The first thing you should do is stop using that AC unit immediately to prevent further exposure. Do not attempt to clean the heavily contaminated filter yourself, as this can release a large concentration of spores. Replace it with a new, high-quality filter as a temporary measure. However, the filter is only the first line of defense; the mold you see is likely just the tip of the iceberg, with more growth deeper in the evaporator coils and drip pan.

For a situation with active symptoms and visible growth, a professional inspection is strongly recommended. DIY cleaning often fails to address the root of the problem inside the unit. A proper assessment involves a borescope inspection of the ductwork and evaporator coils to gauge the full extent of contamination. In many cases, a certified remediation is required, which includes containment, HEPA vacuuming, and antimicrobial treatment of the entire system to stop the recurrence. To verify the cause of your symptoms, you could also consider an air quality test; a lab culture can identify the specific mold species present, which is valuable information for understanding your health response.