Burning incense releases products into the air just like any other type of fire burning. When a stove windows or chimneys look black and dirty and smell bad, this is the result of these products that are released.
A simple experiment proves that the same thing happens with burning incense. If a smooth material is held for a while above an incense stick that is burning, the surface of the material first becomes sticky and after a while it turns brown. The name of this brown substance is creosote. Creosote is a mixture of all sorts of non-combusted organic chemicals, such as acrolein and aldehydes. If a person is exposed to inhaling fire released products, the same creosote forms in their lungs.
The same experiment can be repeated with burning different things, incenses or tobacco for example.
There was research done in Taiwan regarding a connection between incense and cancer. It has been concluded that the monks who had spent a long time in monasteries had a higher risk of cancer, due to their long lasting exposure to the burning sandalwood fumes. Sandalwood the type of incense the commonly use. The conclusion was that prolonged exposure to incense can be harmful for human health.
People are exposed to inhaling exhaust fumes on a daily basis. Life in urban areas enhances the risk of cancer by gases that cars produce. Even the statue of Ramses II had to be taken away from Cairo to prevent being ruined by the gases produced by factories and cars.
Human respiratory organs are equipped with vibratory hairs. These hairs prevent dirt from coming into the respiratory organs. Only prolonged exposure to smoke can make the body become unable to handle dirt. Creosote can lock up the lungs. This is what happens to smokers often.
Many Taiwanese monks were, unfortunately, exposed to smoke all of their lives. Even without scientific research, simple common sense says that this can not be healthy.
It is not recommended to keep the nose directly above an incense stick, because it can cause the nasal organ to freeze up and stop functioning properly. It is better to let the smoke quietly dissolve through space.
Ventilation is very important while using incense. Occasions when the room is full of smoke should be limited or avoided.
Incense from shops is usually a mixture of wood, perfume oils and charcoal. This composition and the quality of materials used are specially related to health issues.
In cases of allergic reactions to essential oils or signs of asthma appearing, it is recommended to avoid the use of incense altogether.