Protecting your eyes from the sun can go a long way toward preventing acute ocular diseases such as photokeratitis and snow blindness, and chronic ones like cataracts and pterygia. But, while people often take precautions to protect their skin, they rarely protect their eyes with the same fastidiousness. A new study noted that the UV index isn’t a great measure for the amount of UV damage to the eyes and proposed a new instrument to help the public better pinpoint ocular UV exposure.

The instrument includes a rotating model head with UVB sensors that record intensity at the crown and eye area. The sensors span eight azimuths from sunrise to sunset during multiple climatic conditions throughout the year. The researchers obtained UV dose intensities from their instrument to create an ocular UV index and compared it with the UV index.

They found that UV exposure to the crown of the head increases with the sun’s altitude, whereas UV exposure to the eyes is greater at lower solar altitudes. “The ocular UV index levels were higher than recorded UV index levels in the summer under low solar altitude in the early morning and mid-to-late afternoon and were markedly higher all day in winter when solar altitude remains low,” the researchers wrote.

“This may be because the UV sensor on the crown mainly measured overhead solar UV and those on the eyes were influenced by the horizontal effects of the scattering component of UVB in winter when the sun altitude was lower,” the researchers explained. “When the mannequin [modeled after a Japanese female] faced the sun (summer, spring and autumn), ocular exposure slightly decreased around the time of maximum solar culmination but then increased again, so there were two peak times of high irradiance.” They noted that previous studies have found steep bimodal peaks of UVA (315nm to 400nm). The present study measured only 280nm to 310nm of UVB.

They concluded that the UV index doesn’t provide a good warning system when it comes to ocular solar exposure. They say their proposed ocular UV index will be useful for warning the public about ocular UV exposure in addition to providing another means of researching UV-induced ocular diseases.

Hatsusaka N, Seki Y, Mita N, et al. UV index does not predict ocular ultraviolet exposure. Trans Vis Sci Tech. 2021;10:1.