It's always a pleasant surprise when you're sitting on the beach or hiking through a national park only to find that the mosquitoes, sand flies and ticks you expected aren't biting. But, it's not so nice to spend $20 or more on a full bottle of bug spray, only to find that its potency has worn off when it's time to use it.
Fortunately, the good news is that while most bug sprays and insect repellants do expire, it takes a very long time. And the expiration date — if there is one on the bottle — doesn't really mean much other than "it may not work quite as well."
What Bug Spray Lasts the Longest?
How long your bug spray lasts mostly comes down to its ingredients that keep the bugs at bay, also known as the active ingredients. DEET (N,N-diethyl-meta-toluamide) is the active ingredient in many strong bug repellents, especially those used by hikers or campers in deep woods or jungles and has, theoretically, an infinite shelf life.
That's because the amount of DEET doesn't have anything to do with how well insect repellent works; it determines how long it works. According to the University of Michigan School of Medicine, a bug spray with 20 percent DEET will protect for about five hours; spray with 7 percent DEET will protect for two or three hours. Apparently, that's why you should skip sunscreen-bug spray combos; sunscreen should be applied way more often than bug spray.
That said, most brands, such as Ranger Ready Repellant and Off! recommend you toss any bug spray that's older that three years, though Sawyer brand promises a 10-year shelf life when stored correctly.
Natural bug sprays, which primarily use essential oils as the active ingredients, are likely to lose their effectiveness quicker than chemical and synthetic sprays. The most commonly used essential oils are rosemary, lemongrass/citronella (two- to three-year shelf life), thyme (three to four years), and peppermint (four to five years). So depending on the percentage of those oils in the repellent, you may begin to see decline in effectiveness after about two years.
A quick — albeit not foolproof — way to test a natural repellent is to spray a few squirts and smell the air. If it doesn't have much of a scent anymore, it's probably not as strong as it once was. It won't hurt you, but it just may not work quite as well.
Interestingly, there are no federal laws requiring food or cosmetics/lotions to have expiration dates (except for baby food and formula), though many skin-applied repellents are registered with the Environmental Protection Agency (EPA). That means the company making them provided the EPA with technical information on the safety of the product and how effective it is against mosquitoes and/or ticks.
Why Don't They Have Expiration Dates?
So if your bug spray does have an expiration date, it's voluntary and may be related to other ingredients in the mix, like essential oils (which can start to become less effective after two years) or benzaldehyde (which only lasts about 18 months and is common in bee repellents).
Just keep in mind, there aren't many consequences of using an old bug spray other than it not being quite as effective as a new one, so you may find yourself with a few more bug bites than you'd like.
However, if you have some you haven't used in a while, the pump and tube inside the bottle might be a little sticky with semi-coagulated spray, so you might need to rinse it well before you use it. And if you have a cream or rub-on bug spray (especially one with a clear bottle), be sure to store it out of direct sunlight, or it'll eventually begin to dry up.