A mosquito coil is a low-cost, slow-burning insect repellent, typically shaped into a spiral, widely adopted across the globe. This form of pest control is popular in tropical and subtropical regions where mosquito-borne diseases are prevalent. The coil’s primary function is to release an insecticide into the air as it smolders, offering a localized protective zone against biting insects. This article examines the scientific mechanism of mosquito coils, their effectiveness, and the associated health risks.
The Science Behind Mosquito Coils
A mosquito coil works by coupling a slow-burning matrix with active insecticidal compounds that are vaporized by the heat. The coil is composed of a combustible binding material, often dried plant matter like sawdust or coconut shell powder, which allows it to smolder for several hours without an open flame. This smoldering action is a form of thermal vaporization, slowly releasing the active ingredients into the surrounding air via the smoke.
The insecticidal component is primarily a synthetic pyrethroid, such as allethrin, d-allethrin, esbiothrin, or metofluthrin, which are analogs of natural pyrethrins derived from chrysanthemum flowers. These pyrethroids are neurotoxins that target the nervous system of insects, causing a rapid “knockdown” effect followed by paralysis or death. The concentration of the active ingredient ranges from 0.3% to 0.4% of the coil’s total mass.
Some coils also include synergists, like piperonyl butoxide (PBO), which enhance the effectiveness of the pyrethroid by inhibiting the mosquito’s natural detoxification enzymes. The smoke’s odor can also act as a simple repellent, deterring mosquitoes from entering the immediate vicinity.
Quantifying Effectiveness
Scientific studies confirm that mosquito coils can significantly reduce mosquito activity. Pyrethroid-based coils are designed to both deter mosquitoes from biting and incapacitate those that enter the treated area. Field studies have demonstrated that burning a coil can reduce mosquito biting rates by over 50% in areas with minimal air movement.
In semi-enclosed spaces, a coil’s insecticidal impact is more pronounced due to the higher concentration of vaporized chemical. For instance, in laboratory settings, a single coil containing d-allethrin can achieve a high percentage of mosquito-landing reduction, even at distances up to 7 meters, after an hour of continuous burning. The initial efficacy is observed within the first 10 to 15 minutes of ignition near the coil, with the protective radius expanding as the chemical disperses.
Efficacy is highly dependent on environmental factors, particularly air movement. In open, outdoor spaces, a light breeze can help distribute the repellent smoke, but stronger winds quickly dissipate the active ingredients, drastically reducing the effective coverage area, which is typically a radius of only 1 to 2 meters in calm conditions. The concentration of the active ingredient and the dispersal rate are also highly variable between different product brands and formulations.
The primary role of the coil is to provide a temporary zone of protection against bites, rather than complete mosquito eradication. Relying solely on coils to prevent mosquito-borne diseases is not recommended, as a single bite can still transmit a pathogen. Studies on whether coil use reduces the incidence of diseases like malaria or dengue have been inconclusive or showed no strong evidence of prevention.
Safety Profile and Health Risks
The most significant health concern associated with burning mosquito coils is the smoke they generate, which is a source of inhalable particulate matter (PM). Burning a single mosquito coil can release particulate matter equivalent to burning between 75 and 137 cigarettes, posing a substantial risk, especially when used indoors. The smoke contains fine particles, specifically PM2.5 and PM10, which can penetrate deep into the lungs and potentially trigger or exacerbate respiratory conditions like asthma.
Beyond particulate matter, the smoldering process releases toxic compounds, including carbon monoxide (CO), sulfur dioxide (SO2), and volatile organic compounds (VOCs) such as formaldehyde and benzene. Formaldehyde, a known human carcinogen, is released as a byproduct of combustion, and its concentration from a single coil can be comparable to that of numerous cigarettes. Exposure to these pollutants, especially in poorly ventilated environments, is linked to chronic respiratory irritation and an increased risk of long-term health issues.
While the active pyrethroid insecticides are considered low-toxicity to mammals due to their rapid metabolism, chronic, low-level exposure is still a concern. Pyrethroids can cause acute symptoms in humans, such as eye and skin irritation, headache, and dizziness. Certain household pets, such as cats and fish, are particularly sensitive to pyrethroids, and exposure to the coil smoke can be highly toxic to them.
Some coils have also been found to contain octachlorodipropyl ether (S-2), which can degrade into bis(chloromethyl)ether (BCME), a lung carcinogen. Due to the complex mixture of combustion byproducts and active ingredients, public health authorities advocate for strict regulatory oversight and cautious use.
Proper Use and Environmental Factors
To mitigate the health risks associated with smoke inhalation, mosquito coils should only be used in well-ventilated areas, ideally outdoors or in semi-open spaces. Never use a coil in a closed room, especially while sleeping, as this allows the toxic smoke and chemical vapors to accumulate to hazardous levels. This allows the smoke and active ingredient to disperse effectively without excessive human exposure.
For outdoor use, the coil should be placed upwind of the people it is meant to protect, allowing the smoke plume to drift toward the area of interest. This placement maximizes the repellent effect while keeping the source of the smoke at a safe distance from occupants. Limiting the duration of exposure reduces the cumulative health risk, so the coil should be extinguished once protection is no longer needed.
Environmental concerns include the potential for pesticide runoff and harm to non-target insects. The active pyrethroids are toxic to insects, not just mosquitoes, and their introduction into the environment through wastewater or improper disposal contributes to broad pesticide exposure. Proper disposal of the ash and unburnt coil material minimizes the risk of environmental contamination.