Hong Kong (CNN) Almost 2 million people in Hong Kong --- about a third its adult population -- are estimated to have experienced symptoms of post-traumatic stress disorder (PTSD) during months of social unrest in the city, according to research published Friday in The Lancet medical journal.

Researchers from the University of Hong Kong surveyed 18,000 people between 2009 and 2019 in what they said was the world's largest and longest study of the population-wide impact of social unrest on mental health.

Of those surveyed, 32% reported signs of PTSD in recent months, according to the researchers. PTSD is an anxiety disorder that manifests after a traumatic experience and can cause sleeplessness, irritability and traumatic nightmares.

Hong Kong's pro-democracy supporters take part in an anti-government march on New Years Day.

Protests erupted in Hong Kong in June over a now-scrapped extradition bill that would have allowed suspects to be sent to mainland China. The demonstrations have since evolved into a broader pro-democracy movement calling for greater civil liberties and police accountability.

As the protests have continued, they have grown more violent and disruptive. Two protesters have been shot with live rounds, a man was set on fire, and another man died after being hit with a brick during clashes. More than 16,000 rounds of tear gas have been fired by police, with about 7,000 people arrested.

Police arrive to clear pro-democracy protesters in the Kowloon district of Hong Kong on New Year's Eve.

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