respirable diatomaceous earth particles (Seixas 1997).

Vigliani (1948) reported extremely high dust concentrations of 400 – 500 part per cubic centimeter of total dust particles for two diatomite processing plants. 13 out of 20 workers had developed silicosis after an average exposure for 5 to 10 years to dusts of mainly calcinated diatomaceous earth. In this study it is also pointed out that adverse effects are cause most probably by cristobalite formed from amorphous silica through calcination.

This is an accepted fact today and exposures are described as mg/m³ of respirable crystalline silica. In the case of calcinated (heating at ca. 1000°C) diatomaceous earth, the crystalline silica is predominantly cristobalite. Amounts of cristobalite range from 10 to 100 % depending on the temperature, duration and addition of soda ash (flux calcination). Amounts of quartz and/or tridymite are usually minor and also depend on the source of the diatomite and the calcination conditions.

Vigliani describes special signs of diatomaceous earth induced pneumoconiosis as general wasting, shortness of breath, cough and diffuse, reticular (early stages) or conglomerated (later stages) fibrosis of the lung accompanied by compensatory emphysema, enlarged root and mediastinal lymphnodes and in late stages spontaneous pneumothorax. In severe cases as described by Vigliani also lung function parameters are adversely affected. Comparable adverse effects have been noted by Smith 1963, Dutra 1965 and Beskow 1978

The correlation of silicosis and adverse effects on lung parameters is unclear. While the results from Ebina 1952 point into this direction, neither Motley 1956 and 1960 nor Harber 1998 could show a significant correlation between these parameters.

Jorna 1993 showed that exposure to raw diatomaceous earth does not lead to silicosis, but that generally exposure to dust can lead to reduction of lung parameters.

 

(Quantitative) Risk Assessment of lung cancer and other lung disease after crystalline silica exposure

Several epidemiological studies have been conducted to assess the correlation between occupational exposure to calcinated diatomaceous earth and development of lung disease. Ebina 1952 describes 12 cases of silicosis in 106 workers, but the exposure is unclear. Rafnsson determined the cumulative dust exposure for 118 workers of an Icelandic diatomite mine and plant. Exposure concentrations were sometimes 0.5 to 0.75 mg/m³. No case of silicosis was found, and only one lung parameter showed positive correlation to both exposure indices (total dust index and cristobalite index). Rafnsson 1997 describes a retrospective cohort study where 1346 workers of an Icelandic diatomite mine and processing plant where analyzed. Due to a lack of measured exposure data, work time was the only measure for exposure. This was correlated to incidences of cancer. No positive correlation was found between incidences of lung cancer and work time in this population. Statistically significant correlation between work time and number cancer incidences was only found for skin and brain cancer, the latter of which cannot be explained by a reasonable mode of action. The positive correlation for skin cancer is based on very few cases and therefore arguable.

All above studies described above have deficiencies either in reporting of methodological detail, in exposure assessment or in statistical power and are therefore of limited relevance.

Checkoway et al (Checkoway 1993 and 1997) have published a number of successive epidemiological studies which analysed quantitatively the risk of lung disease after crystalline silica exposure based on a dataset of annual health surveillance data from 2342 workers and exposure data from 1948 – 1988 from a diatomite mine and processing plant in Lompoc California (Studies of Okawa 1977 and Cooper 1984 are not discussed here as the data used in this study is covered in more detail and a broader context in the studies by Checkoway et al) to allow finally the calculation of life time excess risks for cancer (Rice 2001, for morbidity from silicosis and for mortality from lung diseases other than cancer for different exposure limits.

Statistical power of the studies is high and exposure data was assessed for each individual and is therefore relatively reliable even though for earlier decades 1940's to 1950's the exposure could only be estimated based on a few measurements. Data on smoking habits and asbestos exposure in the early decades are weak. Nevertheless a big effort has been made to assess the influence of both potential confounders in Checkoway 1997 and in the following studies, showing that both have no significant influence on the analyzed adverse effects.

Rice 2001 reports a life time excess risk for lung cancer of 19 for every 1000 workers (95 % CI 5 - 46) assuming a 45 year period of occupational exposure at the momentary recommended exposure level (REL) of 0.05 mg/m³.

Park 2002 reports a life time excess risk of 54 for every 1000 workers for death from LDOC assuming a 45 year period of occupational exposure at the momentary recommended REL.

In addition a life time excess risk of 75 for every 1000 workers for morbidity of silicosis was determined for this exposure level and exposure time.

Summarizing the result of Checkoway and coworkers for the lung cancer, morbidity from silicosis and for death from lung diseases other than cancer a clear correlation with exposure to calcinated diatomaceous earth was seen and for all three the excess lifetime risk is far above the acceptance level of 1 in 1000 if an exposure at the recommended exposure limit (REL) of 0.05 mg/m³ (NIOSH) is assumed for a work time of 45 years.

Based on these results the authors recommended a lowering of the current REL.

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