Which of the following should be monitored during a chemical service for potential irritation?

Monitoring the client's comfort level is crucial during a chemical service for potential irritation because discomfort can be an early indicator of an adverse reaction to the chemicals being used. Keeping track of how a client feels throughout the process allows the stylist to assess whether the hair and scalp are tolerating the treatment well. If the client experiences any pain, burning sensations, or unusual discomfort, it can prompt immediate action, such as rinsing the product or adjusting the application technique to prevent further irritation.

While other factors like hair length, color fading, and scalp temperature are important in their own right, they do not directly indicate the risk of irritation during a chemical service. Hair length primarily affects the amount of product used or the technique applied, color fading relates to the longevity of a hair dye or texturizer, and while scalp temperature can be of concern, it does not reflect the client's subjective experience, making the comfort level a more comprehensive measure to monitor throughout the service.