In November 2012, the town of Umea in northern Sweden began installing phototherapy lights at bus stops to help combat the shorter days and lack of sunlight.

A study published in the medical journal Neuropsychiatry and Clinical Neurosciences in 2005 estimated that winter depression in Sweden affected 8 percent of the population.

Wang Jian, a leading psychiatrist at Beijing Huilongguan Hospital, a mental institute in the capital, defines the mental impact of weather phenomena, including smog, as "ecological pressure".

"This, like social and spiritual pressure, could heighten negative feelings, fear and anxiety for both the healthy and those with mental problems," Wang said.

"For mental patients, particularly those with depression and neurosis, smog can trigger some symptoms and worsen the situation, he said.

"We've seen some extreme cases where people with depression have committed suicide due to bad weather," he said.

But he pointed out that not all patients with mental health problems are affected by bad weather.

"Learning more about the link could help us to avert mental health risks posed by bad weather," Wang said.

The Huilongguan hospital plans to conduct surveys on the issue among patients.

"The healthy might also be included in such surveys, particularly those working long hours outdoors like traffic police," Wang said.

Worldwide, studies on smog and its impact on mental health have begun to appear in recent years but remained limited.

Research in 2011 by Ohio State University in the US found that exposure to smog causes depression and learning problems.

Scientists from the university's neuroscience department have concluded that smog alters the brain's composition, and can lead to loss of memory and depression.