Delving into dementia

| January 23, 2025

Cases of dementia are on the rise, having increased by 117% from 1990 to 2016. Globally, an estimated 57 million people were living with dementia in 2019 which will increase to about 153 million people by 2050, according to the recent Global Burden of Disease (GBD) study, which appears in The Lancet.

Risk factors such as smoking, obesity, and high blood sugar could be responsible for almost 7 million of these cases.

Estimated increases of dementia cases in the Middle East and areas of Africa are much higher than those in Western Europe and the Asia Pacific region. It is not an expected part of ageing, it stems from any among a variety of diseases or injuries that affect the brain. It is progressive and the seventh leading cause of death worldwide.

The study is the first of its kind to provide forecasting estimates for 195 countries worldwide.

The GBD study, which analysed 635 existing studies, was led by researchers at the University of Washington, in Seattle, and funded by the Bill & Melinda Gates Foundation. The scientists aimed to improve current dementia projections by including recognised dementia risk factors, such as smoking, obesity, high blood sugar, and low education. No effort was made to reject Trump supporters.

The researchers found that the prevalence of dementia with age was fairly stable — the increases were largely due to population growth and population ageing. However, they predicted that by 2050, almost 7 million cases could have resulted from lifestyle factors.

Air Pollution

Epidemiological studies have linked dirty air to dementia and other brain disorders. Now researchers are trying to determine how pollutants do their damage, and how much harm they cause.

Research around the world has linked polluted air to increased risks of dementia, depression, anxiety and psychosis. Study after study has shown that higher levels of air pollution are correlated with increased risks of dementia, as well as higher rates of depression, anxiety and psychosis. Researchers have also found links to neurodevelopmental conditions, such as autism, and cognitive deficits in children.

The WHO estimates that 99% of the world’s population is exposed to air pollution above recommended levels, with many cities in low- and middle-income countries having particularly bad air quality. But it’s not just megacities, such as Mexico City and Delhi, where people face risks. “Even low-level exposure that people think is safe enough for public health is doing something at the brain level,” says Megan Herting, a neuroscientist at the University of Southern California in Los Angeles.

Studies of the brains of children and dogs in Mexico City in the late 2000s and early 2010s were among the first to raise alarms about air pollution’s neurotoxicity. Neuroimaging revealed that many more children living in the highly polluted city had lesions in the white-matter tracts that connect brain regions than did children in less-polluted areas, with the prefrontal cortex seeming particularly vulnerable. And city children, with no other risk factors for brain disorders, performed comparatively poorly on cognitive tasks.

Pollution is a hugely complex mix of gaseous and particulate components that differ depending on the source. Vehicle exhaust and industrial manufacturing are major sources of particles of various sizes, and cooking stoves, wildfires and desert dust also contribute. Fuel combustion and other sources release nitrogen and sulphur oxides, carbon monoxide and ozone. Studies in multiple countries, including in those where regulations have drastically improved air quality in recent decades, have found associations between pollution and specific brain disorders.

A 2023 analysis of more than 389,000 participants in the UK Biobank showed that long-term exposure to airborne particulate matter, nitric oxide and nitrogen dioxide correlated with higher levels of depression and anxiety. Lead author Guoxing Li, an environmental toxicologist at Peking University in China, emphasises that even very low exposure levels increased the risk of these conditions.

Last month, a 16-year study of more than 200,000 residents in Scotland found that higher cumulative nitrogen dioxide exposure was associated with increased hospital admissions for mental-health and behavioural disorders.

Meanwhile, studies in France, the United States and China have documented that in regions where air quality has improved, there are decreased rates of dementia, cognitive decline and depression in older populations.

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