Do Mosquito Coils Work? The Science Behind How Mosquito Coils Repel and Kill Mosquitoes

Yes, within limits. A burning coil releases a small amount of insecticide, usually a pyrethroid, into the air. The vapor drives mosquitoes away from a small area around the coil and knocks down some of those that stay. How well it works depends on the product, how close you are and how much the air moves.

Mosquito coils are one of the oldest and most widely used mosquito repellents in the world, and households across Asia, Africa, Latin America and the Pacific Islands use them to keep mosquitoes away. If you are new to coils, start with our complete guide to mosquito coils.

National regulators, such as the US Environmental Protection Agency, register coil products one by one, so check the registration number on the pack.

In this article:

• How mosquito coils work
• How far protection reaches
• How long protection lasts
• What affects effectiveness
• Coils compared with other methods
• Different mosquito species
• How to get the best results
• Why coil quality matters
• Questions people ask

A mosquito coil is a pressed spiral of wood powder or charcoal powder that carries a small dose of an insecticide, usually a synthetic pyrethroid such as allethrin, esbiothrin, metofluthrin or dimefluthrin; our article on the pyrethroid actives inside a coil explains each one. Once the tip is lit and the flame blown out, the coil smolders slowly toward the center. The heat vaporizes the active ingredient, which drifts into the air with the smoke.

The vapor acts on mosquitoes in three ways:

1. Repellency. Mosquitoes sense the vapor and avoid flying into the treated air. This is the main protective effect, and it starts within minutes of lighting.
2. Irritation. Mosquitoes that enter the vapor fly erratically, become disoriented and tend to leave before they bite.
3. Knockdown. Mosquitoes that stay in the vapor long enough absorb enough insecticide to be paralyzed and fall, and at a high enough dose they die.

Together these effects protect everyone near the coil, not only the person who applies a repellent.

Not far. A coil protects a small zone around it, not a whole house or garden. As a rough guide, expect protection within about 2 meters of the coil in still air, and less in a breeze or near an open door, where the vapor is carried away quickly. Coils made with highly volatile actives such as metofluthrin or dimefluthrin spread their vapor more readily.

In practice:

• Place the coil within about 2 meters of the people you want to protect, near the door, window or seating area where mosquitoes come in (indoors, where moving air carries the smoke out of the room), never beside a bed.
• Shelter it from the wind: moving air thins the vapor and shortens the burn.
• For a larger outdoor area, use several coils around its edge rather than one in the middle.

A coil protects only while it burns. How long it burns depends on its size, formulation and density and on the conditions around it, so check the burn time printed on the pack. Our guide to mosquito coil burn time and coverage area compares the common sizes.

Unlike a skin repellent, a burning coil keeps working for hours without reapplication. In a well-made coil the vapor stays effective from start to finish, because formulators add antioxidants such as BHT (butylated hydroxytoluene) that stop the active ingredient from breaking down as the coil smolders. Larger (jumbo) coils burn longer and are made for outdoor commercial and agricultural use.

Burn a coil only for as long as you need it, and put it out before you leave the area or go to sleep. Most coils can be put out partway through and relit later.

Several things you can control change how well a coil works:

• Air movement. Still or gently moving air keeps the vapor around the coil; wind thins it and carries it away before it can work. Outdoors in wind, shelter the coil or use more than one.
• Room size and ventilation. Indoors, coils belong only in well-ventilated rooms. In a large space, such as a ventilated living room or veranda, or under a high ceiling, the vapor is more diluted, so one coil protects less of it.
• Coil quality. Coils made with accurately measured actives, the right binder ratio, even compression and good base materials burn evenly and release their active steadily. For brands, this is why the choice of partner for OEM and private-label mosquito coil manufacturing matters.
• Mosquito species and resistance. Some mosquito populations have become partly resistant to pyrethroids. Formulations with a synergist such as PBO (piperonyl butoxide) are designed to help against resistant mosquitoes.
• Placement. A coil near the door or window where mosquitoes come in protects better than one in the middle of a room or on a high shelf.

