A New Jersey man accidentally blew up his apartment on Monday while attempting to spray for bugs. The blast and ensuing fire destroyed his home and damaged an apartment above. Is household bug spray a major hazard?

Not unless you're using it in extremely large quantities. The chemical ingredients in a spray that actually kill insects—things like permethrin and imiprothrin—are not flammable. But a can of Raid also contains significant amounts (PDF) of highly flammable fluids such as propane and butane. These compressed gases, which make up about 40 percent of the spray, help to push the liquid insecticide out of the can and into the air. If the air in a room contained exactly the right mixture of propellants and oxygen, an explosion might occur.

A flammable gas is most dangerous when it's present in a specific range of concentrations, bracketed by what are referred to as its lower and upper explosive limits. If the atmosphere is "too lean," there's simply not enough flammable vapor in the air for an ignition to occur. An atmosphere that's "too rich" contains so much flammable vapor that it squeezes the available oxygen out at points like window cracks and doorways. (Since oxygen is required for combustion, this reduces the chance of an explosion.) But at any concentration between these limits, you've got a volatile situation. Something like a pilot light, a cigarette, or even static electricity could trigger a blast. To be on the safe side, it's best to not even exceed a quarter of the lower explosive limit.