Last summer may have left you with a ton of half-full bottles of sunscreen. But the question is: How do you know if they've gone bad?Cosmetic dermatologist, Dr. Mervyn Patterson from Woodford Medical explains what's the issue with sunscreen going bad. How long does it last? And most importantly, how do you know if it's expired?First, does sunscreen expire?"Most sunscreens do expire, and because of this, a lot should come with an expiration date stamped on the box," explained Patterson. "It is very important not to ignore this, as the chemicals do degrade and may well lose their protective effect."But what if you've thrown away the box — and with it — the expiration date? As a rule of thumb, Patterson says most suntan lotions will last for "at least a year of use."How to tell if it's gone badThe warning signs to look for are smell and texture. If your sun protection has expired, the formula will have "started to separate." "If it's looking different in terms of texture or smell from when you first bought it, then things may not be well with the product. In this case, it's best to simply discard," says Patterson.What happens if you use expired sunscreen by accident?Short version? It's pretty likely you'll burn and the formula itself could trigger a reaction from your skin. "If a sunscreen product has deteriorated, then in theory there is a risk of chemical alteration of the ingredients." What does this mean? There's a greater likelihood of "contact reactions" with the skin, which means it could be irritating. There will also be a "fall in sun protection," hence the burning. Ouch.So what causes it to expire? These are the triggers to avoidMost sunscreens have been designed to withstand a lot, "However if the containers are left in direct sunlight even the most stable of formulations can degrade," Patterson continues, "Storage is important. Leaving your sunscreen baking in the sun, whether it's on the beach, or in the back window of your car, may trigger degradation of the formula, and that renders the product useless." The best place to store it is somewhere shaded, preferably cool and dry.If in doubt, throw out the old stuff and pick up a new bottle — it's worth protecting your skin no matter what.