Dear student,
When plastic straw undergoes combustion it will produce harmful gases that are toxic to our body. This is the reason that plastic combustion is not done because it will produce carbon monoxide which has the ability to displace oxygen from our blood and binds in place of oxygen which leads to suffocation and failure of the heart. Many other gases are also released during combustion such as carbon dioxide, dioxins, furans, mercury, and polychlorinated biphenyls (better known as BCPs) into the atmosphere, and poses a threat to vegetation, and human and animal health.
Regards