Mycotoxins in HVAC ducts and air systems

Mycotoxins in HVAC systems, explained

How mycotoxins get into the system

There are two basic routes, and many houses have both.

• Growth inside the equipment. In cooling season, the evaporator coil pulls moisture out of the air and it drips into a drain pan. A pan that doesn’t drain, a clogged condensate line or a dirty coil stays wet for long stretches. Dust on those surfaces gives mold something to eat.
• Growth on duct liner. Some ducts and air handlers are lined with fiberglass insulation. Its rough, porous surface traps dust and holds moisture if condensation forms.
• Leaky returns in damp spaces. Return ducts running through a crawlspace, basement or attic can pull air from those spaces through gaps and joints. If there’s mold there, its particles get drawn into the system.
• Dust from the living space. Every return grille pulls in room dust. If rooms are contaminated, so is the air the system is moving.

Once contaminated particles are in the air stream, the system distributes them. Some pass through the filter, some settle inside the ducts, and some blow out of the supplies into every room. That’s the mechanism behind the related question, can mycotoxins spread through HVAC?

Where contamination collects

Component	Why it matters	Usual approach
Evaporator coil and drain pan	Wet during cooling season. The most common place for growth inside a system.	Clean, confirm the pan drains, clear the condensate line
Blower and cabinet	Collects dust and can hold moisture	HEPA vacuum and clean
Return plenum and ducts	Pulls in dust from the house and from any leaky spaces	Mechanical cleaning under negative air. Seal leaks.
Supply ducts	Carry fine particles to every room	Mechanical cleaning under negative air
Fiberglass liner, duct board, flex duct	Porous, hard to clean once contaminated	Replace where contaminated
Registers and grilles	Collect visible dust at the room end	Remove, clean and reinstall

What public guidance says

EPA’s consumer guidance on duct cleaning says routine cleaning isn’t needed for every home, but it recommends considering it when there’s substantial visible mold growth inside hard-surface ducts or on other components of the system. The same guidance says insulated ducts that get wet or moldy can’t be effectively cleaned and should be removed and replaced, and it urges caution with chemical biocides and sealants. The duct cleaning industry’s trade group, NADCA, publishes a cleaning standard that calls for the system to be kept under negative pressure while it’s cleaned. Our process follows the same logic.

Testing the system

Visual inspection shows dirty coils and dusty ducts, but it can’t tell you whether the dust carries mycotoxins. Dust samples from the return plenum and a main supply trunk, compared with samples from the rooms, show whether the system holds more or less than the living space. A system that’s much higher than the rooms is likely a source or a reservoir. A system that matches the rooms is probably carrying dust from elsewhere, and the source search moves on. Mycotoxins in house dust explains how those samples are collected.

Keeping the system clean afterward

• Keep indoor humidity in a normal range, especially in summer.
• Have the coil, drain pan and condensate line checked during routine service.
• Change filters on schedule, and make sure they fit without gaps.
• Seal return leaks in crawlspaces and attics.
• Correct moisture in any space ductwork runs through.

For the full HVAC project, from coil to register, see HVAC mold remediation. If you’re weighing a filter upgrade or a portable unit, do HEPA filters capture mycotoxins? covers what filtration can realistically do.

We clean mold and mycotoxins out of buildings and their mechanical systems. We don’t diagnose or treat medical conditions. If you notice symptoms that seem tied to your heating or cooling system, please talk with a doctor. With your permission, we’ll share test results with them.