What is a common symptom of an allergic reaction to hair products?

A common symptom of an allergic reaction to hair products is typically a rash or irritation. This occurs because allergic reactions often provoke an immune response that manifests on the skin, leading to redness, swelling, and sometimes hives or bumps in the affected area. This can happen when the skin comes into contact with ingredients in hair care products, such as fragrances, preservatives, or dyes.

The other choices reflect conditions that could arise for various reasons but do not specifically correlate with allergic reactions. Excessive dryness may occur due to other factors, such as environmental conditions or the overuse of certain types of products, but it doesn’t indicate an allergic reaction. Headaches can result from strong odors or chemical irritations but aren't a direct symptom of skin allergies. Hair loss may be associated with various scalp issues or stress but not directly as a symptom of an allergic reaction. Therefore, a rash or irritation stands out as the most relevant and common symptom to identify.