What is the key reason for barbers to avoid certain harsh chemicals?

Barbers should exercise caution when using certain harsh chemicals primarily because they can lead to client allergic reactions. Allergic reactions can vary in severity, sometimes resulting in discomfort, irritation, or even serious health issues for clients. This risk underscores the importance of conducting patch tests and being aware of a client's previous experiences and sensitivities before applying any chemical products.

While hair damage is indeed a concern and can be a result of using harsh chemicals, the immediate health implications associated with allergic reactions make this a more critical reason to avoid such products. Additionally, the efficacy of the chemicals or the time they take to use, while relevant to the practice, do not directly address the health and safety concerns for clients as effectively as the potential for allergic reactions does.