Candle burning in residences and in other locations in indoor environments is a widely-practiced and increasingly popular phenomenon. This is also true for incense as well.

Unfortunately, candle/incense burning is not as innocuous as North Americans and Europeans perceive. Candles of both the scented and unscented forms emit a variety of byproducts on burning. These byproducts may be generic (common to all combustion processes) such as carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), aldehydes, unburned/partially burned hydrocarbons and particulate matter (soot particles).

They may also be specific to the product. Candle specific by-products include a variety of aldehydes, alcohols and esters which are responsible for the odor/aroma associated with particular candle types. In some instances the candle wick may contain lead. Though U.S. manufacturers are prohibited from using lead wicks, they are nevertheless present in the U.S. candle market (due to foreign imports).A study of candles in Washington, D.C./Baltimore area revealed that 3% of candle types found in local stores contained lead wicks.

When lead is heated, it volatilizes to produce very fine particles of lead oxide which are easily inhaled and deposited in lung tissue. In the lung lead is leached from deposited particles and enters the blood stream. High blood levels can cause a variety of acute/chronic cardiovascular and nervous system symptoms. Lead exposure can continue for weeks/months after candle burning is discontinued( as a result of the resuspension of lead particles deposited on building surfaces).

The nature of gas-phase emissions from scented candles is largely unknown. There are few scientific studies that have attempted to characterize what the odor-producing compounds are either in new candles or those which are being burned. It is likely that candle manufacturers use multiple dozens of odor-producing compounds which produce scents associated with their products. The safety/toxicity of these compounds is unknown. In candle burning a number of new odor-producing compounds are also likely to be produced. For some individuals exposure to the active ingredients in a variety of scented products such as perfumes, colognes, aftershave lotions, soaps are reported to cause illness symptoms. These are largely of an anecdotal nature (not based on scientific studies). As a consequence of such concerns, anti-scent activists are trying to persuade governmental bodies and employers to establish "no scents" policies.

The pollutant of most public health concern associated with candle and incense burning is soot particles. Soot particles are very tiny and most can be easily inhaled and deposited in lung tissue. Such particles can be highly irritating. They have the potential to cause localized irritation of the respiratory airways and lung tissue. Because of such irritation/inflammation, they may pose a special risk to asthmatic children and adults. Soot particles generated by candle burning also contain significant quantities of polycyclic aromatic hydrocarbons (PAHs), a group of compounds which are well known for their cancer-causing potency.

As you can see, the acute illness you experience around lighted candles may be due to a variety of candle-produced airborne contaminants. Because of acute and chronic exposures, burning candles in one's home on a day-to-day basis or even several times a week (for several hours or more) is not particularly wise.