What is smoke and why do some fires seem to have more smoke than others?

Smoke is a collection of tiny solid, liquid and gas particles. Although smoke can contain hundreds of different chemicals and fumes, visible smoke is mostly carbon (soot), tar, oils and ash.

Smoke occurs when there is incomplete combustion (not enough oxygen to burn the fuel completely). In complete combustion, everything is burned, producing just water and carbon dioxide. When incomplete combustion occurs, not everything is burned. Smoke is a collection of these tiny unburned particles. Each particle is too small to see with your eyes, but when they come together, you see them as smoke.

Smoke in a wood fire

Wood is made up of:

water

volatile organic compounds – a compound is volatile if it evaporates (becomes a gas) when it is heated

carbon

minerals in the tree’s cells, like calcium, potassium and magnesium (which are non-burnable and become ash).

When you put wood on a hot fire, the smoke you see is the volatile organic compounds (hydrocarbons) evaporating from the wood. They start to evaporate at about 149°C. If the fire is hot enough, the hydrocarbons will burst into flames. Once they burn, there is no smoke because the hydrocarbons are turned into water and carbon dioxide.

Charcoal

After the fire has been burning for a while, most of the hydrocarbons (gases and smoke particles) have been released, and all that is left is charcoal, which is almost pure carbon with some minerals. The hot charcoal slowly burns with a red glow. There are no flames because charcoal will only produce carbon dioxide, which cannot be burned any further, unlike other vapours. Very little smoke is produced at this stage. The quicker a fire is reduced to glowing charcoal, the hotter it will be and the less smoke it will produce. The carbon combines with oxygen to produce carbon dioxide until all that is left at the end of the fire is the ash – the minerals.

Smoke is dangerous

Smoke inhalation is the primary cause of death in victims of indoor fires. Nearly 75% of home fire victims die because of the effects of the smoke rather than the fire.

Depending on the house contents, the smoke generated can be extremely toxic or irritating. For example, burning plastics often produce soot and poisonous gases like carbon monoxide and hydrogen chloride.

Another danger is that smoke contains flammable compounds. With increased oxygen, these can ignite either through open flames or by their own temperature. This leads to a backdraught or flashover effect.

Smoke also obscures visibility. This makes it difficult to evacuate from a fire. Many deaths occur because people (including firefighters) become disorientated in smoke and can’t find their way out of a building.

Smoke can often cause more damage to a building than flames or the heat of the fire. Smoke will leave widespread stains and smells that are difficult to remove.