Know why drunk drivers see halos as they zoom past city lights, leading to traffic accidents? It's because alcohol disturbs the tear-film that covers the surface of their eyes, leading to deterioration of vision.
Alcohol consumption markedly impairs night-vision because it increases the perception of halos (luminous circles) and other visual night-time disturbances, a new study shows.
"This is because ethanol from alcoholic drinks passes into the tear and disturbs the outermost layer of the tear-film - the lipid layer - facilitating the evaporation of the aqueous part of the tear," said principal author of the study Jose Juan Castro from University of Granada, Spain.
In an eye with a deteriorated tear-film, the quality of the image that forms in the retina also deteriorates.
Moreover, this deterioration of vision is significantly greater in subjects with breath alcohol content in excess of 0.25mg/litre - the legal limit for driving recommended by the World Health Organisation (WHO).
To reach this conclusion, researchers evaluated retina-image quality and night-vision performance following alcohol consumption in 67 participants.
To assess visual performance in low-illumination conditions, the researchers used a visual test known as a "halometer".
The results, published in the Journal of Ophthalmology, showed that following alcohol consumption, the perception of halos and other night-time visual disturbances increased.
Researchers also found that the optical quality of the image the eye produces deteriorated.
"Alcohol consumption and low-illumination conditions are factors present in many traffic accidents, hence people must to be made aware of the effects of alcohol consumption, especially on vision," Castro said.