In a study of the entire working population of Denmark, exposure to respirable crystalline silica — common in jobs in the construction industry — was linked to an increased risk of developing autoimmune rheumatic disorders, including scleroderma and rheumatoid arthritis.
The dose-dependent nature of the association, which indicates a causal relationship between breathing in crystalline silica and developing these chronic diseases, warrants an assessment of current exposure limits on the job, according to researchers.
Crystalline silica is a mineral found in soil, sand, and stone that is used in the production of concrete, bricks, glass, and other industrial materials. Respirable crystalline silica are particles small enough to be inhaled, which are produced when materials containing the mineral are processed, crushed or moved. Exposure can lead to silicosis — marked by lung inflammation and scarring — and lung cancer.
Respirable crystalline silica also has been linked to autoimmune rheumatic diseases, specifically in workers who are frequently exposed to the particles. Research done in Sweden in 2015 found that crystalline silica exposure may increase the risk of rheumatoid arthritis in men and scleroderma in men and women.
Now, researchers at Aarhus University Hospital, in Denmark, and their collaborators investigated the relationship between work-related exposure to crystalline silica and the development of scleroderma, rheumatoid arthritis, systemic lupus erythematosus, and small vessel vasculitis.
The team used nationwide data from the total Danish workforce — a group of 3 million men and women. Workers who started jobs between 1979 and 2015 were assigned an estimated annual exposure level.
Of the 1,541,505 male workers included in the study, 4,673 had one or more autoimmune rheumatic diseases. These included 3,490 cases of rheumatoid arthritis, 252 cases of scleroderma, 255 of systemic lupus erythematosus, and 749 of small vessel vasculitis. Among the 1,470,769 female workers, there were 12,268 cases of these diseases — including 9,190 of rheumatoid arthritis, 746 scleroderma, 1,821 of systemic lupus erythematosus, and 869 cases of small vessel vasculitis.
A total of 17% of men and 3% of women had at some point experienced job-related exposure to respirable crystalline silica. Men had a higher median cumulative exposure level than women, of 60 micrograms per cubic meter (mcg/m3-years) compared with 33 mcg/m3-years.
High exposure levels were associated with older age and smoking. Men had an elevated risk of being diagnosed with autoimmune rheumatic diseases in the time period from 2005 to 2015 than from 1979 to 1984.
In addition, men with respirable crystalline silica exposure were at a higher risk of developing one of the four rheumatic diseases. That increased risk was based on exposure intensity and duration of exposure, the results showed. In addition, men with such exposure had a greater risk of developing each of the studied diseases individually — with scleroderma and rheumatoid arthritis having the most pronounced difference.
Increased cumulative exposure corresponded with increased risk for each disease, particularly scleroderma and rheumatoid arthritis. A latency effect was observed such that cumulative exposure from 20 years before had a greater effect on risk than more recent exposure. Similar but less pronounced exposure-risk patterns were observed in women.
The researchers noted that many of the individuals who had developed autoimmune rheumatic diseases had been exposed to levels of crystalline silica that were far below the workplace exposure limits that are still in place today.
“We observed increased risks of several of the studied autoimmune rheumatic diseases at mean exposure intensity levels well below the current European occupational exposure limit … indicating that this limit provides insufficient protection of workers exposed to crystalline silica,” the investigators wrote.
Among the study’s limitations, they said, are a relatively small number of exposure cases, particularly in female workers, and the potential for false positives in patient records.