Organic peroxides are “extremely flammable,” noted the company in a statement on its website Thursday. Arkema said it expected more fires to start and planned to allow the chemicals to burn themselves out.

“They're actually very marvelous,” said chemist William Carroll, an adjunct professor of chemistry at Indiana University who has worked with organic peroxides for many years. “You use them in a way that they fall apart and do so at very specific temperatures.” One organic peroxide can be tailored to break up at 30 degrees Celsius [86 degrees Fahrenheit], while another will do so at 40 degrees. The trick is to keep them safely below those temperatures — sometimes frozen solid, or, in the case of the Arkema plant, stored as chilled liquids.

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When peroxides heat up, they react. Thursday, around 2 a.m. local time, “the materials did exactly what they're supposed to do,” Carroll said. “They decomposed.”

What's in a peroxide

A peroxide compound contains two linked oxygen atoms. The peroxide you're perhaps most familiar with, hydrogen peroxide, is a chain of two oxygen atoms capped on both ends with a hydrogen atom. Peroxides are useful because the chemical bond between the oxygen atoms is so unstable. Oxygen greedily snatches electrons from other atoms. This rapaciousness makes oxygen a flighty partner when bonded to itself.

Michelle Francl, a chemistry professor at Bryn Mawr, has an antique glass container labeled “peroxide” in her office. A laboratory safety officer who caught sight of the jar almost bolted until Francl gave her assurances that it was scrubbed clean. Had it contained dry peroxide, she said, the jar could have combusted at a jolt.

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“Peroxides are basically tiny little molecular canisters of oxygen,” Francl said. “They’re designed to break apart and be highly reactive. It’s like having a whole bunch of fragile glass molecules.” People have used hydrogen peroxide's fizzy instability to bleach hair, disinfect wounds — not always a recommended technique — and, at high concentrations, propel rockets.

Organic peroxides substitute the hydrogen caps for clusters of carbon plus other elements. There are many kinds of organic peroxides, with almost as many uses. Benzoyl peroxide, as an acne medication, tears apart proteins in skin bacteria. Arkema advertises a wide range of peroxide products: not just to defeat zits but also to kick-start polymer reactions, to improve the color of resins, to create plastics for pipes and countertops.

Your plastic kitchen countertop, of course, isn't in danger of spontaneous combustion. When manufacturing plastics, the reactive peroxide is a spark, not a building block. Once the peroxide kicks off the a chain reaction, the resulting molecules braid together in a stable polymer. The beauty of peroxides is their ability to keep the reaction going, churning out free radicals. “Every time a radical touches something it turns it into a radical,” Francl said. “It’s like zombies.”

Why Texas?

It's no coincidence that Texas is thick with chemical plants, situated alongside natural gas and oil refineries. Where there are fossil fuels there are carbon byproducts, a key component of organic peroxides. You might hear oil and think of fuel. But chemists think of “lots of little carbon frameworks. Like Lego,” Francl said.

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And the choice to construct the chemical plant in a rural area was no accident, either, Carroll said. You won't find such a factory in downtown Houston. “They're acutely aware that they're working with a reactive material,” he said, “that has to be handled in a certain way.”

Past accidents

The Arkema plant is far from the first to have an accident with organic peroxides. Between 1980 and 2001, there were about a dozen accidents severe enough to trigger reviews from the U.S. Chemical Safety Board, Francl noted, including a previous event in Crosby in 1998. A handful took place in Pasadena and other nearby Texas cities.

On Thursday, a deputy with the Harris County Sheriff’s Office sought treatment for inhaling fumes, The Washington Post reported. Other organic peroxide accidents have resulted in injuries. In 2008, in Edmonton, Alberta, nine people were hospitalized after a peroxide holding tank caught fire. (Terrorists have put organic peroxides to the most horrific use. Bombing attacks in Manchester, London, Paris and Brussels involved triacetone triperoxide or TATP, a compound also known as “mother of Satan.")

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If there is a silver lining to the peroxide accident Thursday, the chemicals' reactivity means that they won’t stick around in the environment. The chemicals will burn too quickly for floodwaters to sweep any of the liquid away. “This is an acute event, not a chronic one,” Carroll said.

What's left after such a chemical reaction? Smoke, but mostly carbon dioxide — and more water.