Incense, used primarily for religious, medicinal and meditative purposes, was found to create air quality environments hazardous to human health, according to reports by New Scientist magazine.
A set of chemicals called polycyclic aromatic hydrocarbons are of primary concern. While organic and often aromatic, PAH include carcinogenic chemicals such as the components of benzene and the chemical used in mothballs.
A 2001 study by the National Cheng Kung University of Taiwan surveyed the air quality inside a poorly-ventilated temple and found benzopyrene levels that are 40 times greater than what is found inside of the homes of people who smoke cigarettes. The air quality in the temple was almost as bad as standing next to a busy intersection filled with car exhaust.
A similar study done in The Netherlands found that the air quality of two Maastricht churches had 20 times the amount of chemicals compared to an intersection.
The lead researcher for the Taiwanese study, Ta Chang Lin, expressed concern to New Scientist about the quality of the air in places of religious ceremonies.
"The concentrations of PAHs inside the temple depends on how many visitors come to worship that day. During some major ceremonies, hundreds or even more than a thousand sticks are burnt at the same time," Lin told the magazine.
The air quality in the temple was below standards set in Taiwan.
PAH chemicals are released from the burning of certain organic items, ranging from incense sticks to steaks on a barbecue grill.
According to Dr. Ben Kim, a natural health advocate, the studies serve "as a good reminder to allow fresh air to circulate throughout one's living space," he writes on his web site. Kim recommends keeping windows open in the summer "whenever possible" and to try to keep windows open in the winter if weather and temperature permits.
Assuming the neighborhood you live does not have polluted air, it is "beneficial to one's health to simply open a few windows for a minute or two each day to allow some fresh air to enter the residence," he writes.
Incense is used ceremonially in the Buddhist, Christian and other religions.