The end of summer will bring no respite to the international force of doctors, epidemiologists, and insect control experts working to quell the global Zika epidemic. Not only have more countries recently been added to the list of local outbreaks (including Singapore and the British Virgin Islands), but the end of summer doesn't mean the end of hurricane season. And with big storms come fresh mosquito breeding grounds (aka pools and puddles of standing water).

The Centers for Disease Control and Prevention, and their counterparts in Mosquito-Control districts across the South, have a strategy for curbing mosquito populations post-storm; it's the same one they're using to combat Zika in Florida, and in recent months it's been the source of controversy and complaint. That controversy centers on naled, an insecticide that has been banned in the European Union but is still used in the U.S., in part because many mosquitoes have become resistant to other insecticides.

Last month, when the CDC tried to persuade Puerto Rican officials to spray the chemical in response to their Zika crisis (going so far as to send a shipment of naled to the island without notifying the island's governor), residents protested so much that one health official resigned and the CDC was forced to apologize and take its shipment back. Protests have been less forceful in Florida, where naled is being used to combat local outbreaks, but some residents are still worried.

Here's what you need to know about this aerial insecticide.

What Is Naled?

Naled is a chemical insecticide used mostly for controlling adult mosquitoes. It’s an organophosphate, which means that it works by inhibiting an enzyme in the central nervous system (it’s mode of action is similar to that of Sarin gas).

Naled is a non-residual insecticide: it kills mosquitoes on contact, in mid-air—but breaks down quickly once it lands on surfaces. To work, the chemical has to be sprayed in the atmosphere (as opposed to on targeted surfaces) at regular intervals. It's normally deployed by low-flying planes (roughly 100 feet above ground), which release a superfine aerosol that contains a small volume of active ingredient (called ULV or ultra-low volume spray). Usually, an ounce of the insecticide is used per acre of land (that's about two tablespoons over a football field’s-worth of area).

Naled has been EPA-approved since 1959 and, according to the agency, is routinely sprayed over some 16 million acres of the mainland U.S. It has also been sprayed after hurricanes and floods, including Katrina. According to the No Spray Coalition, roughly a million pounds of naled is sprayed every year in the U.S., almost all of it aerially.

The CDC has found that, while Aedes mosquitoes are increasingly resistant to the more commonly used (and generally safer) pyrethroid insecticides, like permethrin, they are still susceptible to naled. In field tests in Puerto Rico earlier this year, the chemical killed 100 percent of the female Aedes that were exposed to it.

Is It Safe?

No one disputes that naled can be toxic, even fatal, at very high doses. In India, where environmental regulations are far laxer than they are in the U.S., at least 25 children were killed in 2013, by exposure to extreme quantities of the chemical. The European Union banned naled back in 2012, citing a "potential and unacceptable" risk to human health and the environment, and a lack of sufficient evidence of efficacy.

The EPA has banned naled in pet collars, restricted its use to licensed professionals, and asked farmers and governments to voluntarily eliminate it from their arsenals. But the agency emphasizes that when used as directed (i.e. when sprayed in small quantities by a licensed professional), the chemical poses no risk to human health: an aerial spray campaign deploys so little naled, and it breaks down so quickly in any case, that area residents will hardly encounter it at all.

The EPA has classified the insecticide as non-carcinogenic, based in part on studies of thousands of people who conducted agricultural spraying with naled and other insecticides. According to the agency, those studies did not find an increased risk of cancer in that population, even 7 to 11 years later. The EPA says it also conducted risk assessments for naled and found that for every scenario they considered, exposure to the chemical several-hundred-thousand times below any risk threshold.

Still, some inndependent scientists, and several watchdog groups, including the Natural Resources Defense Council, worry that even the small amount of naled used in aerial spraying could, over longterm repeated exposure, pose some threats. They point to studies in lab animals that have shown a wide range of harmful effects from naled and its by-products, including impaired fetal brain development.  

How Can I Minimize My Exposure?

While the EPA says that you don’t need to leave an area during spraying, and that no special precautions are needed, if you want to play it safe, you can take these common-sense precautions:
• Check your local health department’s website, as well as your local newspaper to see if naled is being sprayed in your area. Both should post notices several days before any insecticide spraying is set to take place.
• Take children’s toys, and pet dishes inside before spraying is set to begin. Close your windows and turn off your AC during spraying, and wipe down any outdoor furniture (including slides and barbecue equipment) afterward. Also, wash any fruits and veggies from the garden before eating them.

Does Naled Pose Risks to the Environment?

