Top Five Arson Fire Myths

Fire expert John Lentini outlines most-common myths in fire investigations.

May 5, 2010— -- The only thing worse than losing your loved ones in a fire is being falsely accused of setting the blaze. That's what some experts say happens around the country when the wrong fire investigator looks into the possibility of arson.

"If you survive a fatal fire, you've got a very good chance of being charged with setting it," John Lentini, one of the nation's leading fire experts, told "20/20."

If five percent of the nation's half-million building fires every year are suspicious, according to Lentini, that means there would be 25,000 chances to charge someone mistakenly with arson.

"It's one of the few crimes where the state actually has to prove that a crime took place. And if they do that with bad evidence, and the jury believes that it's a set fire, many times there is no doubt about who did it," Lentini said.

"20/20" examines several cases in a report to air Friday where people claim someone was falsely charged, convicted or even executed based on bad fire science and puts those theories to the test.

"I can turn just about any fire into an arson fire if that's what I want to do," said Lentini.

Lentini, who has conducted over 2,000 fire scene inspections, outlines below what he considers the most-common arson myths in 2010.

Arson Myths

The Five Most Common Arson Myths in 2010

1. The lowest and deepest charring indicates the point of origin in a fully involved room.

2. In a fully involved room, an experienced investigator can identify patterns produced by ignitable liquids on the basis of visual observation alone.

Experts say that when a fire breaks out, a phenomenon called flashover can occur. Flashover is a transition point at which heat causes almost everything in a room to catch fire. When it happens, the natural patterns of the fire can be obscured or destroyed. V-shaped burn patterns, which often occur after a flashover, can be misinterpreted to indicate arson.

3. Flammable liquids burn at a higher temperature than ordinary combustibles.

It's a common misconception that gasoline burns at a higher temperature than wood. It's actually the amount of ventilation that determines the temperature of the fire, not the nature of the fuel.

4. Spalling or flaking of concrete, especially in a "puddle" shape, is an indicator of the presence of burning ignitable liquids.

Fire tests have shown that "puddle" shapes can occur after a flashover.

5. Heat rises and fire always burns upward. Floor level burning is therefore an indication of an incendiary fire.

The idea that a fire will not burn downward unless it has "help" is a simplistic explanation of fire behavior that doesn't take into account the flashover phenomenon. It was widely believed that burning on the floor, particularly under furniture, indicated an origin on the floor, and pointed toward arson.

Additional Myths Still in Favor in Some Quarters Today

6. Multiple low burns, or multiple V-shaped burn patterns, even if they are burned together, indicate multiple origins.

7. Using models, it is possible to calculate fire behavior precisely.

8. A narrow V-pattern indicates a rapidly burning fire, whereas a wide V-pattern indicates a "normal" fire.

9. Although flashover and full room involvement can generate ambiguous patterns, flashover is rare. (Flashover is a transition point at which you go from having a fire in a room to a room on fire).

10. A melted aluminum threshold is unusual in a "normal" fire, and tends to indicate the presence of ignitable liquids.

Myths That Have Been Largely Discredited, And Are Only Used By Those Profoundly Unaware Of The Science

1. "Crazed" glass, broken throughout, indicates that the glass was rapidly heated. Further, the size of the crazing can provide information about the origin.

2. The size and appearance of char blisters can provide information about what was burning, and how rapidly it was burning.

3. An unconfirmed canine alert constitutes valid evidence of an accelerant.

4. The temperature of a fire follows a "standard time-temperature curve."

5. The heat release rate of a fire can be predicted by knowing the weight of combustible fuels in a room.