Exposure to moderate to high levels of silica increases the risk of sarcoidosis and seropositive rheumatoid arthritis (RA), a new study of Swedish iron foundry workers shows.
The study titled, “Risk of sarcoidosis and seropositive rheumatoid arthritis from occupational silica exposure in Swedish iron foundries: a retrospective cohort study,” was published in the journal BMJ Open.
The research was led by Per Vihlborg, from the Örebro University’s Faculty of Medicine and Health, in Sweden.
Although sarcoidosis has no known cause, it is believed to be induced by environmental factors in individuals with a genetic predisposition. Data from studies in animals and humans support the idea that silica or silica dust may be one of those factors. However, evidence associating silica with sarcoidosis is still scarce.
Exposure to silica has been associated with a number of diseases, such as lung cancer, chronic obstructive pulmonary disease, pulmonary tuberculosis, and RA. Silica causes the potentially fatal lung disease silicosis, which is now rare in Sweden due to legislation minimizing exposure and requiring mandatory health checks in exposed individuals.
Silica has also been suggested to play an important role in inflammatory diseases. Silica combined with smoking has been associated with an increased risk of developing seropositive RA. Because both sarcoidosis and RA are inflammatory disorders, scientists hypothesized that the environmental factors that cause them may be similar.
The team analyzed the impact of silica exposure on the prevalence of sarcoidosis and RA in a cohort of exposed male workers from 10 Swedish iron foundries. Their results were compared with the general Swedish population.
The final cohort included 2,187 individuals who had been employed for at least one year before 2005. Most of the workers started employment in the 1970s or ’80s, but some had started in the ’30s. Nearly 40% had worked in the foundries for over 10 years.
In total, only seven cases of sarcoidosis were reported, distributed across the foundries. The results revealed that moderate to high levels of airborne silica exposure increased the risks of sarcoidosis and seropositive RA. This dose-response relationship parallels findings from other studies.
Of note, the scientists observed that the mean levels of airborne silica dust in the foundries decreased significantly between the 1970s and 2000s. As the prevalence of smoking has also decreased over time, “the proportion of smokers (or ex-smokers) may be highest in the highly exposed group, which could affect the prevalence of RA,” the authors wrote.
The strengths of the study include its long-running data set, as the first measurements began in 1968, and the fact that diagnoses were based on the national non-primary outpatient visits registry, which guarantees greater accuracy compared to questionnaires.
Conversely, the low number of sarcoidosis cases is a weakness.
Nonetheless, the team concluded that individuals with high exposure to silica dust have an increased risk for sarcoidosis and seropositive RA, compared with non-exposed and less-exposed groups.
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