Final answer:
High-strength bleaches and those not designed for scalp use should be avoided as they can cause irritation or burns; semi-permanent colors are safe but cannot lighten hair.
Step-by-step explanation:
The types of lighteners that cannot be used on the scalp are generally those that are too strong or harsh, which can potentially cause irritation or burns. While semi-permanent hair color cannot lighten hair and is generally safe for scalp application, chemical lighteners, such as bleach, must be used with caution. Bleaching is indeed a permanent process that removes the natural pigment from the hair. High-strength bleaches or those not designed for scalp use should be avoided to prevent scalp irritation.