For centuries, people have burned materials from aromatic plants to ward off pests, a practice rooted in ancient traditions across different cultures. This historical method evolved into modern incense sticks and coils specifically marketed to keep mosquitoes away. To understand the true efficacy of these products, it is necessary to examine the mechanisms by which the smoke and its chemical components interact with the mosquito’s sensory systems.
Mechanism of Mosquito Repellency
Incense and coils repel mosquitoes through a combination of physical and chemical actions. The most immediate effect is the thick smoke produced by the smoldering material, which creates a physical barrier in the air. This dense cloud of combustion byproducts can confuse the mosquito, making it difficult for the insect to accurately locate a potential host by disrupting its flight path and visual cues.
The more significant action involves the chemical compounds released into the air as volatile organic compounds (VOCs). Mosquitoes primarily hunt using their sophisticated olfactory system to detect host cues like carbon dioxide, lactic acid, and other compounds from human breath and skin. The VOCs released by the burning incense or coil can interfere with the mosquito’s olfactory receptors, essentially “masking” the appealing human scents or overwhelming the insect’s ability to process those signals.
Evaluating Specific Repellent Ingredients
The ingredients used in mosquito-repellent incense fall into two broad categories: naturally derived essential oils and synthetic insecticides. Natural incenses often rely on botanically derived compounds such as citronella, lemongrass, and neem oil. Citronella oil releases a strong aroma that activates certain olfactory receptor neurons in the mosquito, creating a repellent effect. However, the efficacy of these natural compounds is often short-lived, with citronella oil providing moderate protection for only about two hours before requiring reapplication or replacement.
In contrast, commercially available mosquito coils typically incorporate synthetic compounds known as pyrethroids, which are chemically similar to natural pyrethrins found in Chrysanthemum flowers. Pyrethroids, such as allethrin or prallethrin, are powerful neurotoxins that target the insect’s nervous system. These chemicals work by binding to and prolonging the opening of voltage-gated sodium channels in the mosquito’s nerve cells, leading to hyperexcitation, tremors, and a rapid effect known as “knockdown”. This insecticide action, combined with a spatial repellency component, makes synthetic coils highly effective at clearing a localized area of mosquitoes.
Practical Effectiveness and Limitations
The practical effectiveness of incense is highly dependent on the environment. Incense and coils are primarily designed to repel mosquitoes in a small, localized area, and their performance drops significantly in open or breezy conditions. Wind rapidly disperses the smoke and the active VOCs, preventing the formation of a concentrated protective cloud necessary for chemical interference or physical barrier creation.
These products are best used in relatively still air, such as on a porch or covered patio, and are not a substitute for personal, topical repellents. Unlike sprays or lotions applied directly to the skin, which maintain a consistent barrier, the concentration of active ingredients from burning incense fluctuates and diminishes with distance from the source. While a coil may deter mosquitoes in its immediate vicinity, it generally fails to provide the broad, sustained protection needed for larger outdoor gatherings or windy settings.
Health and Safety Considerations
A significant concern with burning mosquito-repellent incense is the resulting indoor air pollution. The smoke generated from smoldering coils and sticks releases fine particulate matter (PM2.5) and volatile organic compounds (VOCs). These ultrafine particles are small enough to penetrate deep into the lungs, posing a risk to respiratory health.
Studies have shown that burning a single mosquito coil can release a mass of PM2.5 equivalent to burning 75 to 137 cigarettes, along with formaldehyde emissions comparable to 51 cigarettes. The smoke also contains known or suspected carcinogens, including polycyclic aromatic hydrocarbons (PAHs). This is particularly problematic when coils are used indoors or in poorly ventilated spaces. Safe use requires adequate ventilation to minimize inhalation of these toxic combustion products.