The case against having your ducts cleaned: Here’s the thing: Dust that settles in your ventilation ducts typically stays there unless disturbed, and is usually harmless. The official advisory of the U.S. Environmental Protection Agency concludes: “Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle ... levels in homes increase because of dirty air ducts. 

This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. ... Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health.

More at source: SF Gate

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