For naturally straight or wavy hair or hair with minor frizz and damage:  0%(Formaldehyde free)  / a little damaged  For medium curly hair or hair with some visible damage: 5% formalin For extremely curly hair or hair with a lot of damage: 12% formalin  DO NOT: 1) If  you hair is golden blonde, do not choose keratin Formaldehyde free (0%) or it will change the hair color to orange, please choose keratin 5% formalin or others 2) Use standard store-bought shampoos not suitable for keratin treatment.