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Specifically, are they both dangerous to human health?

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There are thousands of species of mold known to Science, with different species being found in different parts of the world. There isn't any one "crawlspace" mold or "air duct" mold, it's just whatever spores of whatever species happened to take a foothold there. Which one is more dangerous will depend on the particular species of mold and the sensitivity of the people that are exposed to it.

In general, though, you're better off not having them growing in your home because even if you don't have a really dangerous one growing now, you obviously have the right conditions for mold so a dangerous one could start growing too.

For more information, have a look at this Wikipedia article on Mold health issues

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I suppose having mold spores in your HVAC will spread around the house more than one relegated to the crawlspace. But that does not determine how bad they are for your health. –  auujay Sep 1 '11 at 22:01

Mold detection and identification can be done in two ways. Visible mold can be collected on a test tape, and air borne spores can be collected with an air sample kit. Either way, samples must be sent to a lab for microscopic inspection and ID.

Mold and mold spores can range from common everyday varieties that rarely have negative effects on humans to very toxic species that can cause serious illness. Most common molds are classified as irritants and the effects will vary depending on how sensitive the people who contact it are, or if they have allergies or other respiratory conditions.

In probably 90% of the tests I send to the lab, the results come back as mild irritants. In some cases, especially in moldy walls or floors around bathroom fixtures where leaks have gone unmitigated, Black mold is common and this is dangerous if spores are allowed to go airborn.

I repeat, however, the only reliable way to identify dangerous molds is to have samples analysed by a professional lab.

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As has been said, there are thousands of species of mold, and determining exactly what you may have, and whether it's dangerous in general, is a job for the pros.

If you are sensitive to mold spores, whether through allergies, asthma or respiratory disease, or you have a suppressed immune system due to cancer treatment, autoimmune disease or HIV, it doesn't matter what species of mold it is, or how much there is; they're all bad. That said, you will not find a house more than a few years old that you could do a standard air screening of and find zero mold. Mold is a natural consequence of human habitation and a terrestrial habitat. You can only minimize it.

To do so, simply understand that most harmful molds need three things; water, simple organic material (like the paper in drywall, or food wastes, or the nutrients in soil), and low light levels. You usually can't reduce the organic component or the light levels unless you want to remove all the drywall from your home (not a great look), but you usually can control the moisture element. Run the A/C in your home (and make sure it drains properly). Make sure all plumbing is watertight and leak-free. Clean up spills immediately. Check your basement or first floor for any signs of water or water damage (softened wallboard, peeling or bubbling paint, and cracked or crumbling mortar or brickwork are all bad signs)

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