I have got a lot of work to do outside, why can't I just wear my normal sunglasses?

Normal sunglasses are only intended to offer filtering protection and will not offer any protection against impact. The optical properties are also not tested in the same way as safety eyewear. 

Additionally, normal sunglasses are likely to be more susceptible to damage by potential impact or scratches, over a safety spectacle with F impact rating and K rated scratch resistance.

Safety spectacles marked 5-2.5 and including F and K markings, for example, have been assessed against EN 172, European Standard for Sunglare filters for Industrial use and EN 166 the main eye protection standard, offering UV and impact protection, and scratch resistance, as well as meeting mandatory requirements for optical quality and robustness.