'Change your sunglasses every TWO years': UV rays damage lenses over time - raising your risk of vision loss

Sunglasses should be replaced at least every two years, scientists warned today. 

Rays from the sun damage lenses over time - meaning they gradually let more UV light through and provide less protection. 

Without adequate UV filters, the eye is exposed - and the damage can cause long-term, irreversible harm to vision.

Brazilian researchers have now called for standards that test the quality of sunglasses to be revised, so that safe limits for the filters can be established. 

They say a new test would guarantee sunglasses were safe to wear for a period of two years.

Exposure to sunlight varies across the world, but tropical countries have the highest amounts as UV levels are extremely high in summer and remain high in the winter.

Therefore, sunglasses worn in the southern hemisphere may need replacing more often than in those worn in the northern half. 

Lenses may also break more easily a result of consistent exposure to sunlight, experts say.

And wearing sunglasses which don't offer proper protection can cause oedema - distorted vision from a swollen eye. 

It can also result in cataracts - where the lens of the eye becomes cloudy, affecting vision - and pterygium - the growth of pink, fleshy tissue on the white of the eye which can interfere with sight.

Prolonged UV exposure can also damage the macula - the part of the retina responsible for the majority of vision.

Professor Liliane Ventura, of Sao Paulo University, said: 'Ocular health is a serious concern worldwide, but particularly in tropical countries where UV indexes are extremely high in summer.

'In most countries in the southern hemisphere... sunglasses standards are not quite appropriate for the ultraviolet conditions. 

'Sunglasses play an important role in providing safety, and their lenses should provide adequate UV filters.' 

The test currently used in Europe, Australia, New Zealand and parts of South America and calculates how badly the lenses deteriorate as a result of exposure to the sun.

It exposes sunglasses to a sun simulator for 50 hours from a distance of 30cm.

The 450 W lamp is the equivalent to two days in a natural environment on a summer's day, or four in winter. 

But Professor Ventura claimed the current test is 'ineffective' in its present form and doesn't accurately measure the quality of sunglasses.

The findings were published in the journal Biomedical Engineering OnLine.