A mosquito coil is mosquito-repelling incense, usually shaped into a spiral, and typically made from a dried paste of pyrethrum powder. The coil is usually held at the center of the spiral, suspending it in the air, or wedged by two pieces of fireproof nettings to allow continuous smoldering. Burning usually begins at the outer end of the spiral and progresses slowly toward the centre of the spiral, producing a mosquito-repellent smoke. A typical mosquito coil can measure around 15 cm in diameter and lasts around 8 hours. Mosquito coils are widely used in Asia, Africa, and South America.
Pyrethrum was used for centuries as an insecticide in Persia and Europe, and the mosquito coil was developed around the 1890s by a Japanese business man, Eiichiro Ueyama. At that time in Japan, people usually mixed pyrethrum powder with sawdust and burned it in a brazier or incense burner to repel mosquitoes. Initially, Ueyama created incense sticks mixed from starch powder, dried mandarin orange skin powder, and pyrethrum powder. However, the bar-shaped mosquito sticks quickly burned up in about 40 minutes, too brief for a long repelling action. In 1895, his wife Yuki proposed making the sticks thicker and longer, and curling them in spirals. In 1902, after a series of trials and errors, he finally obtained the desired incense burning effect out of a spiral-shaped mosquito repellent. The method involved cutting from a thick bar of incense to a certain length and manually winding it. This method continued to be used until 1957, where mass production was made possible through machine punching, making a far larger manufacturing scale possible. After the Second World War, his company, Dainihon Jochugiku Co. Ltd, set up joint-venture firms in various countries, such as China and Thailand, to produce products suited to local conditions.
Active ingredients found in mosquito coils may include
- Pyrethrum - a natural, powdered material from a kind of chrysanthemum plant; performance moderate
- Pyrethrins - an extract of the insecticidal chemicals in pyrethrum
- Allethrin - sometimes d-trans-allethrin, the first synthetic pyrethroid
- Esbiothrin - A form of allethrin
- Dibutyl hydroxyl toluene (BHT) - an optional additive to prevent pyrethroid from oxidizing during burning
- Piperonyl butoxide (PBO) - an optional additive to improve the effectiveness of pyrethroid
- N-(2-ethylhexyl)-bicyclo-(2,2,1)hept-5-ene-2,3-dicarboximide (MGK 264) - an optional additive to improve the effectiveness of a pyrethroid
Modern day Mosquito coils burn without flame for around eight hours. In quantitative tests, they provide about 80% protection. Mosquito coils are also inexpensive and easy to use. They are portable and fit into normal household practices of lighting candles or incense.
Several hazards are known to be caused by mosquito coils. In 1999, sparks from mosquito coils ignited a fire that swept through a three-story dormitory building at a summer camp in South Korea; 23 people, including 19 children, died in the blaze. Recent studies showed that the smoke generated from a burning mosquito coil is of certain health concerns – one burning mosquito coil produces the same amount of particulate mass (diameter up to 2.5 μm) as 75-137 burning cigarettes would; and the emission of formaldehyde from one burning coil can be as high as that released from 51 burning cigarettes.
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