Candles provide ambiance, light, and fragrance in many homes. Because they involve an open flame, a common safety question concerns the release of harmful gases, particularly carbon monoxide (CO). CO is a dangerous, odorless, and colorless gas that can be toxic even at low concentrations. Understanding the combustion process of a candle is essential for assessing the potential risk of CO indoors.
The Direct Answer: Carbon Monoxide Production
The short answer is that yes, a burning candle does produce carbon monoxide, but typically only in trace amounts that are not considered a health concern under normal use. Any process involving combustion will create some amount of CO as a byproduct. Properly burning candles release negligible levels of the gas, usually about 0.05 to 0.1 parts per million (ppm) per hour. For perspective, the Environmental Protection Agency’s standard for outdoor CO is 9 ppm averaged over eight hours. The minute quantities released by a candle are quickly diluted in a standard room. Dangerous CO levels are associated with malfunctioning appliances like furnaces or vehicle exhaust, not typical candle use.
The Chemistry of Incomplete Combustion
The creation of carbon monoxide is a direct result of incomplete combustion. When a candle burns, the wax is drawn up the wick, vaporized, and reacts with oxygen in the air. The ideal reaction, called complete combustion, occurs when there is an adequate supply of oxygen (O₂) to convert the hydrocarbon fuel entirely into carbon dioxide (CO₂) and water vapor (H₂O). When the supply of oxygen is limited relative to the fuel, the combustion becomes incomplete. This incomplete process results in the formation of byproducts, including solid carbon particles (soot) and carbon monoxide (CO). The characteristic yellow color of a candle flame is a visual indicator of incomplete combustion, coming from tiny, superheated carbon particles suspended in the flame. When these particles escape the flame without being fully burned, they condense as soot, which increases the likelihood of CO generation.
Practical Factors That Increase CO Risk
Several practical factors can disrupt a candle’s efficient burn, pushing the reaction toward incomplete combustion and increasing CO output. Poor ventilation is the primary environmental factor, restricting the constant supply of fresh oxygen the flame needs to burn cleanly; burning a candle in a small, enclosed space can lead to a higher accumulation of combustion byproducts. The physical condition of the candle is also an important variable a user can control. For example, an untrimmed or excessively long wick draws up more wax than the flame can efficiently consume, creating a larger, smoky flame that starves the core reaction of oxygen. This excess fuel leads to more soot and a higher rate of carbon monoxide production, and a flame that flickers excessively due to drafts or struggles to burn cleanly (tunneling) is a visual sign of unstable combustion.
Beyond CO: Other Candle Emissions
While the carbon monoxide risk from a candle is minimal under typical conditions, the flame releases other byproducts that can affect indoor air quality. The most common and visible of these is particulate matter, or soot, which are fine carbon particles resulting from incomplete combustion. These microscopic particles can be inhaled, and excessive exposure to them may irritate the respiratory system, particularly for individuals with asthma or other lung sensitivities. Candles, especially those made from paraffin wax, which is derived from petroleum, can also release trace amounts of volatile organic compounds (VOCs). These compounds include chemicals such as toluene, benzene, and formaldehyde, which are released when the wax or synthetic fragrances burn. While the quantities are generally low and often fall within safety limits, using high-quality waxes, such as soy or beeswax, and ensuring good ventilation can help minimize the presence of these emissions in the indoor environment.