New research reveals that green spaces can significantly lower mortality risks in type 2 diabetes patients by reducing harmful air pollutants like PM2.5 and nitrogen dioxide, offering a promising strategy for urban environments.

Association of residential air pollution and green space with all-cause and cause-specific mortality in individuals with diabetes: an 11-year prospective cohort study. Image Credit: LedyX / Shutterstock

In a recent study published in the journal eBioMedicine, researchers in China examined the impact of exposure to residential air pollution on cause-specific mortality rates among type 2 diabetes patients and the mitigating effects of green spaces in residential areas through a prospective cohort-based analysis.

Background

Diabetes is a rapidly growing global health concern, with over 500 million cases by 2021 and a global prevalence of 6.1%. The diabetes rates in China are especially high at 12.4%, according to 2018 statistics, leading to significant healthcare burdens and costs. Diabetes is also a risk factor for mortality due to other diseases, such as cardiovascular disease, making it imperative to understand all the risk factors for diabetes completely.

Studies have shown that exposure to air pollution is a key risk factor for type 2 diabetes, contributing to 20% of diabetes-related mortality in 2019. Furthermore, green spaces have been linked to lower mortality rates associated with non-communicable diseases such as diabetes and cardiovascular disease due to the various health benefits green spaces offer in terms of unpolluted air and spaces to exercise. The study also highlights that the protective effect of green spaces was partly due to their ability to lower PM2.5 and nitrogen dioxide levels by 23.8% and 26.6%, respectively.

However, there is a shortage of studies that have quantifiably examined the impact of air pollution on mortality rates in the diabetic population or explored the mediating effects of green spaces on this association between air pollution and diabetes.

About the study

In the present study, the researchers attempted to address this gap by conducting a long-term follow-up of a prospective cohort of newly diagnosed type 2 diabetes mellitus patients in Shanghai to assess the increase in mortality risk due to air pollution exposure. The study also examined how green spaces might reduce the mortality risk due to air pollution exposure among type 2 diabetes patients.

The prospective cohort was part of the Shanghai Standardized Diabetes Management System, which registers all the type 2 diabetes cases in the city. It consisted of over 170,000 individuals over the age of 20 who were diagnosed with the disease between 2011 and 2013. The type 2 diabetes diagnosis was based on post-oral glucose tolerance test levels, fasting plasma glucose levels, or the use of diabetes medications.

The study focused on all-cause and specific-cause mortality, including mortality due to respiratory diseases, cardiovascular disease, cancer, and diabetes-associated complications. The deaths were assessed using the Shanghai death registry.

Satellite-based models were used to assess air pollution exposure, and the measured pollutants included fine particulate matter below 2.5 microns in diameter (PM2.5), nitrogen dioxide, and inhaled particulate matter below 10 microns in diameter (PM10). The study also utilized restricted cubic splines to model non-linear exposure-response relationships, particularly for nitrogen dioxide (NO2), which showed significant effects only at concentrations above 45 µg/m3. The use of cross-validated models allowed the pollution levels to be estimated at a resolution of 1×1 km with high accuracy.

The Normalized Difference Vegetation Index, derived from satellite data, was used to quantify the green spaces surrounding the participants’ residences at a resolution of 250 meters. The pollution exposure data were averaged for a year, while the greenness was calculated annually.

Furthermore, baseline questionnaires provided information on covariates such as age, sex, body mass index (BMI), alcohol consumption levels, smoking status, socioeconomic indicators, and physical activity levels. Additionally, sensitivity analyses were conducted to adjust traffic noise levels and exclude early deaths due to the coronavirus disease 2019 (COVID-19) pandemic.

Results

The study followed the 174,063 participants registered in the prospective cohort for a period of 7.9 years and reported that 22,205 participants died during this period from various causes, including respiratory diseases, metabolic diseases, cardiovascular diseases, and cancer.

Exposure to fine particulate matter PM2.5 was significantly associated with an increased risk of all-cause mortality. However, exposure to particulate matter below 10-micron diameter was not found to be significantly associated with any other cause of mortality except cancer. The strongest associations with PM2.5 exposure were observed for peripheral vascular disease (HR: 2.70) and gastrointestinal cancer (HR: 2.44).

Furthermore, while PM2.5 exposure was found to increase the risk of mortality due to respiratory, cardiovascular, and metabolic diseases, as well as cancer, the strongest associations were with peripheral vascular disease, gastrointestinal cancer, and lung cancer. The relationship between nitrogen dioxide exposure and mortality was non-linear, with significant effects observed only at concentrations above 45 µg/m3.

Although the impact of nitrogen dioxide exposure was less than that of PM2.5 exposure, it was associated with an increased mortality risk from cardiovascular disease and cancer. The mortality risk from nitrogen dioxide exposure followed a non-linear response curve, significantly increasing at concentrations above 45 micrograms per cubic meter.

Residential green spaces were linked to a 6% to 24% decrease in mortality from various causes except respiratory diseases. The most significant decrease in mortality risk was observed for peripheral vascular disease.

The protective effect of green spaces was attributed mainly to their ability to lower the concentrations of nitrogen dioxide and PM2.5, with a mediating effect of 26.6% and 23.8% decrease, respectively. Furthermore, the beneficial effects were observed to be greater in urban areas, potentially due to these areas facing higher pollution levels. The study also found a significant negative interaction between nitrogen dioxide and green spaces, but no significant interaction between PM2.5 and green spaces was observed.

Conclusions

To summarize, the study found that long-term exposure to PM2.5 and nitrogen dioxide in areas with high air pollution was linked to an increase in mortality risk among type 2 diabetes patients. Green spaces were found to mitigate the harmful effects of air pollution by lowering pollution levels. The findings emphasized the importance of maintaining green spaces, especially in urban areas, where the interaction between green space and nitrogen dioxide exposure was most significant.