Squamous Cell Carcinoma: This is the most direct association with elevated squamous cell carcinoma antigen (SCC-Ag) levels. Given the age and good past health, if there's a strong suspicion or evidence pointing towards a squamous cell carcinoma, particularly in areas like the head and neck, lung, or skin, this would be the primary consideration.

Human Papillomavirus (HPV) Infection: HPV is known to cause an increase in SCC-Ag levels, especially in the context of cervical cancer in females, but it can also be relevant in males, particularly with the rise in oropharyngeal cancers associated with HPV.
Chronic Inflammatory Conditions: Certain chronic inflammatory conditions, such as chronic dermatitis or other skin conditions, can lead to elevated SCC-Ag levels due to the ongoing tissue damage and repair process.
Benign Tumors or Cysts: Some benign conditions, like squamous cell papillomas or certain types of cysts, might also elevate SCC-Ag levels, although this is less common.

Lung Cancer: Although less common in a 30-year-old, lung cancer, especially squamous cell carcinoma, is a critical diagnosis not to miss due to its potential for aggressive behavior and the importance of early intervention.
Head and Neck Cancer: Similar to lung cancer, head and neck cancers, particularly those of squamous cell origin, are crucial to identify early due to their significant impact on quality of life and prognosis.
HIV/AIDS: Individuals with HIV/AIDS can have a variety of opportunistic infections and cancers, including those that might elevate SCC-Ag levels. The immune-compromised state makes timely diagnosis critical.

Paraneoplastic Syndromes: Rarely, elevated SCC-Ag can be associated with paraneoplastic syndromes related to cancers that do not directly produce SCC-Ag, such as certain neuroendocrine tumors.
Genetic Conditions: Some rare genetic conditions might predispose to elevated tumor markers, including SCC-Ag, although these would be exceptionally uncommon and typically part of a broader syndrome.
Idiopathic Conditions: There might be rare, idiopathic conditions where SCC-Ag is elevated without a clear underlying cause, requiring careful monitoring and possibly further investigation.