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How come when you spray fly spray by the smoke alarm it goes off?

TECHNOLOGY: Pretorius, aged 8 from Hampton Hill School, Tawa, Wellington asked this question.

Thanks Pretorius for that interesting and difficult question. I myself wonder why my kitchen smoke alarm goes off when I fry eggs. I’d disconnect it, but that’s not a very smart choice. So, in looking into this question, I found out that lots of things can set off a smoke alarm. The first and important thing to know is that a smoke alarm doesn’t really exactly detect smoke. Instead it detects heavier particles of a certain weight floating in the area, which you mostly find in smoke.

Thus, just having a really humid house can set off a smoke alarm as the water in the air makes it seem smoky – this is a very humid house and not likely you would find one like that in New Zealand! Strong chemical odours like paint or terps (paint thinner) can set off a smoke alarm, because when you smell those odours you are actually smelling the tiny particles that float up off these chemicals.

So, why does fly spray set it off? Well, it has an odour (nasty, don’t smell it), and that odour comes from the small chemical particles in the spray, which are the chemicals used to kill flies – which is what you are supposed to use it for! Equally, the spray is made of tiny water droplets, like humidity. So, what happens is that the chemical particles and the added water moisture from the spray likely combine to set off the smoke alarm when you spray fly spray near it or at it.

Our expert

Distinguished Professor Geoff Chase from the University of Canterbury answered this question. He is a Fellow of Royal Society Te Apārangi, which means he’s one of the top experts in his field of engineering. 

For more information on our expert, visit his profile: Geoff Chase