Current national and regional standards require that sunglasses provide levels of UV protection linked to the ratio of transmitted light to incident light, which decides the category of the lenses. But the average Australian's exposure to the sun may deteriorate sunglasses over time, and the lenses may become lighter, altering the category under which they are classified and how often you should be replacing them.
This deterioration may also affect how "shatterproof" the lens is.
Revision of standards is needed to test sunglasses quality and establish safe limits for the lenses' UV filters, according to research.
The aging test, used in Europe, Brazil, New Zealand and Australia, calculates the extent to which the lenses' category deteriorates as a result of exposure to the sun. The test exposes sun glasses to a sun simulator for 50 hours at 30cm from a 450 W lamp. The lamp exposure is equivalent to two days in a natural environment on a summer's day, or four days in winter.
Lenses should provide adequate UV filters, because insufficient protection could lead to pathological modifications to the cornea and to the internal structure of the eye. This could cause edema (swelling of the eye which can distort vision), pterygium (growth of pink, fleshy tissue on the white of the eye that can interfere with vision), cataract (clouding of the lens of the eye) and retina damage.
"50 hours of exposure to the sun simulator equates to 23.5 hours of exposure to natural sun in Sao Paulo in Brazil," Researcher Liliane Ventura said. "Most Brazilians replace their sunglasses every two years. To test the sunglasses are safe to wear for these two years, with the assumption they are worn for a period of two hours a day, they should be tested for 134.6 hours at a distance of 5cm".
Exposure will vary among world latitudes, with tropical countries being of most concern, as UV indexes are extremely high in summer and remain high in the winter. Moral of the story here is, sunglasses worn in Australia -- and the rest of the southern hemisphere -- may need replacing more often than in those worn in the northern hemisphere, the research concluded.