Final answer:
Keratin-based hair straightening treatments release formaldehyde and volatile organic compounds when heated, due to residual monomers and the disruption of hair's molecular structure.
Step-by-step explanation:
When silicone polymers and formalin in keratin-based straightening treatments are heated to high temperatures, they release substances such as formaldehyde, which is a toxic gas, and other volatile organic compounds (VOCs). These emissions occur because the formation of long polymer chains is not perfectly stoichiometric, leaving some residual monomer. The heat provides kinetic energy that causes protein molecules in the hair to vibrate more rapidly, disrupting weak bonds and ultimately changing the hair's structure while potentially releasing small molecules. These treatments also break the disulfide bonds in hair that give it its natural curl, which explains why the straightening effect is temporary.