How does a mosquito coil work?
A mosquito coil is a slow fuse wound into a spiral: nearly 70 cm of pressed powder that an ember crosses at 1.4 mm a minute, warming a few milligrams of insecticide into the air just ahead of itself. See why it is a spiral, what the vapour does to a mosquito's nerves, and why opening a window cuts the smoke and the protection by exactly the same share.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Two spirals, a stand and an ember. A coil comes as a flat disc that is really two spirals nested together; you snap them apart and stand one on a metal prong. It is a slow-burning base of sawdust, coconut-shell powder or charcoal powder, a binder, dye and fragrance, and the part that matters: a synthetic pyrethroid insecticide. Indian labels show 0.005% to 0.1% for the common coils, so a 13 gram coil carries only a few milligrams.
- A night-long fuse in a 12 cm disc. The length of a spiral is very nearly the area of its ring divided by the spacing of its turns: about 68 cm for a common coil. The ember creeps at about 1.4 mm a minute, so the coil lasts about eight hours. The glow itself would destroy the insecticide, but heat runs a few millimetres ahead of it, and at 150 to 300 °C the active turns to vapour and leaves before the fire arrives.
- Gates held open. A mosquito finds you by the carbon dioxide in your breath from 10 to 50 metres, by your shape from 5 to 15 metres and by your warmth in the last metre. Her nerves fire using sodium channels that open for a thousandth of a second. A pyrethroid holds them open, so one touch becomes a burst of signals: at a faint dose she is restless and fails to bite, at a higher dose she is knocked down. Insect channels grip pyrethroids about a thousand times more strongly than mammal ones, and a warm body and a liver do the rest. Cats and fish are sensitive.
- The window cuts both. A coil adds two things to a room: the vapour, and smoke. Chamber tests found 63 to 127 mg of fine particles (PM2.5) an hour from ordinary coils, enough to lift a shut 30 m³ bedroom into the thousands of micrograms per cubic metre, against a WHO guideline of 15 for a day's average. One coil gives off as much PM2.5 mass as 75 to 137 cigarettes, which measures the dust released, not what a sleeper breathes. Fresh air removes the same share of smoke and vapour: there is no setting that clears one and keeps the other.
- The same idea without the ember, and without the chemical. Liquid vaporisers and mats warm a similar active with a small electric heater, so there is no smoke from burning. Nets, screens, fans and emptying standing water add nothing to the air at all, and a treated bed net is the WHO's main recommendation where malaria spreads. Coils do cut bites, but a Cochrane review found we do not know whether they prevent malaria. Ultrasonic gadgets have failed every fair field test.
- Rules, care and resistance. In India a coil is an insecticide product under the Insecticides Act, 1968: it must be registered, and the pack must show the active, its strength and a registration number. Use it on its stand, away from bedding, with some fresh air. Where pyrethroids are used heavily, mosquitoes carrying a slightly different sodium channel survive and multiply; this knock-down resistance has been found in India's malaria and dengue mosquitoes. India reported about 2.6 lakh malaria cases and 2.3 lakh dengue cases in 2024.
| Term | Meaning |
|---|---|
| Mosquito coil | A spiral of pressed, slow-burning powder that carries a little insecticide and smoulders for hours. |
| Pyrethroid | A synthetic insecticide modelled on pyrethrins, the natural insect-killers of pyrethrum daisies. |
| Archimedean spiral | A spiral with evenly spaced turns; its length is about the area of its ring divided by the spacing. |
| Smoulder | Burning without a flame: a glowing ember creeping through a solid. |
| Sodium channel | A protein gate in a nerve's membrane that opens briefly to make a voltage spike. Pyrethroids hold it open. |
| Spatial repellency | Mosquitoes avoiding or leaving a space because of a chemical in its air. |
| PM2.5 | The mass of particles finer than 2.5 micrometres in a cubic metre of air. |
| Air changes per hour | How many room-fulls of fresh air replace the old air each hour. |
| Knock-down resistance (kdr) | A small change in an insect's sodium channel that makes pyrethroids bind less well, spread by selection. |
Two hundred years from a daisy ground into powder to a spiral that billions light at dusk, and the science still catching up with it.
- 1886 · Seeds cross the Pacific (Eiichiro Ueyama and H. E. Amoore, Wakayama, Japan)
- 1890 · The first mosquito incense stick (Eiichiro Ueyama, Wakayama, Japan)
- 1895 · Yuki Ueyama suggests a spiral (Yuki Ueyama, Wakayama, Japan)
- 1897 · The mosquito is caught carrying malaria (Ronald Ross, Secunderabad, India)
- 1902 · The spiral coil goes on sale (Eiichiro and Yuki Ueyama, Osaka, Japan)
- 1949 · Allethrin: the first man-made pyrethroid (Milton S. Schechter and colleagues, United States)
- 1963 · The electric mat: vapour without an ember (Fumakilla, Hiroshima, Japan)
- 1967 · How it jams a nerve (Toshio Narahashi, Duke University, United States)
The full story, with 31 moments, charts, people and 43 sources: glassbox.how/e/mosquitocoilclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
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It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
mosquitocoilclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
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python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/coil.js |
This box's own helpers: the spiral, heat, room, nerve and resistance sums, chart boards, particles, the 3D coil and stand |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
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