Dusty Trades: Inorganic Dust Exposure During Military Service May Be an Occupational Risk Factor for RA

Exposure to inorganic dust has been linked to increase the risk of developing rheumatoid arthritis (RA) and other autoimmune conditions. This association between dusty environments and RA has been documented in miners, as well as individuals with silica-exposed vocations, such as stone cutters. Additionally, research has revealed patients with RA have immunological citrullinated targets in their lungs and synovial tissue, which suggests dust may be associated with anti-cyclic citrullinated peptide (CCP) seropositive RA.¹

Military Service as a Dusty Trade

Many military activities, such as welding, abrasive blasting, grinding and polishing metals, vehicle and other maintenance work, earth moving and other construction, as well as explosives detonation, entail inorganic dust exposure. Up until now, however, researchers have not assessed whether exposure to inorganic dust during military service is a risk factor for RA and other autoimmune conditions.

Assessing such an association is challenging because military personnel, generally, tend to be healthier than the general population. This healthy-soldier effect appears to erode over time, which means veterans from the Afghanistan and Iraqi wars may now be vulnerable to exposure-associated diseases. If an association between dust exposure during military service and a risk for RA were to be documented, the findings could influence the healthcare of military personnel and veterans through primary prevention and targeted follow-up surveillance.

Results from a recent, large-scale study indicate dust exposure during military service represents an occupational and environmental risk for a future diagnosis of RA or other autoimmune condition. Dust exposure among military personnel was associated with a 10% increased risk of developing RA compared with military personnel who had minimal to no exposure to inorganic dust. David Ying, MD, a rheumatologist at the San Francisco Veterans Affairs (VA) Health Care Systems, and colleagues suggest their analysis may be relevant for prevention activities. Their findings were published in the July issue of ACR Open Rheumatology.¹

Study Details

The study comprised a large sample (N=438,086) of veterans of the Afghanistan and Iraqi wars. It focused on veterans of Operation Enduring Freedom, Operation Iraqi Freedom and Operation New Dawn who received healthcare through the VA system. The majority (88%) of the cohort was male, 63% were white, non-Hispanic, and 68% had smoked. The study was unique because it did not rely on participant recall, but instead employed a job exposure matrix (JEM) based on military occupation codes to assign risk linked to military duties. These military occupation codes are nine-character codes used in the service personnel system of the U.S. Army and Marine Corps to identify and rack and manage every job performed by military personnel. Almost half (44%) of the veterans were classified as having likely or somewhat likely exposure to dust.

The cohort included 1,139 cases of RA, with 40% of these positive for either rheumatoid factor (RF) or anti-CCP. Also, 467 cases were patients with systemic lupus erythematosus (SLE) and 180 cases were patients with other autoimmune diseases, such as systemic sclerosis. The investigators found that, in addition to the 10% increased risk of RA in individuals exposed to military dust compared with those who had minimal to no exposure to inorganic dust, other covariates included in the model were also statistically associated with an increased risk for RA. These covariates included age at first VA appointment, being female, a history of smoking and Hispanic ethnicity.

The investigators also documented that military dust exposure was associated with a 23% increased risk for systemic sclerosis, vasculitis or inflammatory myositis. In contrast, dust exposure was protective with regard to systemic lupus erythematosus. When the researchers analyzed the relationship between dust exposure and disease in women and compared that with the relationship in men, they found dust was statistically protective for eight years for women but was not statistically protective for men. The authors addressed this unexpected finding in their discussion, noting, “Only 12% of our cohort was female, and, moreover, work exposures even within JEM [job exposure matrix] categories likely differed systematically by sex. This complicates the interpretation of autoimmune diseases for which sex is a powerful risk factor and may account for the statistically significant protective effect of exposure for SLE among women [but not men].”

When the researchers disaggregated the patients according to serologic status, they found dust exposure was a statistically significant risk factor for seronegative RA, but not for seropositive RA. When they used models stratified by years of service, they found dust exposure in those with four to eight years of service was most strongly associated with a risk for RA and no statistical association among those with either fewer or more years of service.

An Occupational Laboratory

“Nobody has looked exactly this way at the risk of RA as a consequence of military operations writ large,” says principal investigator Paul D. Blanc, MD, MSPH, professor of medicine at the University of California, San Francisco.

Although he acknowledges the excess risk is modest, he points out that because it applies to many people, it’s noteworthy. “When you have a patient with a new diagnosis of rheumatologic disease, you should take an occupational history,” Dr. Blanc says. Such information may be useful in obtaining worker compensation from the military. He explains that although at the civilian level a 10% increased risk may not be justification for workers’ compensation, exposure to agent orange is associated with a similar magnitude of increased risk to multiple diseases and the military provides workers’ compensation for medical conditions associated with this exposure.^2,3

Dr. Blanc says the study underscores the value of looking at large groups of military personnel to gain insights into exposures that occur in many occupations. “We didn’t rely on people’s self-reported exposure,” he emphasizes, adding, “Recall is not an issue here.”

Dr. Blanc acknowledges the study used a heterogeneous categorization, which he explains is not an exact science, making the fact that the researchers were able to document the association “all the more remarkable.” The team also documented the expected association between smoking and RA, a finding Dr. Blanc describes as reassuring because it served as a type of positive control.

The study findings underscore the fact that the association between “dusty trades” and RA needs further investigation. Dr. Blanc called for further analysis of autoimmune diseases that are rarer and more difficult to study.

Lara C. Pullen, PhD, is a medical writer based in the Chicago area.

Reference

1. D Ying, G Schmajuk, L Trupin, et al. Inorganic dust exposure during military service as a predictor of rheumatoid arthritis and other autoimmune conditions. ACR Open Rheumatol. 2021 Jul. 3(7):466­–474.
2. Agent Orange exposure and VA disability compensation. U.S. Department of Veterans Affairs. 2021.
3. Institute of Medicine. Veterans and Agent Orange: Health Effects of Herbicides Used in Vietnam. National Academies Press. 1994 Feb 1.