Two studies published in international journals have reported a worrying link between air pollution levels and the incidence of type 2 diabetes in Chennai and Delhi. The study is notable not because the findings are new – they aren’t unprecedented – but because they have found that the link, which has been indicated in Western countries and more recently in China as well, also holds in urban India.
Indian cities have consistently dominated the tops of lists of places with the worst air for residents, with air quality frequently several times higher than the limits set by the World Health Organisation.
The new studies are part of the Centre for cArdiometabolic Risk Reduction in South Asia (CARRS) Surveillance Study. Here, researchers roped in 6,722 adults in Chennai and 5,342 in Delhi and tracked their health through questionnaires and blood samples, with which they checked for fasting plasma glucose (FPG) and glycosylated haemoglobin (HbA1c), at specific intervals from 2010 to 2016.
The researchers also developed air pollution and exposure models using, among other things, satellite data and emissions inventories.
Based on their findings, the researchers reported that 10 μg/m3 (micrograms per cubic metre air) difference “in annual average PM2.5” could be related to a 9-36% higher risk of developing type 2 diabetes. They have interpreted the long-term follow-up of study participants to mean that the link between type 2 diabetes and air pollution is “not due to intermittent episodes of high pollution levels” but “long-term exposure to ambient PM2.5”.
They also reported that for every 10 μg/m3 increase a month in PM2.5 levels, FPG increased by 0.21-0.58 mg/dL (milligrams per decilitre) and HbA1c by 0.012-0.024 in Delhi, and FPG increased −0.36-1.39 mg/dL and HbA1c 0.01-0.06 in Chennai. Over six months, a 10 μg/m3 change in PM2.5 levels resulted in a rough doubling of both ranges in Delhi, but couldn’t be associated with a statistically significant result in Chennai.
The paper published in BMJ Open Diabetes Research & Care also said that “hypertensive participants… were more susceptible to developing type 2 diabetes against long-term exposure to PM 2.5 in Chennai,” whereas “younger participants were more susceptible to developing [the disease] in Delhi”.
There is some epidemiological wisdom as to how ambient PM2.5 concentrations ‘outside’ the body can affect processes ‘inside’. For example, one 2016 study in mice reported that “short-term exposure to PM2.5 induces vascular insulin resistance and inflammation triggered by a mechanism involving pulmonary oxidative stress”.
A commentary accompanying the publication of this paper noted that “any other condition involving oxidative stress may increase the susceptibility to harm from PM 2.5”.
The other paper, published in the journal Hypertension, stated that the “data strongly support a temporal association between high levels of ambient air pollution, higher systolic blood pressure, and incident hypertension”.
India already has a large burden of noncommunicable diseases, which accounted for 64% of the national disease burden in 2021. And poor air’s links with respiratory and heart health have stoked concerns of a public health crisis, even as new research has revealed its effects on disparate parts of life.
For example, a model-based study published in The Lancet Planetary Health in 2021 estimated that increasing PM2.5 exposure by 10 μg/m3 could also increase the risk of pregnancy loss by 3% in India, Pakistan, and Bangladesh.