Expert: George Maxwell - 1/2/2008

Question
Hi George

I would like to know why cigarette smoke activates fire alarms but marijuana smoke does not. What substance is it in the cigarettes that causes fire alarms to trigger that is absent in marijuana?

Thank you.

Answer
Thank you for your question.

Both types of smoke can activate a smoke detector - it's just that in some cases, a detector responds quicker to tobacco than to marijuana.

It's not primarily a chemical difference, because smoke detectors work on a physical basis:

There are two main types of detector - ionisation and photoelectric.

The ionisation detector contains a tiny radioactive source which makes the air in the detector body electrically conductive. When smoke particles enter, they interrupt the flow of electricity, causing a drop in current. A microchip detects this, and sets off the alarm.

The photoelectric detector contains a light source and a light detector. Smoke particles interrupt the beam of light, causing a drop in current at the light detector. Again, the unit senses this, and the alarm is initiated.

The ionisation type responds more quickly to invisible combustion producs, and the photoelectric to thick, visible smoke.

Both tobacco and marijuana are more likely to activate an ionisation unit - they give off light smoke and a large amount of invisible combustion products. Tobacco in a cigarette burns at a higher temperature than marijuana; this higher temperature breaks down the tobacco in to smaller ionised particles, which might set off an ionisation detector more quickly.

In my experience as a fire officer, it's more likely to be the sensitivity of the detector than the nature of the burning material that determines what sets the alarm off. A university in my local area recently upgraded its fire alarm system to include sensitive ionisation units. On one occasion, we responded to an alarm from a student bedroom, and could find nothing except a telltale smell of "exotic" cigarettes...

Hope ths helps - Happy New year!