Again, the EPA and CDC say no—but others disagree. In some studies, naled proved highly toxic to honeybees, but only in the first day or so after exposure. The EPA and CDC say that aerial spraying can be done in a way that minimizes that risk: by only spraying between dusk and dawn, when bees are more likely to be in their hives, and by alerting beekeepers to spraying plans so that they can either cover their colonies during spraying, or move them to untreated areas if possible. (In South Carolina, beekeepers say they weren't given any notice of the planned aerial spraying. Local officials say that's not true). 

The EPA acknowledges that naled also poses a risk to some aquatic invertebrates, like shrimp and water fleas, but says that it “displays low toxicity” in birds and mammals, and that it’s safe to spray over lakes and rivers. The agency says that aerial spraying does not pose long-term risks to the environment, even if spraying happens more than once, because the chemical breaks down quickly.

The No Spray Coalition and others say that studies show naled is, in fact, toxic to birds, fish (especially trout), and many types of insects.

Are There Alternatives to Naled?

There are. The FDA has recently approved a U.S. trial of genetically modified mosquitoes designed to combat Zika—but that trial has yet to launch, and any widespread use of this approach is still years away.

Other biological control measures—including one that relies on infecting mosquitoes with a common bacteria that prevents them from transmitting viruses to humans—are being tested in some places, while others, that have shown promise in the past, have yet to be deployed against Zika.

“Decades ago, in Florida, they used elephant mosquitoes, which eat the larvae of Aedes mosquitoes, to control disease outbreaks like this,” says Consumer Reports senior scientist Michael Hansen, Ph.D. “Why not try that now?”

The CDC says that no immediately available option will work as well or as quickly as naled to control the current outbreaks. In early August, the agency announced that “aggressive mosquito control measures,” such as eliminating breeding grounds by removing standing water, were failing to curb the spread of Zika in Wynwood, Fla., the Miami neighborhood where locally acquired cases of the disease were first reported on the U.S. mainland. “Point source reduction is very, very effective,” says CDC-entomologist Janet McAllister. “But it’s also very labor-intensive, especially when you’re talking about a large, densely-populated area. You need resources to go door-to-door.” (The agency is set to run out of funds to combat Zika in the next few weeks, unless Congress acts in time). 

Low-flying aircraft can cover a large area quickly and more thoroughly than any other method, McAllister says: 10-square-miles in a few hours, compared with several days to cover the same area from the ground.  

Why Are Aedes Mosquitoes So Hard to Kill?

Part of the problem is the mosquito itself: Aedes aegypti, which transmits Zika as well as Yellow Fever, Dengue, and Chikungunya, is considered “the cockroach of mosquitoes” because of how resilient it is. It can breed in just a bottle-cap’s worth of water; and its eggs can survive for up to a year in unfavorable conditions, simply by going dormant. To make things even tougher, female Aedes aegypti don’t lay all their eggs in one place; they will deposit one batch, of 60 to 80 eggs, over many different sites, a breeding strategy meant to ensure the survival of at least some offspring from every batch.

But another problem has to do with our slapdash mosquito control policies. In the late 1940s, the Pan American Health Organization launched an aggressive campaign against the Aedes aegypti in Latin America, that involved DDT-spraying inside homes, and aggressive community control measures. By 1962, the mosquito (and the Dengue virus along with it) had been eliminated from 18 countries. In 1965, the U.S. launched an all-out war against Aedes, meant to eradicate the bug for all time. But the government dropped that program four years later when political will lagged and funding dried up.

What causes insecticide resistance? Mosquitoes and other insects can develop resistance to a given chemical in a number of ways: They can learn to adapt their behavior to avoid the chemical (for example, in the past mosquitoes have learned to avoid walls inside houses, when DDT was sprayed there). They can also develop physiological resistance—new enzymes that break the chemical down, for example—through random genetic mutations. The more an insect population is exposed to a given chemical, the more likely it is to develop resistance, because each subsequent exposure kills off more of the susceptible bugs, which helps the resistant ones thrive.

Will naled finally rid us of these disease-carrying insects? Some experts have been critical of the decision to spray naled against Aedes aegypti, because the mosquito has a knack for hiding in places where aerial spray may not reach, like inside garbage containers and under houses.

In 1987, naled failed to stop a Dengue outbreak in Puerto Rico, and experts concede that there’s no guarantee it will work now, though they say that new technologies—including modern spray nozzles and super-fine mists—have made aerial spraying more effective. “We’ve learned a lot even just in the past two years about the physics of small droplets,” McAllister says. “And we have found that you actually can kill mosquitoes, even underneath raised houses, by spraying naled.”

In Wynwood, Fla., the CDC says it has measured 90 percent reductions in mosquito populations after aerial spraying. The area designated as a transmission zone has been substantially reduced in recent weeks, and if no additional cases are reported by September 19, the neighborhood will be considered Zika-free. 

As with any pesticide, though, the more successful naled is, and the more we use it, the greater the chance there is for mosquitoes to develop resistance to it, too.