Cardiometabolic Risk Factors May Boost Effects of Air Pollution, Chinese Study Says

Long-term exposure to high levels of air pollution seems to spur the development of cardiometabolic risk factors as well as cardiovascular disease, new research suggests. Investigators say the findings could be a step toward understanding how to optimize preventive interventions for those most vulnerable to poor air quality.

“Our results indicate that people with cardiometabolic risk factors such as hyperbetalipoproteinemia, hypertension, and type II diabetes, might be more susceptible to the cardiovascular effects of air pollution compared to people without these risk factors,” Guang-Hui Dong, MD, PhD (Sun Yat-sen University, Guangzhou, China), said in an email.

The study, published online March 11, 2019, in JAMA Network Open, was conducted in 15,477 Chinese adults with exposure to a range of levels and individual components of air pollution.

Although the exact mechanisms are unclear, Dong and colleagues say regular exposure to air pollution “could elicit systematic inflammation and oxidative stress, trigger autonomic nervous system imbalance, and cause insulin resistance and abnormal epigenetic changes. These mechanisms are involved in elevation of blood pressure, glucose levels, and body mass index and in lipid metabolism disturbances, which are consequently capable of instigating CVD events,” they write.

However, the presence of cardiometabolic risk factors failed to fully explain all of the increased risk for CV disease in the study, making it unclear whether the pollutants themselves could have some direct effects on the development of disease or if they may have some cumulative effect when added to other unknown factors.

Commenting on the study for TCTMD, C. Arden Pope III, PhD (Brigham Young University, Provo, UT), said while data from China are extremely important given the high levels of daily exposures there to air pollution, making sense of the associations is still a work in progress.

“It's extremely interesting and it does provide additional evidence that long term exposure to ambient air pollution contributes to both cardiovascular and cardiometabolic disease,” Pope said. “The downside is it’s difficult to separate out the interactions between the cardiometabolic risk factors, air pollution, and cardiovascular disease.”

Lipids May Be Especially Sensitive to Pollutants

For the study, Dong and colleagues led by Bo-Yi Yang, PhD (Sun Yat-sen University), collected self-reported CVD and risk factor information. They also estimated daily concentrations of aerodynamic particulate matter in the 33 communities where study participants lived based on a variety of data sources. All of the 15,477 subjects had lived in one of the communities for at least 5 years.

The prevalence of CV disease was 4.8%, while the prevalence of cardiometabolic risk factors ranged from 8.6% for hyperbetalipoproteinemia to 40.5% for overweight or obesity.

All air pollutants studied were significantly associated with increased odds of CV disease, but the researchers noted that some relationships among specific pollutants that were stronger than others. Hyperbetalipoproteinemia, in particular, was associated with exposure to daily concentrations of particles with aerodynamic diameter of ≤ 1.0 μm (PM1.0), as well as nitrogen dioxide (NO2). A 10-μg/m3 increase in PM1.0 was associated with a 36% increased odds of having hyperbetalipoproteinemia. The researchers say the findings suggest that lipid metabolism may be especially sensitive to long-term exposure to air pollutants.

To TCTMD, Dong said that in large cities, PM1.0 and NO2 are main air pollutants that merit attention.

“In particular, PM1.0 should be targeted, as it remains not regularly monitored so far, and a corresponding air-quality standard for PM1.0 is not developed by any country or organization,” he noted. “Nevertheless, the evidence on PM1.0 and health remain scarce, and future well-designed studies should be performed to confirm and extend our findings.”

Yang et al also found that having any existing cardiometabolic risk factor increased the odds of CV disease compared with not having a risk factor. Additionally, sex-stratified analyses suggested that most of the associations of air pollutants with cardiometabolic risk factors were stronger in men than in women. A 10-μg/m3 increase in PM1.0 was associated with a 15% higher odds of hypertension in men compared with a 4% higher odds in women, for example. For hyperbetalipoproteinemia, the association with air pollutants was strongest in men, whereas hypertriglyceridemia showed the strongest associations with most air pollutants in women. Age-stratified analyses showed stronger associations of air pollution with type II diabetes and being overweight or obese in younger participants compared with older participants.

According to the study authors, less than 20% of the associations of air pollutants with CV disease could be attributed to cardiometabolic risk factors, with the exception of body mass index and LDL cholesterol levels. In subgroup analyses, participants who had exposure to air pollutants at or above mean levels and presence of a cardiometabolic condition had the highest odds of CVD relative to the other groups.

Air Pollution Puzzle Not Yet Solved

To TCTMD, Dong said that beyond the need for longitudinal and/or intervention studies, more environmental factors including indoor exposures and traffic noise should be considered and controlled for in future studies to comprehensively estimate the individual effects of air pollution on CV health. He and his colleagues say the current findings “may have important public health implications in identifying sensitive populations who are more susceptible to hazardous effects of air pollution.”

But before that can be done, Pope said much more work is needed to further understand how air pollution fits into the puzzle.

"We know that air pollution clearly is associated with cardiovascular disease and these cardiometabolic risk factors are clearly associated with cardiovascular disease, but the interactions between air pollution and these risk factors and cardiovascular disease are still pretty complicated,” he said.

Pope noted that when it was first suggested that air pollution contributed to cardiovascular disease many people did not understand how it was possible. “But now we’re getting more and more evidence that air pollution contributes to a broad range of disease,” he observed. “It would be easier if we could say that people with cardiometabolic disease are more susceptible to air pollution, but it’s not quite that simple.”