There are three things that mold spores need to survive – food source, moisture, and a stable temperature. Your duct systems provide all three. Dust and dirt enter through the filter providing a food source. The furnace keeps a stable temperature and the humid conditions provide the required moisture.
Molds can cause serious health hazards as they are able to trigger asthmatic and allergic reactions. Molds are risky – both living and dead as they produce harmful toxins. And it has been proven that long term exposure to even low levels of mold toxins can lead to permanent chemical sensitivity to some common environmental compounds.