Bug spray is a common tool for outdoor protection, but its complex mixture of ingredients often raises questions about fire safety. Flammability concerns stem from the highly volatile compounds used in the spray. Clarifying the fire risk requires distinguishing between the product when it is first applied and the chemical residue left behind after the solvents have evaporated.
The Immediate Fire Hazard
The primary fire risk occurs during application and immediately afterward, while the product is still wet. This hazard is attributed to the volatile organic compounds (VOCs) and aerosol propellants used to deliver the active ingredient. Many aerosol formulations rely on extremely flammable liquefied petroleum gases like propane, butane, or isobutane.
These propellants have flash points far below room temperature, such as isobutane’s flash point around \(-117^\circ\text{F}\). Non-aerosol pump sprays often use alcohol-based carriers, like isopropanol, which also has a very low flash point of approximately \(53^\circ\text{F}\). The vapor plume created during spraying or the wet film on the skin can easily ignite when exposed to an open flame or spark, resulting in a brief, intense flare-up. This risk dissipates quickly as these highly volatile solvents and propellants evaporate.
Flammability of the Dried Chemical Residue
Once the solvents and propellants have completely evaporated, the high risk of flash ignition diminishes significantly. The remaining residue is primarily the active repellent ingredient, such as DEET or Picaridin, mixed with non-volatile carrier oils. These compounds are combustible, meaning they will burn if subjected to a continuous, direct flame or intense heat, but they are not highly flammable like the wet spray.
The difference lies in the flash point, which is the lowest temperature at which a liquid produces enough vapor to form an ignitable mixture with air. The active ingredient DEET has a relatively high flash point of \(203^\circ\text{F}\) (\(95^\circ\text{C}\)) in its pure form. Since this temperature is much higher than ambient air temperature, the dried residue does not readily produce enough flammable vapor to ignite spontaneously. The residue can still act as a fuel source, particularly if the spray has saturated clothing fibers.
Heavy application, especially onto clothing, creates a concentrated film of combustible material that poses a sustained fire risk if it contacts a flame. This is relevant for materials like synthetic fabrics, which can absorb the repellent and potentially increase the intensity or duration of a fire once ignited. Even after the product feels dry, a lingering combustibility risk remains, which is why warnings advise caution near ignition sources.
Chemical Composition and Fire Risk
The fire hazard of an insect repellent product is directly linked to the chemical properties of its various components. Bug sprays contain a combination of active ingredients, carrier solvents, and, in aerosol forms, propellants, each contributing differently to the overall fire profile. The immediate danger is driven by the propellants and solvents, which have extremely high volatility and low flash points.
Liquefied propellants like propane and butane are stored as liquids under pressure, and their rapid release creates a highly combustible vapor cloud. Carrier solvents, such as isopropyl alcohol, are designed to dissolve the active ingredient and facilitate an even application. Their low flash points make the wet mixture instantly flammable, but these components are designed to evaporate quickly, removing the immediate fire threat.
In contrast, the active repellent compounds, such as DEET and Picaridin, are non-volatile oils or synthetic molecules with much higher flash points. While DEET itself is classified as combustible, its flash point is high enough that it only poses a fire risk when exposed to direct flame or a sufficiently hot heat source. Picaridin is even less volatile, meaning its vapors are not a source of flash ignition, but the material itself will burn if it reaches its combustion temperature.
Safety Measures for Application and Storage
Minimizing the fire risk begins with observing proper application techniques. It is important to maintain a safe distance from all ignition sources, including lit cigarettes, grills, campfires, and pilot lights, while actively spraying the product. After application, allow the repellent to dry fully on the skin and clothing before approaching any heat source to permit the volatile solvents and propellants to dissipate.
When treating clothing, apply the product outdoors and let the fabric thoroughly air-dry before wearing it. This prevents the trapping of flammable vapors and reduces the risk of the residue acting as a sustained fuel source. Proper storage also plays a role in safety. Aerosol cans and containers of flammable liquids should be kept in a cool, dry area away from direct sunlight and potential sources of high heat or sparks, as storing cans in a hot vehicle can lead to dangerous pressure build-up and explosion.