A house in Middle Swan was blown up by roach bombs yesterday.
What happened? Are these things dangerous?
The short answer is “no” if they are used correctly. They contain insecticide, water, and a hydrocarbon propellant. The way they work is you click the valve at the top open, it locks in place, and they dispense the entire contents into the air. The insecticide then settles on all hard surfaces, and kills any cockroaches in the vicinity for the next few months.
As it comes out of the can, it is not flammable as it contains water. What happens then, however, is that they separate – the water (and insecticide) fall to the ground, whereas the air fills with the flammable hydrocarbon gas (normally propane or butane).
The only question then is how much is there in the air? Normally, there is not enough in one can to reach a high enough concentration (the Lower Explosive Limit) to be explosive. What happens, however, is people think that if one can will work, then five cans will work better.
What’s wrong with that? That makes sense doesn’t it? Well, five cans contain five times as much insecticide as one can, of course. Unfortunately, they also contains five times as much flammable gas. Frequently, this will put enough gas into the air to reach the lower explosive limit and the result is rather spectacular.
Consequently, there are many stories, particularly in restaurants, of places getting blown up because they used too many cans at once, and mythbusters quite elegantly showed what can happen.
So the lesson is, read the instructions on the can. They will tell you what the area coverage of a single can is, and you don’t need to use any more than this. In other words, one can at a time!
But if you do want to use more, do it over a few days – don’t let off too many cans at once, or you won’t have a house any more!
Incidentally, 25 years ago when people were less concerned about safety than they are today, fly sprays were a lot more flammable – the legendary Pea-Beu Tri Kill is a case in point!