wet conditions

To avoid inappropriate anxiety on the part of any readers we state up front that it is normal for some dust and debris to accumulate on the inside of heating or air conditioning ducts, and this material is not necessarily a hazard to building occupants. But when ducts have also been wet, or when ducts are constructed of materials like fiberglass insulation that have been damaged or can't be cleaned, some potential health or respiratory issues may be present, as we discuss further here.

Rust flakes from rusty heating or air conditioning ducts themselves are unlikely to be much of a health hazard - these particles are pretty big, not easily airborne, and probably won't be found at high levels in indoor air except in unusual circumstances. But rust in ducts is a problem indicator, showing quite clearly that the duct system, more importantly the insulation, has been wet.

The combination of organic debris within a duct system and water (indicated by rusty ducts or duct registers) indicates a possible risk of mold or bacterial hazards within the system. Since blowing air through the system can pick up and distribute these hazards to occupants of the building, wet or previously-wet duct work is a potential health hazard to building occupants.

Important factors you should know:

The materials used to insulate the duct interior: Fiberglass and similar materials form a "sticky" surface that tends to hold debris and also to act as a sponge holding moisture, possibly increasing the risk of mold or bacterial hazards, and not easily cleaned.
The frequency of wet conditions: Recurring wetting obviously increases the risk of pathogens in the duct work, though we've seen significant contamination as a result of a single building flooding event which wet the ducts
The health vulnerability of individual occupants in the building: Elderly, immune-impaired, asthmatics, infants, people with allergies, people with COPD or other respiratory illness, are more likely to be bothered sooner by contaminated insulation that is produced by wet conditions in the air conditioning or heating system.

Humid and wet crawlspace environments results in rust on ducts, pipes, post jacks, and beams made of metal. The moisture in the wet crawlspace will evaporate in to the atmosphere causing the relative humidity to increase. This humidity increase will rust the ducts and pipes over a period of time. Ironically, even a dry crawlspace floor can have potential problems with rust as well. Most crawlspace are vented through the foundation to allow air in and through the crawlspace. Crawlspace Vents bring in hot humid air in the summer. Most cooler surfaces such as sheet metal ductwork, copper pipes, steel beams, metal post jacks, and even insulation will condensate due to the ventilation in the crawlspace. The "sweating" of the ducts and metal surfaces will cause rust over time. Have you ever seen your air ducts on a hot and muggy day? They are sweating just like you are outside.

Wet or Corroded HVAC Ducts are also a health concern. The main components of house dust, which will collect in a duct system, include fabric fibers, skin cells, starch fragments and other organic debris. The combination of organic debris within a duct system with moisture (indicated by rusty ducts or duct registers) indicates a possible risk of mold or bacterial hazards within the air conditioning or heating system. Since blowing air through the system can pick up and distribute these hazards to occupants of the home, wet or previously-wet duct work is a potential health hazard to the homeowners.

Many homeowners will wrap their ducts with fiberglass insulation in an effort to stop the condensation. While this may work in stopping some of the condensation, it does not address the real problem. Plus, the moisture in the insulation will possibly harbor mold colonies. These mold spores are then transferred to the upper levels through the seams and holes in the duct work. The only sure method to prevent rust on duct systems, pipes, beams, and jacks is to dry the crawlspace permanently.

Important factors you should know:

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The materials used to insulate the duct interior: Fiberglass and similar materials form a "sticky" surface that tends to hold debris and also to act as a sponge holding moisture, possibly increasing the risk of mold or bacterial hazards, and not easily cleaned.
*

The frequency of wet conditions: Recurring wetting obviously increases the risk of pathogens in the duct work, though we've seen significant contamination as a result of a single building flooding event which wet the ducts
*

The health vulnerability of individual occupants in the building: Elderly, immune-impaired, asthmatics, infants, people with allergies, people with COPD or other respiratory illness, are more likely to be bothered sooner by contaminated insulation that is produced by wet conditions in the air conditioning or heating system.