Car emissions may trigger Parkinson's disease, research suggests.
The progressive condition is caused by a loss of nerve cells in the part of the brain responsible for producing the chemical dopamine, which helps co-ordinate movement.
Exactly why this occurs is unclear, however, the disease is known to run in families and is increasingly being linked to environmental factors – like pesticides, traffic and air pollution.
With the evidence being "inconclusive", scientists from the University of Ulsan in South Korea analysed more than 78,000 Seoul residents.
Based on the participants' addresses, the team estimated their exposure to pollutants released in vehicle emissions.
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Results reveal those with the highest exposure to nitrogen dioxide were 41% more likely to develop Parkinson's over the next eight years than the residents with the lowest exposure.
Nitrogen dioxide "primarily gets in the air from the burning of fuel", but is also released "from cars, trucks and buses".
"Air pollution is a significant public health hazard," said study author Professor Sun Ju Chung.
"More than 80% of urban area residents are exposed to levels that exceed limits set by the World Health Organization.
"Recently, it has been identified to be associated with neurodegenerative diseases through systemic inflammation, oxidative [internal] stress and direct invasion into the brain."
Around 145,000 people are diagnosed with Parkinson's in the UK, with one in 37 expected to be told they have the disease in their lifetime.
In the US, nearly 1 million are living with Parkinson's, expected to rise to 1.2 million by 2030.
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Persistent exposure to air pollution and traffic is increasingly being linked to dementia.
To better understand whether these may also trigger Parkinson's, the Ulsan scientists analysed more than 78,000 adults who lived in Seoul between January 2002 and December 2006.
None of the residents, average age 54, had been diagnosed with the disease at the start of the study. They were then followed-up once a year from 2007 to 2015.
Over these eight years, 338 of the participants were diagnosed with Parkinson's.
"Exposure to NO2 [nitrogen dioxide] was associated with an increase in risk", the scientists wrote in the journal JAMA Neurology.
Parkinson's was not linked to other emission pollutants, however, like carbon monoxide, sulfur dioxide or particulate matter – microscopic substances that can become lodged in the lungs.
Nevertheless, the scientists concluded: "This finding suggests the role of air pollutants in PD [Parkinson's disease] development, advocating for the need to implement a targeted public health policy."
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