Novel Study Links Air Pollution to Increased Risk for RA Flares


In patients with rheumatoid arthritis, exposure to air pollution is associated with both elevated levels of C-reactive protein (CRP) and increased risk of arthritis flares, according to a novel longitudinal study presented at the annual European Congress of Rheumatology.

The data revealed “a striking association between air pollution and increased CRP levels and risk of an arthritis flare,” reported first author Giovanni Adami, MD, DSc, of the rheumatology unit at the University of Verona (Italy).

The excess risk of elevated CRP and flares began “at very low levels of exposure, even those below commonly used thresholds for risk to human health,” he added.

Study Details

Researchers collected data on 888 patients with RA from numerous patient visits in the context of more than 13,000 air pollution records. The CRP levels and RA flares were evaluated in the context of air pollution monitoring that is performed on a daily basis at several sites in the city of Verona where the study was conducted. Verona is an industrial city in northern Italy that has high but variable levels of air pollution based on factory activity and weather conditions.

Patients with RA who provided clinical data for this study were matched by their proximity to specific air pollution monitoring sites. By linking CRP levels and disease activity to air pollution levels over multiple follow-up visits, the design allowed the RA study participants “to serve as their own controls,” Adami explained.

At each patient visit during the study, CRP levels were measured and disease activity assessed. Patients were considered to have elevated CRP when levels were 5 mg/L or higher. The presence of an RA flare was defined by a 1.2-point increase or more in 28-joint Disease Activity Score using CRP (DAS28-CRP).

Both the CRP level and the presence or absence of a flare were evaluated in relationship to the patient’s specific local air pollution levels in the prior 60 days.

Increased levels of CRP, a surrogate for inflammatory activity, and increased disease activity, were both associated with elevated exposure to air pollutants prior to an office visit. These associations remained statistically significant when evaluated by specific air pollutants such as carbon monoxide (CO), nitrogen oxides (NO2, NO), small particulate matter (PM10; particles ≤ 10 mcm), and ozone (O3).

The relationship between increased exposure to air pollution contaminants and elevated CRP was supported by a dose effect. In the case of PM10, for example, the odds ratio of having elevated CRP was increased by only about 25% (OR, 1.25) when mean levels were 30 mcg/m3 or lower in the period prior to the office visit. This rose incrementally for higher mean levels of PM10, reaching 70% (OR, 1.70) for levels > 50 mcg/m3.

The researchers detected statistically significant differences in mean and area-under-the curve (AUC) values of most air pollutants in the 60 days prior to office visits when patients had a flare versus when disease activity was low. For example, the difference in mean and AUC levels in the period prior to a flare relative to a period with low disease activity was significant for CO (P = .001 for both) and NO and NO2 (P = .003 for both), and O3 (P = .002 and P = .001, respectively). For PM10, P values were .011 and .005, respectively.

“Remarkably, we found that the cumulative exposure to NO2 in the 60 days preceding a flare was approximately 500 mcg/m3 higher than the low disease activity visit, an exposure that equates to approximately 200 passively smoked cigarettes,” Adami reported.

Trying to Confirm Causality of Association

Adami’s study is not the first study to link air pollution to risk of RA. Several have suggested that air pollution is a risk factor for developing joint disease, but a recently published study conducted in Kuwait associated greater disease activity with NO2 and another air pollutant, sulfur dioxide (SO2), although not CO, PM10, or O3.

A coauthor of that study, which evaluated pollution in regard to disease activity on DAS score, Adeeba Al-Herz, MD, a rheumatology consultant at Al-Amiri Hospital, Kuwait City, said in an interview, “We proved the correlation between them but not the causality.”

However, she believes that this is an important area of inquiry.

“We are working now on another paper in which we studied a causal relationship between the two, meaning that we are evaluating whether SO2 and NO2 trigger RA activity,” Al-Herz said. That study is now complete, and the manuscript is being written.

The magnitude of the association in these two studies suggest that there might be a clinical message if causality can be confirmed, according to Adami. Although there are many reasons to seek to reduce and avoid air pollution, these data suggest risk of a proinflammatory state might be one of them.

Adami believes that the evidence of an adverse effect on patients with RA is strong.

“In order to reduce the burden of RA, public and environmental health policy makers should aim to diminish gaseous and particulate matter emissions to a larger extent than currently recommended,” he said.

In an interview after his presentation, Adami suggested that the risk of an inflammatory response and increases in arthritis flares from air pollution is not surprising. Previous studies have linked cigarette smoking to both.

“The mechanisms underlying the development of inflammation are very similar. Indeed, the toxic components contained in cigarette smoking are largely shared with diesel exhaust and fossil fuel combustion,” he said.

Although causality between air pollution and arthritis flares cannot be confirmed in these data, a basis for suspecting a causal relationship is supported by “plenty of in vitro and animal studies,” according to Adami.

On the basis of these studies, several mechanisms have been postulated.

“As an example, exposure to air pollution can promote the activation of the bronchus-associated lymphoid tissue (BALT), which can trigger the activation of the transcription factor nuclear factor-kappaB,” he said. This, in turn, can “lead to the secretion of proinflammatory cytokines, such as tumor necrosis factor–alpha and interleukin-1.”

Another theory is that posttranslational modification of proteins in the lung, a process called citrullination, “can lead to production of autoantibodies known to have a pathogenic role in RA,” he added.

Proving a causal relationship, however, is difficult.

“We certainly cannot conduct a randomized clinical trial on that and voluntarily expose some patients to pollution. Thus, we need to rely on observational data,” Adami said.

Of strategies being considered to generate evidence of a causal relationship between pollution and the exacerbation of RA, “we certainly will try to study those patients that move from a highly polluted area to a greener zone and vice versa,” he said. This will allow us “to explore what happens when the exposure to pollution changes dramatically in a short period of time.”

In the meantime, “given what is known to date, I would certainly advise my RA patients to avoid exposure to air pollution,” Adami said. He acknowledged there is no proof that this will help patients to reduce the risk of flares, but there are already many good reasons to minimize exposure to air pollution.

Adami and Al-Herz report no potential conflicts of interest.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.





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