1. How the Insecticide Is Released Before the Coil's Hot Flame Destroys It
The burning tip of a mosquito coil is extremely hot, reaching 700 to 800°C. Yet, according to manufacturer explanations, the insecticide ingredient called allethrin enters the air from a zone about 6 to 8mm behind the burning part, where the temperature is around 250°C. [2] The "exit point" for the ingredient is not the flame itself, but the area just before it.
This phenomenon is similar to how soap scent spreads in a warmed room. When heat is applied, ingredients inside a solid turn into gas and mix with the air. However, with mosquito coils, the ingredient is released from the part at about 250°C, rather than from the overly hot tip itself. [2]
Is smoke irrelevant? The Japan Household Insecticide Industry Association explains that the ingredient volatilizes due to the heat of combustion and spreads along with the smoke. [5] From this, it is reasonable to think that the ingredient is the main active ingredient, while the smoke serves to carry it and make the spread visible.
2. What Is in a Mosquito Coil? Pyrethrin and Pyrethroids
The natural insecticide found in the flowers of *Chrysanthemum cinerariifolium* (*Chrysanthemum cinerariifolium*) is called pyrethrin. "Pyrethroid" is the general name for compounds similar to pyrethrin. [3] Pyrethrin is contained in about 1.3% of the weight of dried flowers, and about 90% of that amount is concentrated in the ovary (the seed pod). [4] Since it is only 1.3% of the whole flower, the amount obtained from 100g of flowers is about 1.3g. It is barely present in the leaves or stems.
Modern mosquito coils are described as being made by mixing pyrethroids with plant-based powders like wood flour and a binder (such as starch), then shaping and drying them into a spiral. [5]
3. How the Mosquito Coil Changed from a Stick to a Spiral
Eiichiro Ueyama, who invented the mosquito coil, began researching chrysanthemum cultivation in 1886. In 1890, he sold stick-shaped mosquito coils, inspired by incense sticks used in Buddhist altars. [1]
However, the stick shape produced thin smoke and released few ingredients, with a burning time of only about 40 minutes. [1] Around 1895, his wife Yuki suggested, "Wouldn't a spiral shape last longer?" This suggestion led to improvements. In 1902, the world's first spiral-shaped mosquito coil was sold. [1]
Today, one spiral coil from the KINCHO company is about 75cm long, weighs 13g, and provides stable effects for about 7 hours. [4] [7] Note that these length and duration figures are specific to this company's product and are not common to all manufacturers.
4. How Pyrethroids Affect a Mosquito's Nerves
Pyrethroids enter pests through their skin or mouth and act on their nerves, causing excitement, convulsions, and paralysis. [3]
Looking closer, insect nerves have small passages called "voltage-dependent sodium channels" that open and close. Normally, these close immediately after a signal passes. However, reports indicate that pyrethroids bind to these channels and interfere with the closing function, keeping them open for longer. [6] As a result, the nerve sends out signals repeatedly.
It is easier to imagine this not as "breaking" the nerve, but as a switch getting stuck in the "on" position.
5. Are Mosquito Coils Safe for Humans? Usage Precautions
The manufacturing company and the Aichi Prefectural Institute of Public Health and Environmental Science explain that pyrethroids that enter the body of warm-blooded animals like humans are quickly broken down and expelled from the body in a short time. [3] The detailed mechanism for why there is a difference between insects and humans has not been confirmed in the materials for this article.
Reading this as "harmless to humans" is an overstatement. The Japan Household Insecticide Industry Association warns that in a closed room, smoke can accumulate, causing mild irritation to the eyes and throat. [5] Safety Note: With an adult, use the coil in its proper holder, keep it away from anything that can catch fire, ventilate the room, and avoid touching the hot coil or holder. [5]
6. How to Investigate Mosquito Coils Further Yourself
What you can do today: With an adult, read the ingredient labels on mosquito coils or insecticide boxes at home. You might find names mentioned in this article, such as "pyrethroid" or "allethrin." You do not need to light the fire.
If you want to know more: Manufacturer articles about insects and factory tour pages are available in Japanese. Read those pages and compare their information with what you learned in this article.
Question: This article does not explain the mechanism by which pyrethroids are quickly broken down in humans. Try placing two mosquito coils or insecticide boxes you have at home side by side and compare whether the names of the active ingredients are different.