Mold in Your AC and HVAC System: What Homeowners Need to Know Before Summer

As temperatures climb and AC units kick back on across the country, most people don't think twice about what's been sitting in their ductwork since fall. Here's what the research says — and what you can do about it before summer gets into full swing.

Turning on your AC for the first time each summer feels routine. But your HVAC system has been sitting dormant for months, and whatever settled into those ducts over fall and winter is now getting pushed into every room in your home the moment you power it back on.

How mold ends up in your HVAC system

Mold needs three things to thrive: moisture, warmth, and a food source. Your HVAC system quietly checks all three boxes. Drip pans collect condensation. Coils stay damp. Dust accumulates in ductwork and provides an organic food source. Over the colder months when the system sits idle, mold spores settle in undisturbed — and begin colonizing.

Sources: U.S. Environmental Protection Agency — Indoor Air Quality Report; Mold Course Chapter 2

The tricky part? You can't see it happening. Mold inside ductwork is invisible to the naked eye, and most standard home inspections don't catch it. Symptoms like fatigue, brain fog, or respiratory irritation often get attributed to allergies or stress — not to what's cycling through your vents.

When mold produces more than spores

Certain mold species don't just trigger allergic reactions — they produce mycotoxins, chemical compounds that can accumulate in your body with repeated exposure. Three of the most common mycotoxin-producing molds found in residential HVAC systems:

• Aspergillus — one of the most common indoor molds, known to produce aflatoxins and ochratoxin A with links to respiratory and immune issues.
• Stachybotrys (black mold) — associated with water-damaged buildings and trichothecene mycotoxin production, often found in ductwork after moisture intrusion.
• Chaetomium — frequently found in damp drywall and insulation, linked to neurological symptoms via chaetoglobosin production.

Mycotoxin exposure symptoms overlap heavily with other common conditions — which makes misdiagnosis a real problem. Common signs include:

Test, don't guess

Testing your home's dust is one of the most direct ways to understand what you're actually being exposed to. HVAC vents accumulate settled dust over time — making them an ideal collection point for everything that's been cycling through your air. A small cut-out of your HVAC filter is all it takes to get a lab-confirmed answer.

Your HVAC checklist before summer

• Replace air filters — aim for MERV-8 or higher for better particulate capture
• Clean drip pans and inspect drain lines, common spots for standing water and mold growth
• Schedule a duct cleaning if it's been more than 3–5 years
• Keep indoor humidity below 50% — mold growth accelerates above 60%
• Test your home's dust with the EMMA combo test to know what's actually in your environment