Does sunscreen degrade in a hot car?

What the evidence says

Yes, hot cars can wreck your sunscreen. Health Canada specifically warns people to keep sunscreen out of direct sunlight and out of places that get very hot, like a car, because extreme heat makes sunscreen less effective at protecting skin.

The Canadian Dermatology Association explains why. Sunscreen contains chemicals that naturally break down over time, and this breakdown speeds up when the product sits somewhere hot, like a car's glove compartment or a beach bag left in the sun. They recommend storing sunscreen at room temperature, below 30 degrees Celsius.

In a closed car on a sunny day, temperatures can climb far above that, sometimes reaching levels that patent and stability research on sunscreen formulas has tested up to 40 to 60 degrees Celsius. At those temperatures, the ingredients that block UV rays, called UV filters, can degrade faster, and the lotion itself may get thin and runny, making it harder to apply evenly.

The practical takeaway for parents: don't leave the sunscreen tube in the car between outings, especially in summer. Keep it in a bag you bring inside, a cooler, or somewhere shaded. If a bottle has clearly been baking in a hot car for a long stretch, changed texture or smell, or is past its expiry date, it's safer to replace it rather than trust it to fully protect your child's skin.

This matters because sunscreen that doesn't work as well means less protection from sunburn and long-term UV damage, which is a real concern for kids playing outside.