While depression and anxiety had long been linked to suicidal thoughts, a new study links asthma and air pollution with this psychological problem. 

Previous studies have reported that individuals suffering from at least one chronic health condition such as heart disease, diabetes, cancer, chronic obstructive pulmonary disease or mental illness are more vulnerable to committing a suicide. 

According to the study published in the American Journal of Psychiatry, asthma and air pollution can increase the risk of suicide. In other words, respiratory problems may worsen an individual's mental state. 

The more severe the asthma, the greater the likelihood of suicide, the study found. 

Any increase in the air levels of particulate matter -- also known as particle pollution -- was reported to be associated with 9 percent higher risk of suicide. The risk was reported to reach as high as 18.9 percent in those with underlying heart disease. 

Substances in particulate matter including lead, mercury, or diesel exhaust are believed to negatively impact the neurological functioning, leading to an increased suicidal rate. 

They may also impair the cognition and boost depression through causing inflammation in the body, the study found. 

"It could be that air pollution worsens patients' perception of their health, which could then cause more psychological problems," said lead researcher Chian-Jue Kuo.