A driver on the way home work gets a flat. He never bothered to check the pressure in his spare tire, and it's flat, too. Luckily his brother bought him a can of Fix-a-Flat last year, and he tossed it in the trunk. He refills the tire and heads to the tire store for a proper repair or a new tire. At the tire shop, the technician puts the wheel on the tire machine, and somehow in the process of deflating the tire or breaking the bead a spark flies, ignites the flammable propellant the tire sealer left behind, and injures or kills him (depending on what version of the story you're listening to).
The fact is that the chemical propellant used in Fix-a-Flat today is a non-flammable gas. It will not explode!
Before we start calling these doomsday screamers idiots, we should get all the facts. A little research revealed that in fact some tire sealing and inflation products did have explosive chemicals inside as propellants. There are a few famous cases involving serious explosions of tire sealing products. Even Fix-a-Flat had explosive qualities until 1999 when they removed the product from shelves and replaced it with a new non-explosive formula. With that info, the guys telling us that Fix-a-Flat will kill us aren't entirely misinformed, just about a decade too late.
Important Note: While we know for sure that Fix-a-Flat is non-explosive, it's very possible that other lesser known tire sealing products on the market still use a propane or butane based propellant. Be especially wary of strange off brands you find in dollar stores or at flea markets -- some of these don't even have to pass Federal muster since they're brought into the country "under the radar." To be safe, stick with a major brand like Fix-a-Flat. If you're not sure, do some research to find out how safe your product is.