In Australia, there is a mandatory standard for the labelling of sunglasses to help the consumer.
In 2003, the Australian Competition and Consumer Commission (ACCC), a consumer watchdog issued a Regulation Impact Statement about the standard. Here is an historical copy, and here it is in its (more difficult to read) official location.
In the document, they discuss the risks of poor-quality sunglasses.
Some are irrelevant to your question:
- "Coloured sunglasses can reduce the ability to see traffic light signals"
- "Sunglasses that are too dark are unsafe for driving"
- "Sunglasses should not be made of flammable materials"
- "Sunglasses should be robust enough to minimise the risk of eye injury"
Some are related:
Solar ultraviolet light causes cataracts
Long term exposure to solar ultra violet radiation is a risk factor for cataracts, a very common eye disorder that results in loss of vision. The prevalence of significant cataracts is 25 per cent in people aged 65-74 and over 40 per cent in those aged more than 75 years.
Cataract surgery imposes a very significant annual cost on the community. Over 120 000 cataract operations are performed in Australia each year. Australia’s cataract surgery rate is 6 300 per million, which is higher than the USA and the UK. About 0.3 per cent of the population suffers from blindness due to cataracts.
Visible light poses little hazard to the eyes at the levels commonly encountered, although long term exposure to short wavelength (blue) light has been implicated in the development of age related macular degeneration. Sunglasses are principally used to reduce glare, which is caused by high levels of visible light or by light in inappropriate places.
Infra-red radiation has wavelengths longer than visible light, and poses little hazard to the eye, as the amount reaching the eye under normal conditions is low. Protection against infra-red radiation is mainly required by people working in industries in which they receive a high exposure, such as people working with molten metals.
Ultraviolet radiation has wavelengths shorter than visible light. Exposure to high levels of environmental ultraviolet radiation is common in Australia. In addition to the well known association between ultraviolet exposure and melanoma, chronic exposure to ultraviolet radiation has been implicated in serious eye disorders including cataract, pterygium and age related macular degeneration. Acute exposure to intense ultraviolet radiation can produce photokeratoconjunctivitis (sometimes known as snow blindness).
It also talks about "Hazards associated with dimension and strength of frames" - i.e. that if the lenses are too small, the protection will be insufficient no matter what the filtering it performs.
Finally, it also discusses price, and shows expensive sunglasses may be poor, and cheap sunglasses may be fine.
Price is not an assurance of safety
Consumers cannot rely on price as an indicator of quality. Past enforcement experience shows that some very high priced sunglasses do not necessarily conform to the Australian Standard.
Price is not really an indication of compliance and it has been found that many cheaper priced sunglasses do comply with the standard.