# How does the presence of chlorine atoms in PVC make it flame resistant?

In a paper that I'm reading, it states that:

The presence of chlorine atoms in polyvinyl chloride make it flame resistant

and I'm just wondering why the presence of chlorine atoms will make it flame resistant. To the best of my knowledge, burning something is just providing a material with enough activation energy such that it can react with oxygen to produce products such as carbon dioxide and water. If so why does the chlorine prevent this process in any way?

Many of the hydrogen halides ($\ce{HCl}$ and $\ce{HBr}$) are able to inhibit the combustion process, as when they are added to a flame they alter the flame chemistry. What will happen is that they react with oxidizing radicals such as hydroxyl radicals to form halogen atoms. The halogen atoms are able to react further but their reaction with fuels such as methane will be less exothermic than the reaction of hyroxyl radicals.
Also, Chemosphere, 1991, 22 (1–2), 67-76 states that alkyl radicals in flames can react with $\ce{HCl}$ to form alkyl chlorides. This will leave behind hydrogen atoms. It has also been stated by M.J. Thomson, A.D. Lawrence and J. Bu (Combustion Science and Technology) that HCl inhibits the combustion of benzene, it increases the carbon monoxide yield and reduces the chain branching which occurs during the combustion.