No single method suits every situation. This is how coils compare with the main alternatives:

Many households combine them: a coil for evenings outdoors or in ventilated living areas, a bed net or plug-in vaporizer for sleeping, and a skin repellent for time outdoors. For detailed comparisons, see coils vs electric vaporizers, spray vs coil and citronella candles vs coils; for homes in Nigeria and Kenya, read coil or net in malaria season.

Coils act on all the main disease-carrying mosquito groups, but how well depends on the species, when it bites and how resistant local populations are to the active ingredient:

• Aedes, including Aedes aegypti, which spreads dengue, Zika, chikungunya and yellow fever, bites mainly during the day, around homes. A coil used outdoors or in a ventilated living area can help while people are up and about, though local resistance to pyrethroids can reduce the effect.
• Anopheles, the mosquitoes that transmit malaria, bite mostly at night, often while people are asleep. A coil can help in the evening, but it must be put out before anyone goes to sleep, so an insecticide-treated bed net is the main protection at night.
• Culex, which can transmit lymphatic filariasis, Japanese encephalitis and West Nile virus, generally responds to pyrethroid coils, although some urban populations have developed resistance. Formulations with a synergist such as PBO are designed for these cases.

For brands and distributors, local species and resistance data should guide the choice of active ingredient and synergist, a question to raise with your mosquito coil manufacturer.

Follow the directions on your product's label first: rules are set by each country's regulator, and some labels allow outdoor use only. Within those directions:

1. Light it 15 to 20 minutes before you need it, for example before you sit down outside for dinner, so the vapor builds up before mosquitoes arrive.
2. Burn it outdoors or in a well-ventilated space. Never burn a coil in a closed room or in a room where someone is sleeping.
3. Place it where mosquitoes come in: near a door, window or gap rather than in the middle of the room, and out of drafts.
4. Keep it low and secure. Many mosquitoes fly low and bite ankles and legs. Use a metal holder on a non-combustible surface, at least 1 m from anything flammable and out of reach of children and pets.
5. Use several coils outdoors, spaced around the area you want to protect, so their vapor zones overlap.
6. Keep the smoke away from people who should avoid it: babies and young children, pregnant women, people with asthma, COPD or heart disease, and cats and pet birds.
7. Put it out before you leave or go to sleep. For sleeping, use a bed net, or a plug-in vaporizer as its label directs.

Choose the coil type for the setting: wood-based, low-smoke coils for well-ventilated indoor living areas and charcoal-based coils for outdoor use. Store unused coils sealed in a cool, dry place, because damp coils burn unevenly.

The difference between a well-made coil and a poor one is not always visible, but it shows when you burn it. A well-made coil smolders evenly from start to finish and releases its active ingredient at a steady rate. A poorly made one may burn unevenly, go out partway through, crumble in the box, give off excessive, irritating smoke or release too little active ingredient to repel mosquitoes.

The difference comes from control at every stage of manufacturing:

• accurate dosing of the active ingredient and even mixing through the coil;
• the right ratio of binder to base powder, so the coil is strong but burns cleanly;
• even pressing, so the density is the same throughout;
• controlled drying, to prevent warping and cracking;
• batch testing of burn time and active-ingredient content.

To see these steps on a production line, read how mosquito coils are made.

Do mosquito coils kill mosquitoes or just repel them?

Both, but mostly they repel. The vapor keeps many mosquitoes out of the area around the coil; mosquitoes that stay in it long enough are knocked down, and some die.

Can I leave a mosquito coil burning while I sleep?

No. Put the coil out before you go to sleep, and never burn one in a closed room or in a room where someone is sleeping. For the night, use a bed net, or a plug-in vaporizer as its label directs.

• Yes, close up: a coil repels and knocks down mosquitoes in a small zone, roughly 2 meters around it, and less in wind.
• The evidence on disease is uncertain: a Cochrane review found it is not known whether burning coils protects people from malaria, and WHO does not recommend coils for malaria prevention.
• Use coils outdoors or in well-ventilated spaces, keep the smoke away from babies, pregnant women and people with breathing problems, and put the coil out before you go to sleep; at night, use a bed net or a plug-in vaporizer as its label directs.