A mosquito coil is a 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.
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
Butylated hydroxytoluene (BHT) - an optional additive to prevent pyrethroid from oxidizing during burning
Piperonyl butoxide (PBO) - an optional additive to improve the effectiveness of pyrethroid
N-Octyl bicycloheptene 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.