Burning incense, as is commonly done during meditation, religious ceremonies or simply for the fragrance, exposes people to a dangerous amount of cancer-causing toxins in the smoke. Researchers collected air samples from one temple in Taiwan, where incense is burned often, and compared them to air samples from outdoors and at a traffic intersection. They found:

• Levels of polycyclic aromatic hydrocarbons (PAHs), a group of highly carcinogenic chemicals that are released when certain substances are burnt, were 19 times higher inside the temple than outside.
• Burning incense created more pollution than at a local traffic intersection.
• Levels of one PAH that causes lung cancer were up to 45 times higher in the temple than in homes where residents smoked tobacco, and up to 118 times higher than in areas with no indoor source of combustion, such as cooking fires.

Although this study was conducted in a poorly-ventilated temple where incense is burned regularly, you are still releasing these chemicals into your home if you burn incense. To keep your cancer risk to a minimum, your safest bet is to not release any extra pollutants into your home, including those from incense, candles, air fresheners or other synthetic fragrances.

Bulletin of Environmental Contamination and Toxicology 2001 Sep;67(3):332-8

NewScientist.com