Smoking more than 20 cigarettes a day can damage your vision, according to a new paper published in Psychiatry Research.

The Centre for Disease Control and Prevention estimates that 34.3 million adults in the US currently smoke cigarettes and that more than 16 million are living with a smoking-related disease, mainly those relating to the cardiovascular system.

The study included 71 healthy people who smoked fewer than 15 cigarettes in their lives and 63 who smoked more than 20 cigarettes a day, were diagnosed with tobacco addiction and reported no attempts to stop smoking. The participants were between the ages of 25 and 45 and had normal or corrected-to-normal vision as measured by standard visual acuity charts.

The researchers looked at how participants discriminated subtle differences in shading and colours while seated 59 inches from a 19-inch cathode-ray tube monitor that displayed stimuli and monitored their eyes. 

The findings demonstrated significant changes in the smokers' red-green and blue-yellow colour vision, suggesting that the consumption of substances with neurotoxic chemicals, such as those in cigarettes, may cause a loss of colour vision. Scientists also found that the heavy smokers had a reduced ability to discriminate contrasts and colours when compared to the non-smokers.

“Cigarette smoke consists of numerous compounds that are harmful to health, 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,” said co-author Steven Silverstein, director of research at Rutgers University Behavioural Health Care. “Our results indicate that 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.”

Silverstein said the findings also suggest that research into visual processing impairments in other groups of people should take into account their smoking rate or independently examine smokers versus non-smokers.