In North America, flame retardants are commonly added into clothing, furniture, electronics and children's products like car seats and cribs.

Most are organohalogens, carbon-based molecules manufactured with multiple bonds to halogen elements like bromine or chlorine.

The retardants are true to their name in that they slow a fire rather than stopping it. In the flames, atmospheric oxygen molecules in the room break into free oxygen radicals. Radicals are molecules with lonely, unpaired electrons looking to form a bond.

Normally, they find that bond in hydrogen atoms in the hydrocarbon fuel—like in a couch or shirt. One product of this reaction is a hydroxyl radical, which then propagates the chain reaction until hydrogen atoms in the fuel source are all used up.

But let's say the couch contains a common organohalogen flame retardant called polybrominated biphenyl ester, or PBDE. When heated, PBDE gives off hydrogen bromide.

This molecule reacts with the hydroxyl radicals in the flames and leaves a bromine radical. Because bromine is more stable in radical form than oxygen, these bromine radicals do not react as easily, slowing down the entire combustion reaction.