Why should incompatible chemical services not be layered carelessly (for example, certain treatments over others)?

Layering incompatible chemical services — for example, applying certain strong chemicals over hair already treated with another incompatible process — can cause severe damage, breakage, or unpredictable reactions, because the hair's bonds and condition may not tolerate the combination. This is why knowing the client's chemical history and doing strand tests is essential before a new chemical service. The cosmetologist must assess compatibility and the hair's condition first. Awareness of chemical-service compatibility and its risks is important safety content on the exam.