Air Pollution: A New Cause of Type 2 Diabetes?

ENDOCRINE DISRUPTOR

Particular interest has been directed to the effects of air pollutants as “endocrine disruptors” leading to type 2 diabetes (T2D). Indeed, a fifth of the burden of T2D worldwide has been attributed to air pollution; 13.4% is derived from ambient PM2.5 and 6.5% from household air pollution. ¹ Studies from the United States and Europe reveal that there is a definite link between incident T2D and ambient PM2.5. A systematic review and meta-analysis of 13 studies from these countries showed that the risk of T2D rose by 8–10% per 10 µg/m increase in exposure and that the association was stronger in females. ² A study from Taiwan showed that compared to the lowest quartile, there was a 1.28 times higher risk of incident diabetes in the second quartile of exposure. ³ However, conclusions drawn from these studies cannot be directly extrapolated to developing countries such as India, as ambient pollutant levels are, in general, much lower in high-income countries.

There have been a few attempts at assessing the risk of diabetes incidence associated with air pollution in developing nations, where the air quality tends to be much poorer. A study from China showed a 15% increased hazard of incident diabetes for every 10 µg/m increase in PM2.5. ⁴ Another study from China reported an increase in fasting plasma glucose (FPG) and glycated hemoglobin (HbA1c) by 0.025 and 0.011 mmol/L, respectively, for every 10 µg/m ³ increase in PM10, and an increase by 0.061 and 0.016 mmol/L, respectively, for every 10 µg/m increase in PM2.5. An analysis of data from nearly 400,000 individuals showed that exposure to higher levels of PM2.5 was associated not only with increased risk of progression from normoglycemia to diabetes but also with mortality risk from baseline, T2D, and its complications. ⁵

INDIAN EVIDENCE

A recent study from India assessed the association of PM2.5 exposure with glycemic markers and incidence of T2D in two large cities, viz , Delhi and Chennai. In this study, Mandal et al., in the Centre for cArdiometabolic Risk Reduction in South-Asia surveillance study, followed up 12,064 adults residing in these two cities over a period of 7 years. Daily average ambient PM2.5 concentrations were obtained based on a hybrid satellite-based exposure model, as well as ground monitoring-based assessments of daily average PM2.5. ⁶ Individuals with normoglycemia at baseline were tested for development of dysglycemia using FPG and HbA1c measurements at the follow-up visits. A 10 µg/m ³ increase in monthly average exposure to PM2.5 was associated with a 0.4 mg/dL increase in FPG and a 0.021 unit increase in HbA1c. An increase in average annual PM2.5 exposure by 10 µg/m ³ was associated with a 22% increased risk of incident T2D.

POSSIBLE MECHANISMS

What could be the biological mechanisms underlying the increased risk of T2D with exposure to air pollution? Air pollution, especially exposure to PM2.5 and nitrogen dioxide, has been shown to adversely affect pancreatic beta cell function as well as insulin sensitivity. ^{7 , 8} Other biological mechanisms underlying this association include immune activation, endoplasmic reticulum stress, central nervous system inflammation, and oxidative stress. ^{9 - 11} Figure 1 summarizes the putative mechanisms between air pollution and T2D.

Fig. 1:

With the growing socioeconomic development worldwide, the detrimental health effects of air pollutants are receiving increased attention. T2D and air pollution are both critical public health concerns worldwide. Especially in India, the burgeoning epidemic of T2D threatens to derail healthcare systems across the country. Thus far, the higher prevalence of diabetes in urban parts of India has been attributed to unhealthy diets, sedentary lifestyles, and increased stress levels. The study by Mandal et al. suggests that air pollution may be an additional factor underlying the increased risk for diabetes in urban Indians. ⁶ Furthermore, this study shifts the focus of modifying environmental triggers for T2D away from individual choices regarding diet and physical activity towards societal and governmental efforts to clean up the air that we breathe.

So what can we do to prevent diabetes knowing the link between air pollution and diabetes?

Firstly, we should try to reduce pollution at the source. We know the major sources of air pollution are either the burning of stubble by farmers, smoke from vehicles, industrial pollution, use of firewood or charcoal in ill-ventilated kitchens, or indiscriminate pollution during festivals like Diwali due to fireworks, etc. All these are potentially modifiable by legislation and education by governmental and nongovernmental agencies. Individuals living in polluted areas, especially those with a high risk of developing T2D (on account of family history/obesity, etc.), should be encouraged to wear good-quality masks as long as the air quality remains poor. These steps are likely to be as easy to implement and as cost-effective as efforts to modify diet and physical activity, and they have the additional benefit of reducing the multisystem morbidity and mortality attributable to air pollution.

These pieces of evidence offer guidance for developing and implementing population- and region-specific policies aimed at lowering ambient air pollution in order to combat the high prevalence of diabetes in low- and middle-income countries like India.