Which symptom would indicate potential chemical burns from a toxic substance exposure?

The presence of skin irritation is a key symptom that indicates potential chemical burns from exposure to a toxic substance. Chemical burns can occur when the skin comes into contact with caustic or corrosive materials. This contact often causes visible damage such as redness, blistering, or other signs of irritation on the skin. The severity of the irritation can reflect the nature and concentration of the chemical involved.

While the other symptoms listed can occur in various contexts, including other medical conditions, they are less specific to chemical burns. Nausea, extreme fatigue, and headaches may be related to systemic effects of toxins or other environmental factors, but they do not specifically imply damage to the skin or indicate direct contact with a harmful chemical. Therefore, skin irritation remains the most direct and indicative symptom of potential chemical burns from toxic substance exposure.