What could make a can of bug spray explode?

Answers to your questions about the news.
July 25 2008 5:05 PM

Can Bug Spray Explode?

The hazards of aerosol insecticides.

Illustration by Rob Donnelly. Click image to expand.

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.

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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.

How many cans of Raid  would it take to hit these levels? * Take a sealed room that's 10-by-10 feet, containing 28,320 liters of air. For propane and similar propellants, the most dangerous concentrations are between about 2 percent and 10 percent. It would take about 566 liters of propellant to exceed that lower limit in our hypothetical room. Since a standard, 17.5-ounce can of Raid contains about 0.27 liters of these liquefied chemicals, which would convert to about 68 liters in gaseous form, you would need to empty at least eight full cans of bug spray before you crossed that threshold.

The man in New Jersey wasn't spraying with cans of Raid. He appears to have exceeded the lower explosive limit for a flammable gas by setting off 16 "bug bombs" in a two-room apartment. This episode of Mythbusters from 2004 confirms that you can blow up a 500-square-foot house by setting off several dozen "roach foggers."

This calculation assumes that all the propellants released into the room are distributed evenly in the air. In fact, the propane and butane are heavier than air and are therefore more likely to settle in one place. As a result, the hydrocarbons in a single can of bug spray might be sufficient to create a small pocket of explosive gas somewhere in a room. Temperature and humidity also play a role: It takes less bug spray to blow up a dry, hot room than a cold and wet one.

High concentrations of bug spray can also be harmful when they're inhaled. These effects vary from person to person, with children and asthmatic or sick adults being most susceptible. There isn't enough insecticide in a can of bug spray to cause much of a problem for humans, but the propellants could be a problem. If you breathed in air that was more than 10 percent propane, you might experience respiratory irritation, hallucinations, vomiting, asphyxiation, or death.

Got a question about today's news? Ask the Explainer.

Explainer thanks Kristy Davis of the University of Virginia, Todd Eberspacher of Stanford University, and Mark Robson of Rutgers.

Correction, July 30, 2008: The original version of this article neglected to mention that the explosion in New Jersey was the result of 16 "bug bombs," and not regular cans of bug spray. It also misstated the number of cans of spray necessary to create an ignition hazard. The original calculation contained several errors and did not convert the liquid volume of propellants into a gaseous volume. As a result, it came out to 50 cans of spray; in fact, it would take only eight cans of spray. ( Return to the corrected sentence.)

Amaka Maduka is a Slate intern.

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