How a Fire Starts and Spreads

Fire is an energy-producing chemical reaction between oxygen in the air and fuel, such as wood or gasoline. It requires heat to start, which could come from a match, focused light or friction. Once the heat reaches ignition temperature, the atoms in the fuel break free of their bonds with each other and become volatile gases (such as carbon dioxide and water) that combine with oxygen in the air to produce flame. The heat produced by the reaction is also released as a form of light, known as incandescence.

The gasses produced by a fire are hot enough to melt metals, boil water and damage many plastics and rubbers. This is why they can cause injuries if someone gets too close to the flames or tries to put them out with water. They can also release toxins such as dioxins and furans, which are harmful to humans.

Once a fire starts, it can spread rapidly because of the transfer of energy in three ways:

Fuel burns, emitting a plume of gases that rises to the ceiling and is visible as smoke. Convection and radiation transfer this heat, igniting more fuel on the surface of the burning material and causing it to grow. When the fuel reaches its ignition temperature and expands, it forms a flame that can ignite nearby gases, spreading the fire.

Eventually, the fuel is used up and the chemical reaction slows down or stops. Then the fire cools, which causes the gases to contract and sink back into the fuel. The resulting pressure builds up inside the building, often causing an explosion of hot vaporized fuel against doors and windows. This is called a backdraft, and can be dangerous to anyone inside the building.

A backdraft can also blow open doors and windows, allowing the flames to spread. This is why it is important to stay out of buildings while they are burning, and to close doors and windows as soon as the fire begins.

If a fire spreads inside your home, contact the local authorities for assistance and follow their instructions carefully. It is especially important to check for smoldering materials such as cooking oil, propane tanks and gasoline stored in your garage, since these may explode.

Most people think of wildfires as destructive, but they are an important part of many ecosystems. In addition to clearing underbrush and reducing the risk of wild animal predation, they can enrich the soil with nutrients, enabling native species to thrive and eliminating invasive/non-native species that are competing for resources. It is also possible to control fires by carefully planned prescribed burns, and these can have positive effects on forest health.