**Core Concept**
Air-borne contact dermatitis is a type of allergic contact dermatitis caused by inhaling allergens that then come into contact with the skin, leading to an immune-mediated reaction. This condition is often associated with exposure to specific allergens in the workplace, such as balsam of Peru, colophony, or chromates.

**Why the Correct Answer is Right**
The diagnosis of air-borne contact dermatitis typically involves a combination of clinical evaluation, occupational history, and patch testing. Patch testing is a valuable tool in identifying the specific allergen responsible for the reaction. It involves applying small amounts of potential allergens to the skin and observing for a reaction over a period of time. This helps to confirm the diagnosis and guide treatment.

**Why Each Wrong Option is Incorrect**
* **Option A:** Biopsy is not a primary diagnostic tool for air-borne contact dermatitis, as it may not provide specific information about the allergen involved.
* **Option B:** Blood tests, such as IgE levels or atopy patch tests, may be used to support the diagnosis but are not definitive diagnostic tools.
* **Option C:** The absence of a positive patch test result does not rule out air-borne contact dermatitis, as false negatives can occur.

**Clinical Pearl / High-Yield Fact**
Remember that air-borne contact dermatitis often presents as a facial or hand dermatitis in individuals with a history of occupational exposure to specific allergens. A thorough occupational history and patch testing are essential for accurate diagnosis.

**Correct Answer: C. Patch testing.**