2. Formaldehyde damage
Formaldehyde is essential to many hair treatments and has been successfully used to straighten hair, but it should be used with care. Formaldehyde is a known carcinogen, so exposure should be kept as low as possible; use it only in a place with adequate ventilation. Keratin treatments using formaldehyde tend to also use heat, and when both are excessive, chemical burn of hair or scalp may result.