Most mosquito fogging operations use one of two chemical classes: synthetic pyrethroids or organophosphates. Pyrethroids, including permethrin, resmethrin, and sumithrin, are the most widely used in the United States. Organophosphates like malathion and naled are older alternatives still deployed in certain areas. Both kill adult mosquitoes on contact, but they work through different biological mechanisms and carry different risk profiles for people, pets, and the environment.
The Most Common Active Ingredients
Pyrethroids dominate modern mosquito fogging. They’re synthetic versions of pyrethrins, natural insecticides found in chrysanthemum flowers. The three you’ll most commonly encounter in U.S. fogging programs are permethrin (sold commercially as Biomist), resmethrin (Scourge), and sumithrin (Anvil). These are applied at extremely low concentrations, typically less than 1/100th of a pound of active ingredient per acre.
Organophosphates like malathion have been used for decades and remain part of some municipal programs, particularly during disease outbreaks. Malathion is considered low in toxicity to mammals but is highly toxic to bees, some fish, and other aquatic life. It works through a completely different mechanism than pyrethroids, which is sometimes useful when mosquito populations have developed resistance to one class.
How These Chemicals Kill Mosquitoes
Pyrethroids target the nervous system of insects by interfering with sodium channels in nerve and muscle cells. Normally, sodium channels open briefly to transmit a nerve signal, then close. Pyrethroids delay that closure, forcing a prolonged flood of sodium into the cell. This lowers the threshold for firing and causes nerves to fire repeatedly and uncontrollably. At high enough concentrations, the sodium overload becomes so extreme that it blocks nerve signals entirely, paralyzing the insect.
Pyrethroids also reduce chloride channel activity in the brain, nerves, and muscles. At higher concentrations, they interfere with GABA receptors, which normally calm neural activity. The combined effect is rapid overstimulation followed by paralysis and death, often within seconds of contact. This is why you’ll sometimes hear the term “knockdown” used in pest control circles.
Organophosphates take a different route. They block an enzyme called acetylcholinesterase, which normally clears the signaling chemical acetylcholine from nerve junctions. Without that cleanup, nerve signals pile up and the insect’s muscles contract uncontrollably. Interestingly, organophosphates can actually enhance pyrethroid toxicity by blocking the enzymes insects use to detoxify pyrethroids, which is why some formulations combine the two classes.
ULV Spraying vs. Thermal Fogging
The two main delivery methods for mosquito fogging chemicals produce very different droplet sizes, and that matters for effectiveness. Ultra-low volume (ULV) spraying uses an air-shear nozzle to break the chemical into tiny droplets with a median diameter around 14.6 microns. About 73% of the spray volume consists of droplets smaller than 20 microns. These droplets are small enough to hang in the air and contact flying mosquitoes but large enough to eventually settle.
Thermal fogging produces even finer droplets by using pulse jet engines to vaporize the chemical (usually diluted with diesel fuel), creating a visible fog. The median droplet size is roughly 3.1 microns, and 100% of the fog volume falls below 20 microns. The smaller droplets stay airborne longer and penetrate vegetation more effectively, but the equipment runs at lower speeds (about 5 mph versus 10 mph for ULV trucks) and uses more total solution per acre.
Despite the visible cloud, thermal fogging doesn’t necessarily deliver more active ingredient. Both methods apply roughly similar amounts of pesticide per acre. The dramatic-looking fog is mostly carrier fluid. ULV spraying is more common in U.S. municipal programs because it’s faster, less conspicuous, and doesn’t require diesel as a carrier.
Growing Resistance in Mosquito Populations
Mosquitoes are evolving resistance to pyrethroids, and it’s a growing concern for public health agencies worldwide. The primary mechanism is called knockdown resistance, or kdr, which involves genetic mutations that alter the sodium channels pyrethroids target. Two specific mutations, known as V1016G and F1534C, have been detected in Aedes mosquitoes (the species that transmits dengue, Zika, and chikungunya) across multiple countries.
Beyond genetic changes in the target site, mosquitoes are also developing metabolic resistance. They produce elevated levels of enzymes, including esterases and mixed-function oxidases, that break down pyrethroid molecules before they reach the nervous system. This means a fogging operation that worked well five years ago may now kill a smaller percentage of the local mosquito population. Some programs rotate between chemical classes or combine pyrethroids with synergists that block the mosquito’s detoxification enzymes.
Safety for People During Fogging
At the concentrations used in mosquito fogging, pyrethroids and organophosphates pose minimal risk to humans. People and dogs break down permethrin far more efficiently than insects do. Still, direct exposure is worth avoiding, and public health departments offer clear guidelines for when fogging trucks pass through your neighborhood.
Stay indoors while spraying is happening and for at least one hour afterward. Close windows and doors before spraying begins. If you have a window-unit air conditioner, turn it off or switch it to recirculate mode so it doesn’t pull in outside air, and leave it that way for at least an hour after spraying ends. If you’re caught outside during fogging, avoid getting the spray in your eyes or on your skin. Rinse your eyes immediately with water if contact occurs, and wash exposed skin with soap and water.
Before fogging starts, bring laundry, children’s toys, pet food bowls, and pets inside. Cover outdoor tables, play equipment, and ornamental fish ponds. Wash any items that were left outside with soap and water. Homegrown fruits and vegetables should be washed thoroughly before eating, even if you didn’t see direct spray contact. People with chemical sensitivities or pre-existing respiratory conditions may want to take extra precautions, such as staying indoors longer or sealing gaps around doors.
Risks to Pets and Wildlife
Dogs tolerate permethrin relatively well, similar to humans. Cats are a different story. Felines break down permethrin much more slowly, and exposure to concentrated permethrin products (45% to 65% formulations, such as certain dog flea treatments) can cause anxiety, loss of coordination, muscle tremors, seizures, and death. The concentrations used in outdoor fogging are far lower than flea treatment products, but keeping cats indoors during and after spraying is still a sensible precaution.
Both dogs and cats that contact treated surfaces may paw at their face, twitch their skin or ears, or roll on the ground. Animals that lick treated areas may drool excessively or smack their lips repeatedly. These are signs of mild irritation rather than serious poisoning at typical fogging concentrations, but they’re worth recognizing.
The bigger ecological concern is non-target insects and aquatic life. Malathion is highly toxic to honeybees and other pollinators, and both pyrethroids and organophosphates can harm aquatic organisms when they drift into waterways. Pyrethroids bind tightly to soil and sediment, which limits their spread but can concentrate exposure for bottom-dwelling aquatic species. This is one reason fogging programs typically spray in the evening, when bees are less active and wind speeds are lower, reducing drift.