The answer is that it all depends. Let me try to explain. Mold needs moisture in order to grow. No moisture; no mold. In attics, moisture can occur from a leak in the roof or from condensation due to elevated levels of humidity (typically from improper ducting from a bathroom exhaust fan). Mold has the possibility of growing on attic sheathing, which is typically a wood product (something mold likes to eat). A surface sample can help determine if these stains are mold growth.
Many times, even if a surface sample does show mold growth, it may not be an indication that it is resulting in excessive levels of mold spores in the living spaces below. Why is that? It’s something called “stack effect”. This means that warm air is more buoyant and rises. Air generally moves from the lower levels of a house up through the top. If mold growth originates in the attic, it is hard for spores to fight against this pressurization. Attics can also (although not always) have a higher level of ventilation which can help remove the spores.
Does that mean that it’s not a big deal to find mold growing in an attic? Healthwise, issues can still occur if an occupant has a hypersensitivity and there is an HVAC system or ductwork in the attic space. Mold spores can be sucked up into the return ductwork and spread throughout the home. Even if the spores are more or less contained in the attic, the very presence of mold indicates a moisture issue which can lead to rot and destroy sections of the roof. Although not a huge health issue, removing rotting wood can be a very expensive project.
We have seen attic mold discovered during a real estate transaction kill the deal. Therefore we recommend people remediate attic mold if for no other reason than to avoid concerns from a nervous purchaser in the future.
So to answer the question about how big a deal it is to have mold in the attic, one needs to contemplate how moisture is entering into this space and how much damage is occurring because of it. Occupants also need to ascertain if there is any possible way that the spores can access the living areas of the house from the attic. Although attic mold is typically not a huge health issue, it does represent possible dangers and excessive costs if not dealt with immediately.