While excessive smoking has been linked to various health issues, including heart disease and cancer, a new study has warned that smoking over 20 cigarettes a day can cause blindness.

The study from the Rutgers University noted that chronic tobacco smoking can have harmful effects on "spatial and colour vision". The findings, published in the journal Psychiatry Research, noted significant changes in the smokers' red-green and blue-yellow colour vision. This suggests that consuming substances with neurotoxic chemicals, such as those in cigarettes, may cause overall colour vision loss.

Heavy smokers also have reduced ability to discriminate contrasts and colours compared with non-smokers. "Our results indicate excessive use of cigarettes, or chronic exposure to their compounds, affects visual discrimination, supporting the existence of overall deficits in visual processing with tobacco addiction," said Steven Silverstein from the Rutgers's Behavioral Health Care.

"Cigarette smoke consists of numerous compounds that are harmful, and it has been linked to a reduction in the thickness of layers in the brain, and to brain lesions, involving areas such as the frontal lobe, which plays a role in voluntary movement and control of thinking, and a decrease in activity in the area of the brain that processes vision," he said.

For the study, the team looked at 71 healthy people who smoked less than 15 cigarettes in their entire lives and 63 people, who smoked over 20 cigarettes a day. The participants were in the 25-45 year age group. The study's findings showed noticeable changes in the red-green and blue-yellow colour vision of the heavy smokers. Previous studies had also pointed to long-term smoking as doubling the risk for age-related macular degeneration and as a factor causing lens yellowing and inflammation.