Socioeconomic Status and Air Pollution: A Double Blow for Diabetes

Abstract

Air pollution has emerged as a risk factor for type 2 diabetes (T2D) in numerous epidemiological studies.^1,2 However, evidence for the role that social factors may play in the association between air pollution and T2D is scant and mixed.^2,3 A new cohort study published in Environmental Health Perspectives examines how socioeconomic characteristics, coexisting diseases, and pollutant coexposures may combine to affect the incidence of diabetes in the context of environmental and health inequality.⁴

Somewhat surprisingly, the researchers found that air pollution was associated with higher risk of T2D among people with medium education and income compared with people with less education and income. Image: © iStock.com/danefromspain.

T2D is a global health problem. An estimated 537 million adults—roughly 10% of the world’s population—were affected in 2021.⁵ That number is projected to rise to 783 million by 2045, with approximately three-quarters of cases occurring in adults in low- and middle-income countries.⁵ Although T2D can be treated (and even reversed),⁶ common complications can lead to heart disease, nerve damage, kidney failure, and stroke.⁷ Air pollution, which has well-established detrimental effects on cardiovascular, respiratory, and brain health,^8,9 has recently been associated with increased risk of T2D, with growing evidence pointing to fine particulate matter (${\mathrm{PM}}_{2.5}$) as one of the culprits.¹⁰ A 2018 report¹¹ estimated that approximately 3.2 million diabetes cases can be attributed to prolonged exposure to ${\mathrm{PM}}_{2.5}$ globally. The study further found that diabetes risk remains significant even at exposure levels deemed safe by the U.S. Environmental Protection Agency.¹²

Within a given region, exposure to air pollution can vary greatly by socioeconomic status, with poorer communities typically exposed to higher levels.¹³ Similarly, diabetes disproportionately affects certain populations, particularly those with low incomes.¹⁴ To examine this intersection, the authors of the new paper conducted a registry-based prospective study of all persons 35 years of age or older living in Denmark from 2005 to 2017. “Our study is unique in that we had access to a comprehensive, high-quality database whereby all of Denmark was our cohort,” says senior author Mette Sørensen. Using nationwide administrative¹⁵ and health registries,^16–18 Sørensen and colleagues collected address histories and sociodemographic data for all Danish residents, as well as information on incidence and pharmaceutical treatment of diabetes. They used an advanced modeling system with very high spatial resolution to assess air pollution levels, estimate residential exposure to well-characterized components—PM_2., ultrafine particles, elemental carbon, and nitrogen dioxide. Relative and absolute risk hazard models were used to calculate the impact on both air pollution exposure and T2D incidence of sociodemographic variables such as education, income, occupational and cohabitation status, and comorbidities.

All the assessed air pollutants were strongly associated with T2D, especially among people 50–80 years of age. In general, the likelihood of developing T2D was higher in men, people with low socioeconomic status, and people with coexisting diseases. “Our most clear finding, and one we expected, was a strong association of having a comorbidity: If you have another disease, you might be at higher risk of diabetes because your system is already challenged,” explains Sørensen. Men experienced much higher risk than women regardless of the pollutant marker examined. Whether this is due to differences in incidence of comorbidities (such as cardiovascular disease), physiological characteristics (hormones, lung size), or some combination of those remains unclear.

Surprisingly, medium education and income levels were both associated with higher risk than low education and income levels. This finding might be explained by a measurement error in which a person who has T2D was not identified or diagnosed, called outcome misclassification.¹⁹ “A caveat of this study is that T2D might not be diagnosed in all affected cases, and there might be a differentiation between groups who seek health care,” explains Ebba Malmqvist, an associate professor at Lund University, who was not involved in the study. Multiple studies have shown that diabetes is more likely to go undetected in people of lower socioeconomic or educational status—compared with wealthier or higher-educated individuals—even when they access health care.^20–22 Overall, it is estimated that 24% of all diabetes cases go undiagnosed in Denmark.²³

“The effect of air pollution was very clear,” says Francesco Forastiere, a visiting professor at the School of Public Health at the Imperial College London and consultant for the World Health Organization Air Pollution Program, who also was not involved in the study. “These findings underline the need to better explore the effect of air pollution on disadvantaged populations and more sensitive subgroups.”

Malmqvist agrees: “Although current studies tend to focus on the general population, it is important that we also identify specific vulnerable groups for the exposure.” Future studies should seek to clarify the adverse health effects of air pollution among groups in which the air pollution–T2D link is stronger, Sørensen suggests, to help environmental protection officials and health care systems find ways to reduce air pollution that also mitigate the increasing incidence of diabetes.

Biography

Florencia Pascual, PhD, is a Durham, NC–based freelance science writer.