Is mold often harmful?

Yes it can be: Molds and mold spores are common in every household.It is when these are present in large numbers, they can present a health hazard to humans. It can cause allergic symptoms and respiratory problems there are many different molds and the worst ones are the toxic black molds. Some molds with toxic properties have been useful to us, example is penicillin, and in low doses may be used to fight infection.

Rarely harmful: Most people exposed to mold are either unaffected or annoyed (runny nose, irritated eyes, cough, nausea, headache). The worse the exposure, the more annoying it is. People with lung disease or poor immunity can get very sick.