Dermatologists reveal what really happens when you use expired sunblock

With temperatures skyrocketing around New Zealand and Canterbury in the grip of a horrendous heatwave, many of us will be slip, slop and slapping on more sunblock than ever before enjoying some of the country's best beaches and outdoor areas. 

Aotearoa's melanoma rates are up there with some of the highest in the world, so it's vital we all wear sunblock every day - especially on our faces - to protect from harmful UVA and UVB rays. 

If you've only been sporadically applying the last of the sunblock from that bottle you purchased a few years ago every time the temperature tops 24 degrees, it's time to check its expiration date. 

Some brands will include a full expiration date, while others will include a date of production, and an expiry symbol - which can look like a little tub with a six, 12 or 24 month symbol inside it. 

If you're using one long past it's expiration, you could be doing yourself more harm than good, 

"A sunscreen that has expired should be discarded, as it is less effective at preventing damage from ultraviolet light... the SPF listed on the label is not guaranteed for expired products, be it chemical or physical sunscreen," dermatologist  Dr Meghan Feely told BuzzFeed News.

If you can't find an expiration date on your sunblock, dermatologists say generally sunscreen should last for up to three years - but if you use it as recommended, you should run out long before then.

If you're unsure about the expiration date, you may be able to tell from its appearance.  

Expired sunscreen sometimes loses its consistency or becomes discoloured and may no longer look, feel, or smell the way it did when you bought it.

"Physical sunscreens deteriorate and chemical sunscreens oxidize," Feely said. 

It's not just an increased risk of burns you're in danger of. 

That old bottle you've opened dozens of times before may also harbour more bacteria, which can lead to skin breakouts and irritations. 

To avoid early oxidation, store your sunscreens in a cool dark place - which can be tough to find in the high temperatures some parts of the country have been seeing.

If it's been sitting in your hot car or at the bottom of your beach bag for several months, it might be time to toss it. 

You can find a full list of the best sunblocks as approved by Consumer NZ in this Newshub article.