The heart of any bottle of bug spray is the repellent chemical, so we started there before we dug into specific products. To cut to the chase, our research led us to picaridin, a repellent chemical endorsed by the CDC and the WHO. Independent tests prove that a 20% concentration of picaridin can repel mosquitoes and ticks for up to 12 hours. But we began our search considering all options, using expert guidance to determine what mattered most when seeking a formula to recommend. Below is an overview of the factors we considered.

EPA approval: This requirement left us with a short list of skin-applied repellents focusing on picaridin, DEET, IR3535, and oil of lemon eucalyptus, or OLE (as well as PMD, its synthetic form). To meet EPA approval, a chemical must undergo extensive testing and evaluation for efficacy, safety, and the potential for environmental hazards. “If it doesn’t have an EPA registration, then it hasn’t been proven,“ Texas A&M’s Swiger told us.

Because they are classified as pesticides, the EPA approves for use not only all repellent chemicals but also every product that includes those chemicals. This approval extends all the way to the specific labeling on the bottle, including efficacy times, application procedures, and storage and handling. In fact, according to the EPA website, this labeling is “legally enforceable … In other words, the label is the law.”

Store shelves are loaded with repellents that are not approved by the EPA. Most of these are based on essential oils, including cedar oil, lemongrass oil, peppermint oil, and citronella oil. The fact is, essential oils make terrible bug repellents. Conlon told us that although many of these formulas have some effectiveness, it lasts for a very limited time. “If you spread garlic all over your body, you’re going to get 20 minutes of repellency, then you’re going to have to do it again,” Conlon said. “Is garlic repellent? Yes, it is. Is it a good repellent? No, it isn’t.” The Environmental Working Group (EWG) states, “While effectiveness varies, and there may be a few exceptions, most botanicals repel bugs for a short time, if at all.”

Safety: Picaridin was created in the 1980s, so it hasn’t been studied as much as DEET, which has been around since the 1940s. But like DEET, picaridin does have a proven track record of safety. According to the NPIC, skin irritation due to picaridin is “very uncommon,” and the chemical “is considered practically nontoxic if inhaled.” Further, the EPA has approved the chemical for use in children as young as 2 months old. For pregnant women, the EWG recommends a picaridin concentration of 20% and a DEET concentration of no more than 20% to 30%. “What studies have been done have found no connection between picaridin use and any neurotoxicity, carcinogenicity, mutagenicity, or anything like that,” Conlon told us.

Effectiveness: A 2018 review published in the Journal of Travel Medicine analyzed 11 studies comparing DEET and picaridin, and concluded that “the studies showed little potential difference between DEET and picaridin applied at the same dosage, with some evidence pointing to a superior persistence for picaridin.” According to the EPA-approved labels, a repellent with 20% picaridin can provide protection for up to 12 hours, while one with 25% DEET provides up to 10 hours. The studies mentioned in the Journal of Travel Medicine analysis offered varying results, with the majority showing 20% picaridin effectiveness up to and beyond nine and 10 hours.

We should also note that the percentage of an active ingredient doesn’t necessarily translate to increased efficacy. It appears that after a certain concentration, a maximum effectiveness is reached, and additional concentration results in longer protection time only. According to the CDC, “DEET efficacy tends to plateau at a concentration of approximately 50%.” The CDC also recommends using a DEET concentration of at least 20% to ward off ticks, implying that lower concentrations have reduced effectiveness against such hardier insects.

Minimal drawbacks: Aside from picaridin, the other EPA-approved repellents we considered have at least one major downside.

DEET (PDF), the most well known, is effective but tends to be oily and smelly. Available “dry” formulas remedy this some, but another issue with DEET is that it can damage some synthetic fabrics, as well as watches, fishing line, cameras, glasses, and anything else made of plastic. Judi Anderson, director of the DEET Education Program, told us, “DEET-based products are not recommended for use directly on certain fabrics, such as rayon, spandex, acetate, or other synthetic fabrics,” adding that “it can affect the finish of many hard surfaces, such as plastic, vinyl, paint, or lacquer.” To demonstrate this effect, Philip Werner, writer at SectionHiker.com, used DEET to dissolve a pair of pants made from synthetic fabric. The American Academy of Pediatrics says DEET is safe for children over 2 months old—in concentrations of no more than 30%—and recommends washing off the repellent once children return indoors.

Oil of lemon eucalyptus (PDF) (also available as the synthetic PMD)is an extract of the eucalyptus tree. For years, OLE was not approved for use on anyone under the age of 3, but recently—due to information submitted to the EPA by a manufacturer of the active ingredient OLE—that age restriction has been removed. But, because of the way the EPA works, it has only been removed on repellents using that manufacturer’s 30% concentration of OLE. This includes Murphy’s Naturals, Natrapel and Repel. According to the EPA-approved labeling, OLE doesn’t last as long as DEET and picaridin. After testing, the biggest drawback for us was the extremely strong odor. Some may not mind it and others might enjoy it, but it’s certainly a factor to consider.

IR3535 (PDF), like DEET, also affects plastics. The EWG writes, “The manufacturer recommends avoiding contact with plastics other than polyethylene and polypropylene.” IR3535 is typically combined with a sunscreen, which is not recommended because sunscreen is applied at a higher rate than repellent, leading to needless overexposure to the repellent chemical.

We also evaluated a number of products containing permethrin, which is particularly useful against ticks (and EPA approved). Unlike the others, permethrin is meant to be used on clothing and gear—think a backpack or tent—but not directly on skin. Permethrin should be used in addition to a skin-applied repellent, not in lieu of one. When properly applied, permethrin is extremely effective, providing protection for roughly six washings. A study published in the Journal of Medical Entomology found that “subjects wearing permethrin-treated sneakers and socks were 73.6 times less likely to have a tick bite than subjects wearing untreated footware.” High concentrations of permethrin, when wet, can be toxic to cats, but once it dries, there’s no issue.

A decent spray bottle: A good repellent is wasted in a bad bottle. The best ones apply repellent in an even coat with limited overspray. Kids are squirmy, so getting repellent on them takes precision, as does applying spray to the neck and shoulder area. Locking caps are a plus, prohibiting accidental sprays and providing one more barrier for a curious child to overcome. We looked at both pump sprays and aerosols. Some manufacturers offer repellent lotions. We have more thoughts on these below.

We did not test for efficacy of the individual repellents, for a few reasons. First, there are simply too many variables to consider to make any sweeping statements on repellency. The US alone has over 176 species of mosquitoes, and, as Conlon has written for the AMCA, “The process of a mosquito questing for a blood meal involves a complex, interconnected cascade of behaviors, each probably having its own cues, be they visual, thermal, or olfactory.” In other words, the way someone smells, what they’re wearing, how much they’re sweating, and weather conditions can all affect mosquito attraction.

We did test for usability, looking at different competitors’ bottles and comparing design details that we noticed when applying (lots of) bug spray firsthand. Is the spray nice and even? Does the repellent bottle have any safety features? Is it likely to activate when stuffed into a backpack? Are there any overwhelming odors?