Spring is just around the corner, and we’re already ready to enjoy the nice weather and outdoor seating areas.

For us, fire investigation experts, this marks the start of the season for outdoor fires caused by smoking-related items.

Each year, the Ministry of Public Safety records approximately 400 fires linked to smoking materials. These fires account for one in seven deaths and millions of dollars in direct property damage.

A cigarette butt that comes into contact with combustible materials—such as paper, cardboard, fabric, certain plastics, wood mulch, or certain types of potting soil—can ignite smoldering combustion, also known as a smoldering fire.

This slow combustion produces no flames and occurs directly on the surface of the material. This type of combustion is similar to the glowing embers of charcoal briquettes in a barbecue. It may produce very little smoke, if any, and is generally a fairly slow process. The material may burn completely.

Furthermore, if conditions permit, the smoldering fire may progress until flames appear. For this to happen, the smoking product must come into contact with combustible material, and the latter must be positioned in such a way as to allow for some heat buildup. Then, a slight supply of air—such as a breeze or the airflow from a fan—will be needed to cause flames to appear, which can then spread to any adjacent combustible material.

The specific case of flowerpots warrants further explanation. The potting soil in planters and flowerpots contains decomposing organic matter—including compost, bark, peat moss, etc.—not to mention added fertilizers. These compounds are all combustible. The mixture is therefore quite porous overall, allowing oxygen in the air and water to reach the plant’s roots more easily. When the mixture is particularly dry, slow combustion can begin within minutes of a cigarette butt being added to the potting soil.

This combustion will spread slowly through the material, causing the potting soil to turn black until it is reduced to grayish ash. Eventually, this combustion will melt the container—if it is made of plastic—and cause it to catch fire, followed by the surrounding structures. A fire can occur just as easily if the potting soil is in a wooden planter.

When a fire caused by a smoking-related item is suspected, it is crucial to confirm with all potential smokers their smoking habits, particularly regarding where the last cigarettes were smoked, when the smoking-related item was used, as well as the type of potting soil and the dimensions of the pot in question, if applicable. The fire investigation expert will then be able to analyze this information and confirm or rule out the likelihood of such a hypothesis.