Metofluthrin

Active

Cyclopropanecarboxylic acid, 2,2-dimethyl-3-(1-propen-1-yl)-, [2,3,5,6-tetrafluoro-4-(methoxymethyl)phenyl]methyl ester

Metofluthrin is a synthetic pyrethroid used as the active vapor in modern battery- and heat-powered spatial repellent devices such as the Thermacell E-series and EX-series and the OFF! Clip-On. It is not applied to skin. A small amount is warmed so it evaporates and forms a protective zone of vapor that repels and disorients mosquitoes before they land. It is EPA-registered for this use. Field trials show a metofluthrin emanator creates roughly a 15 by 15 foot zone (about a 5 meter radius) of reduced biting. Like all pyrethroids it is highly toxic to fish and aquatic invertebrates, so devices should not be used over or beside water.

How it works

Metofluthrin is a fast-acting Type I pyrethroid. It binds to voltage-gated sodium channels in insect nerve membranes and holds them open, causing repetitive firing, loss of coordination, knockdown, and repellency before a mosquito bites. Because it is volatile it works in the vapor phase, protecting an area rather than a treated surface. Its half-life in air is short, roughly 1.7 to 1.9 hours (EPA), which limits buildup and non-target exposure in outdoor use.

Safety around dogs & cats

Cats are the sensitive species for every pyrethroid because they lack the glucuronidation capacity to clear them efficiently (ASPCA, Pet Poison Helpline, VCA), so keep cats away from the device and never let one mouth a warm cartridge or pad. Real-world risk from the outdoor vapor of an emanator is lower than from a skin or spot-on product, and the short half-life in air limits buildup, but cats warrant more caution than dogs, which tolerate pyrethroids well (Merck Veterinary Manual). Do not run a device in an enclosed space with a pet bird. This is not a product to apply to any animal.

CatsUse caution

DogsLow concern

Moderate confidence. Ratings from ASPCA Animal Poison Control, the Pet Poison Helpline, and published veterinary toxicology. This is general information, not veterinary advice. If your pet is exposed, call your vet or the ASPCA Animal Poison Control Center at (888) 426-4435.

Full guide: bug repellents around dogs and cats →

The evidence

Peer-reviewed hut and field trials (for example PMC10531799 and PMC10768102) show metofluthrin emanators meaningfully reduce mosquito landing and biting, including against some pyrethroid-resistant Aedes and Anopheles populations, over a limited zone of a few meters. Protection is spatial and area-based rather than the whole-body coverage of a skin repellent.

Volatile pyrethroid spatial repellents for preventing mosquito bites: a systematic review and meta-analysis
2025Meta AnalysisDefinitive evidence
This is the most rigorous look at the whole category to date: a systematic review and meta-analysis that pooled data from 58 eligible publications on volatile pyrethroid spatial repellents, the airborne-emanator products built on actives such as transfluthrin, metofluthrin, and allethrin. For 50 of those studies the authors combined individual mosquito-level records, more than 1.7 million mosquitoes in total, along with satellite weather data on temperature, humidity, and wind, so they could estimate an overall protective efficacy and see how it shifts by product format, active ingredient, mosquito species, and indoor versus outdoor setting. This is the single best study for setting honest expectations about what these devices deliver on average.
Experimental hut and field evaluation of a metofluthrin-based spatial repellent against pyrethroid-resistant Anopheles funestus in Siaya County, western Kenya
2024Field TrialWell-supported evidence
This study, run in experimental huts and open field settings in Siaya County in western Kenya, tested a metofluthrin emanator against Anopheles funestus, a major malaria mosquito. What makes it especially useful is that this local mosquito population is pyrethroid-resistant, meaning it has evolved to shrug off the pyrethroid insecticides used on bed nets and in sprays. The researchers wanted to know whether an airborne metofluthrin device could still protect people even when the mosquitoes are resistant to that chemical class, measuring how many mosquitoes landed indoors and at increasing distances outdoors.
An Expert Review of Spatial Repellents for Mosquito Control
2020ReviewWell-supported evidence
This is a commissioned expert review, not a single experiment, that pulled together the published evidence on spatial repellents. Spatial repellents are the family of products that put a volatile insecticide into the air around you rather than on your skin, which is the category that includes battery or heat-powered emanators such as Thermacell and the OFF! Clip-On, passive strips, and the older mosquito coil. The review walks through how these devices work, which airborne active ingredients drive them, and what protection they have delivered in controlled and semi-field testing, with the goal of judging whether the category is promising enough for public-health investment.
OFF! Clip-on Repellent Device With Metofluthrin Tested on Aedes aegypti for Mortality at Different Time Intervals and Distances
2016LabLimited evidence
This was a controlled laboratory study, not a backyard trial, designed to map exactly how far the OFF! Clip-On's metofluthrin actually reaches. The researchers exposed caged Aedes aegypti mosquitoes, the yellow fever and dengue vector, at set distances from the running device and at different time intervals, then recorded knockdown and death. The point was to test the intuitive question every buyer has: does one of these clip-on emanators really protect a whole area around you, or only the air right next to it?
Field Evaluation of the OFF! Clip-On Mosquito Repellent (Metofluthrin) Against Aedes albopictus and Aedes taeniorhynchus
2012Field TrialWell-supported evidence
This trial tested the OFF! Clip-On, a small battery-powered fan device that blows air across a metofluthrin-treated pad to build a repellent cloud around the wearer, in outdoor conditions in northeastern Florida. The researchers wanted to know how well a wearable emanator actually protects a person from bites in the field, so they compared biting rates on volunteers using the running device against unprotected controls. They looked at two mosquitoes: Aedes albopictus, the Asian tiger mosquito that is a common backyard biter, and Aedes taeniorhynchus, an aggressive coastal saltmarsh mosquito.

Fewer bitesest.40–70%

Regulatory notes

EPA-registered active ingredient (PC code 109709) for spatial and personal emanator devices. EPA's registration review found that at labeled use none of the agency's Levels of Concern were exceeded for birds, mammals, fish, or aquatic invertebrates, largely because the amount released and the short half-life in air keep environmental exposure low. Devices are labeled for outdoor and well-ventilated use.

Safety notes

This is not a skin-applied product, so dermal sensitization and irritation are not the relevant exposure routes. The main human caution is inhalation of the vapor: EPA classed metofluthrin end-use products as slightly toxic by inhalation, so run the device outdoors or in a well-ventilated space and avoid deliberately breathing the vapor stream. Keep devices away from ponds, streams, and aquariums.