Does it matter what order I apply sunscreen and bug spray?

What the evidence says

Yes, the order really does matter. Health Canada and the Canadian Paediatric Society both say to put sunscreen on first, then wait a little while, then put on the bug spray or repellent lotion.

Why this order? Studies show that DEET, the main chemical in many bug repellents, can make sunscreen less effective at blocking the sun. One clinical trial found sunscreen's SPF (sun protection factor, a measure of sunburn protection) dropped by about a third when repellent was applied on top of it. Other lab research found that sunscreen ingredients can also make skin absorb repellent faster, though this hasn't been shown to cause harm at normal use levels.

Canadian sources like Caring for Kids and Health Canada suggest waiting about 20 minutes after sunscreen before adding repellent, so the sunscreen has time to settle on the skin. The CDC gives similar advice, just without a specific wait time.

Because repellent lowers your sunscreen's protection a bit, and because sunscreen needs reapplying every two to three hours while repellent usually doesn't need reapplying that often, health groups say never buy a combined sunscreen-plus-repellent product. In Canada these combination products are not even sold anymore, so this isn't a real choice, you will be using two separate products anyway.

One more thing that matters more than order: age limits. Children under six months should not have sunscreen or DEET repellent at all, and instead should be kept in shade or protected with mosquito netting. For older babies and kids, follow the label for the right DEET strength and how many times a day it can be used.