A Hong-Kong University study, published in Jama Ophthalmology, suggests smoking around children may harm their vision as their eyes are still developing.  

Specifically, the researchers found that children who were exposed to second-hand smoke had thinning in the choroid, a layer of tissue that contains tiny blood vessels nourishing the eye.  

The findings are concerning, said Dr Luxme Hariharan, a paediatric ophthalmologist at Nicklaus Children's Hospital in Miami. Ages six to eight are crucial to vision development, "Once you're ten, changes can be permanent," she said. "Anything abnormal can cause a permanent problem in the visual pathway as it's forming, that's why this age group is key." 

For the study, lead researcher Associate Professor Jason Yam and his colleagues measured choroidal thickness in 1,400 children aged six to eight, some of whom had been exposed to second-hand smoke. The degree of thinning was directly related to the amount of second-hand smoke children were exposed to, A/Prof Yam said.  

Exposure to smoke from one cigarette per day was linked with roughly a half-micron of thinning (a micron is one-millionth of a meter). On average, children exposed to second-hand smoke had choroids that were eight microns thinner in the centre and six to seven microns thinner at the edges than kids who had not been exposed.  

While further research is required to establish the direct cause of thinning of the corneal thickness, Dr Hariharan encourages parents and families to be careful.