Smoking! ( from on food and cooking )

The temperature of charred wood and smoke is below 1,800°F / 1,000℃

Charred wood and smoke are products of the incomplete combustion of organic materials in the presence of limited oxygen and at the relatively low temperatures of ordinary burning( below 1,800°F/ 1,000℃). Complete combustion would produce only odorless water and carbon dioxide. 

The nature of wood

The wood consists of three primary materials; cellulose, hemicelluloce, lignin

Cellulose and hemicellulose are both aggregates of sugar molecules and form the framework and the filler of all plant cell walls.

Lignin is a reinforcing material that binds neighboring cell walls together and gives its strength. The higher the lignin content of the wood, the harder it is and the hotter it burns.

How burning transforms wood into flavor

Wood component % of dry weight	Combustion Temperature	Combustion By-Products and Their Aromas
Cellulose ( Cell-wall frame, from glucose )  40-45 %	540 - 610°F     280 - 320°F	Furans: sweet, bready, floral Lactones: coconut, peach Acetaldehyde: green apple Acetic acid: vinegar Diacetyl; buttery
Hemicellulose  (cell wall filler, from mixed sugars )  20-35%	390 - 480°F    200 - 250°F	the same as above
Lignin  (Cell wall strengthener, from phenolic compounds )  20-40%	750°F       400℃	Guaiacol: smoky, spicy Vanillin: vanilla Phenol: pungent, smoky Isoeugenol: sweet, cloves Syringol: spicy, sausage-like

The smoke flavor is determined by the variety of wood and the combustion temperature.
The combustion temperature is partly determined by the wood and its moisture content. Maximum flavor production takes place at relatively low, smoldering temperatures, between 570°F - 750°F / 300 - 400℃. (at higher temperatures, the flavor molecules are themselves broken down into simpler harsh or flavorless molecules. )


Smoke flavor itself is unstable
Though smoke helps stabilize the flavors of meats and fish, smoke flavor itself is unstable. The desirable phenolic compounds are especially reactive, and largely dissipate in a few weeks or months.

The toxin in wood smoke!
In the beginning, smoking was not just a way of giving food an interesting flavor; it was a way of delaying their spoilage. Wood smoking contains many chemical compounds; formaldehyde, acetic acid etc( these make the smoking pH acid - pH2.5).
Many of the phenolic compounds in wood smoke are also antimicrobilals, and phenol itself is a strong disinfectant. The phenolic compounds are also effective antioxidants, and slow the development of rancid flavors in smoked meats and fish.

PAHs (polycyclic aromatic hydrocarbons)
This is an antihuman compounds, substances that are harmful to our long-term health. PAHs is formed from all of the wood components in increasing amounts as the temperature is raised. The deposition of PAHs on meat can be minimized by limiting the fire temperature, keeping the meat as far as possible from the fire.


*This is from other article... ( the condition of PAHs formation is different though... )
PAHs are typically formed during the incomplete burning( relatively low temperature?) of organic material including coal, oil, gasoline and garbage. PAHs are also found in crude oil, coal tar, creosote and asphalt. PAHs are associated with human activities (the combustion of fossil fuels) and natural occurrences (such as forest fires), and they are considered to be ubiquitous in the environment at some level. 

( ref. http://www.mass.gov/dep/toxics/pahs.htm)

PAH cancer risk

Liquid Smoke

Liquid smoke is essentially smoke-flavored water. Most of the flavor and preservative compounds do dissolve in water to some extent. Although PAHs are insoluble in water, some of them exist in smoking water and aggregate and sink over time. So it's best not to shake bottles of liquid smoke before use! Leave the sediment at the bottom.