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Air pollution contributing to 3.2 million cases of diabetes worldwide?

A study published in The Lancet Planetary Health suggests that India’s high levels of air pollution have a hand to play in the country’s emerging diabetes. The research attributes 3.2 million cases of diabetes a year to air pollution worldwide. This is a bold claim that could establish a hitherto little-known risk factor as a key explanation – at least in part – for a rising tide of diabetes cases, both in India and Air pollution contributing to 3.2 million cases of diabetes worldwide?globally.

Smog pictured over Connaught Place in New Delhi. Image credit: Ville Miettinen [CC BY 2.0 (, via Wikimedia Commons
Perhaps the most concerning finding of the study is that even air pollution at levels considered safe can aggravate the diabetes burden.

This is bad news for India – one of the most polluted countries in the world – where a combination of lifestyle factors and genetic predisposition already contribute to increasing rates of diabetes among the population. If pollution is the aggravating risk factor the study claims, this heightens the danger for Indians even further.

The idea that pollution and diabetes could be linked is not a new concept. Studies from the last decade have indicated that long-term exposure to pollutants — in particular to PM2.5 (particulate matter with a diameter of less than 2.5 micrometers) — can increase the risk of diabetes.

The new research is the first large-scale study to investigate the claim.  It examines a cohort of US veterans with no previous history of diabetes drawn from numerous databases. The records of these databases were cross checked against the US Environmental Protection Agency’s (EPA) Community Multiscale Air Quality Modeling System of PM2.5.

Individuals involved within the study were assessed for diabetes onset for a median duration of 8.5 years. EPA records of PM2.5 levels were taken from their county of residence at the beginning of the study.


Findings of the study


The study used the Global Burden of Disease study data and methodologies to estimate the attributable burden of disease (ABD) of diabetes to air pollution. It found that an increase in pollution concentration of 10μg/m3 coincided with an increased risk of type 2 diabetes.

This increase was found to rise sharply until pollution rates reached 13.6μg/m3. At this point, the rise was found to plateau. There were insignificant increases past this point.

At just over 10μg/m3, the increased risk-adjusted for other potential risk factors – was found to be eight percent. At 11.9 to 13.6μg/m3, the rise was 13 percent. Up until 22.1μg/m3, the rise in risk factor only went as high as 14 percent. This indicates a drop-off point in risks. It is from this 14 percent figure that the 3.2 million new cases figure is derived.

While a bold claim, these findings were backed by evidence derived from both positive and negative controls. As a positive control test, variables such as the potential for any cause of mortality were assessed. By doing this, PM2.5 levels were shown to cause increases in mortality due to conditions such as lung disease, dependent on concentration of particulate matter.

Likewise, negative controls were also performed. In this case, lower limb fractures were studied. Lower limb fractures are not in any way associated with air pollution, and so, by demonstrating a lack of correlation between this factor and PM2.5 concentration, it is further reinforced that the correlative increase in diabetes cases may indeed be linked to higher PM2.5 levels.

However, even in areas of the world where pollution levels are not considered especially harmful, the risk may still be a present danger.


WHO safety levels


Many nations with previously considered safe levels of pollution will have been elevating the risk of diabetes while being praised for low levels of pollutants. The World Health Organization (WHO) states that an average PM2.5 level below 10μg/m3 across an annual period is safe. Daily, these levels can average as high as 25μg/m3 and still be considered safe. The plateau for diabetes risk elevation, however, occurs at around 13.6μg/m3, meaning that these “safe” levels are still harmful.

This contains numerous implications for India – home to the fourteen most polluted cities in the world.  In many of these metros, pollution levels regularly exceed what is considered safe – often veering into hazardous.

The implication of the study is that India could reduce pollution levels considerably, yet still remain within the risk zone. In Kanpur for example, the average level of PM2.5 is 319μg. To reduce the pollution levels to within a safe limit in the foreseeable future is all but impossible. Even then, a further reduction may be needed to minimise the impact on rates of diabetes.


Geographic distribution


The impact of pollution levels was more notable in low and less economically developed nations and lower-middle income countries (LMICs). This may be due to a combination of factors. The type of fuels burnt, less government schemes to curb pollution, or lack of access to healthcare all portend that the issues arising from higher pollution levels may not be adequately addressed.

Most of the 3.2 million cases caused by pollution are in developing countries. In India, it may account for some of the notable rise in diabetes cases in recent years. Many of its cities are densely populated and dangerously polluted. Hundreds of millions of Indians are at risk of developing the condition.

Even people in rural environments are at risk. Practices such as burning of waste crops and indoor stoves play a decisive role in increasing pollution levels even in rural locations.


What is the biological mechanism?


The biological mechanism in all of this is unknown. If pollution levels — even those previously considered safe — are increasing the risk of diabetes on a global scale, the mechanism by which this occurs is of the utmost importance to understanding how the risk might be minimised.

Several studies have now confirmed that pollution is increasing the risk of diabetes, how this occurs is not known. Small scale studies have however given an indication that may point future studies in the right direction.

Studies in a rat model undertaken in China have revealed that constant exposure to PM2.5 particles elevated the levels of inflammation within the pancreas. Alongside this blood glucose levels were found to be heightened consistently after exposure to high levels of PM2.5.

The implication of this is that inflammation, a known symptom of PM2.5 particles being absorbed into the blood, plays a large role in its effect on diabetes development.

This discovery warrants further research. This is particularly vital, given the potential impact pollution may be having even at otherwise healthy levels.


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