Smoke Production

Contents of Smoke

In forest fires, the two products of complete oxidation Carbon Dioxide (CO2) and water vapor make up over 90 percent of the mass emitted. The other 10 percent contains most of the problem compounds. The major products are: Carbon Monoxide, Particulate Matter, Hydrocarbons and other organic compounds. The more intense the fire, the more complete the combustion process and a bigger percent of the smoke will be carbon dioxide and water vapor.

Carbon Dioxide (CO2)

Carbon Dioxide is an odorless and colorless gas formed abundantly in nature by the decomposition of organic material including wood. It is exhaled by man and absorbed by plants from the air in the process of photosynthesis. It is also use to make carbonated drinks.

Water Vapor (H2O)

The amount of water is important because it interacts with the other combustion products reducing the efficiency of the combustion process. It can also reduce visibility for a short distance.

Carbon Monoxide (CO)

Carbon monoxide is an odorless and colorless gas that is highly toxic and regulated by EPA. Fairly high concentrations are produced but it is very light and is rapidly diluted in the open. As a result, it is not a problem.

Hydrocarbons (HC)

Hydrocarbons and other organic compounds are not produce in sufficient quantities to be a problem.

Particulate Matter

Particulate matter is complex mixture of small solid or tar (liquid) particles. The size, shape, density and other physical properties are highly variable but the individual particles are too small to be seen with the naked eye. The small pieces of partially or unburned wood that fall out fairly rapidly are not included since they cause no visibility problem but they can become firebrands .

These small particles can remain suspended in the atmosphere for long periods, even weeks. They drastically reduce visibility. The small size also penetrates into the lungs, much the same as tobacco smoke, and can aggravate breathing problems such as emphysema. Particulate matter is the major problem with smoke due to the reduction in visibility. These small particles absorb and scatter light washing out the contrast. They can also act as nuclei for the formation of fog but there are generally enough material in the air already to act as nuclei.

Visibility Effects

Health Effect

Combustion Phases

The combustion process results in a rapid release of stored energy.  The three broad phases are:  pre-ignition, flaming, and residual (smoldering). The amount and type of smoke produced will vary during each phase.

1. During pre-ignition, heat is being absorbed by the fuel. The fuel’s temperature is being raised to ignition temperature.  At the same time, all the moisture is being evaporated and driven off. Small amounts of white smoke is produced--predominately water vapor.
2. The flaming phase begins when the fuel reaches ignition temperature and erupts into flames.  The fuel is now going through a chemical change with the release of large amounts of heat and gases.  When mixed with oxygen, the heated gases ignite, rapid oxidation occurs, and large amounts of smoke is produced. The smoke is lifted off the ground by the heat that is being produced. The combustion process is very efficient during this phase. Smoke is mostly carbon dioxide and water vapor.
3. The residual (smoldering) phase occurs when the flaming front has moved on and some of the unconsumed fuel is still going through the combustion process--slowly, with little heat being released. However, a large amount of smoke is produced.  Some small amounts of flaming may be occurring in scattered areas but not enough heat is being generated to continue the smoke column.  Little fuel is being consumed even though a lot of smoke is being produced. It is darker because more particulates are being produced. This phase is caused when heat, oxygen, and flammable gases are reduced to the extent that the flaming phase is not sustained.  Reduced efficiency of the combustion process allows the temperature to drop.  With insufficient heat to produce a convective smoke column, the smoke is concentrated close to the ground.  
4. The glowing phase is when no visible smoke is seen but the fuel is glowing similar to charcoal.  Very little of this phase occurs in open forest type burning and it is not an important factor.  

Question: How does heat lift smoke?

Fuel Loading

If the amount of fuel that is consumed by fire is doubled (and all other factors remain the same), the amount of smoke produced will generally double.  However, there are many other physical and chemical factors of fuel that affect the amount of smoke.

Prescribed fire should burn off litter but not any of the humus. WHY? (Protects soil and would produce more smoke)

Fuel Moisture

Fuel Moisture is the most important of all fuel factors.  Enough heat must be present to drive off all the moisture in the fuel.  If a lot of moisture is present, much of the heat energy is used in converting the moisture to steam.  Consequently, the combustion process lacks intensity. More smoke will be produced because more of the fuel will be consumed during the residual phase.

A.  High Moisture Content

B.  Low Moisture Content


The shape of fuel has a decided effect on the combustion process.  Flat-shaped fuel has a high surface-to-volume ratio.  This allows the moisture in the fuel to escape more rapidly.  Consequently, the fuel will be drier.  It will ignite and burn readily, producing a minimal amount of smoke since a larger percentage of the fuel will be consumed during the flaming phase.  Round fuel has a lower surface-to-volume ratio.  Consequently, it will be “wetter” due to less surface area for the moisture to escape. A larger percentage of the fuel will be consumed during the residual phase, producing more smoke than flat-shaped fuel.


The size of fuel affects burning intensity and smoke production in much the same way as shape.  Small fuel has a higher surface-to-volume ratio than larger fuel that allows the moisture to escape more rapidly and results in being drier than large fuel under the same conditions.


Arrangement of fuel affects smoke production by changing both the rate of drying and the availability of oxygen required for the combustion process.  Compacted fuel dries out slowly and results in a wetter fuel. The result is a low intensity fire with most of the fuel being consumed during the residual phase.

Compacted fuel reduces combustion efficiency

Strip-heading Fire

Firing Technique

Due to the slow moving front, backing fires consume almost all of the fuel during the flaming phase.  Although less intense, more of the heat is held close to the ground and if low fuel, will generally consume as much of the fuel as a heading fire.  On the other hand, a heading fire will consume approximately half of the fuel during the flaming phase and the other half during the residual phase on a typical prescribed burn.  The net result:  Heading fires produce about three (3) times as much smoke and particulates as backing fires!

Backing Fire
However, backing fires take longer and less area can be burned during the middle of the day and early afternoon when the atmospheric conditions for smoke dispersion are best.  Although heading fires produce more smoke, the higher intensity tends to lift it higher and disperse it better.

Spot and flanking fires will produce somewhere between heading and backing fires, depending on the type and condition of fuel.

Smoke Emissions Are Affected By:
Combustions Phases
Fuel Loading
Size and Shape
Oils and Tars
Fuel Moisture
Firing Method


More Intense Fire Result In:
More preheating
More fuel consumed
More burned during flaming phase
Smoke lifted higher
Burning completed earlier
Less residual smoke