Flashover

Too often one hears about a fire that suddenly sweeps through a trailer house or a building and which kills people before they can get out. Considering how slowly a fire spreads through a pile of logs or even loosely stacked lumber, rapidly spreading fires in structures don't seem to make sense. Obviously, there is something about a fire in a confined space that is different from one out of doors.

The key difference is the confinement itself. Once a fire starts, even if initially very small, it releases gases which are hot and perhaps volatile. In a confined space these gases accumulate. As time goes by, the confining space becomes hotter and more gas-filled.

When the temperature rises above 300°F (150°C) many materials start to decompose. Wood, paper, paints, fabrics and plastics then release volatile gases. These gases would drift away from a fire outside, but if the fire is inside, the accumulating gas is a potential horror in the making.

At some point, the gas density and temperature become high enough that flames almost instantaneously spread through the space. This deadly phenomenon is called flashover by fire scientists. Spectacular photographs of a flashover event appear in an article by John F. Kenahan in the January issue of SCIENCE 80.

Plastic materials are particularly copious suppliers of the toxic and volatile gases that lead to flashover. House trailers and other mass-produced structures such as are found in northern construction camps and similar facilities typically have a goodly share of plastic fittings and furnishings. Even in selecting furniture and fittings for use in a comparatively safe wooden house, one does well to think about which types are least likely to contribute to flashover.