How much mold is growing?

Why are molds a concern?
Is there a mold problem?
How can you tell if it is mold?
How much mold is growing?
How to clean mold?
Dealing with an ongoing mold problem.
Preventing mold.
Mold-Proofing your home, basement.
Mold-Proofing your home, bathrooms, kitchen, laundry.
Mold-Proofing your house, closets and bedrooms, exterior.
Should I have my house air tested for mold?
The air feels dry, can I humidify?
What advantages do HEPA vacuums provide?
Is vacuuming with a HEPA or externally exhausted vacuum cleaner recommended for serious mold problems only?
Where do you find a HEPA vacuum cleaner?

One way to know is to estimate the area of the mold.

Mold is considered to cover a "small area" if the patch is no larger than a square meter. There should be no more than three patches, each patch smaller than a square meter. Clean up small areas yourself using a detergent solution, household rubber gloves and a dust mask for protection.

Small moldy areas in homes may become larger over time, if ignored, so it's important to clean up and remove even small patches of mold.

The mold area is considered "moderate" if there are more than three patches, each patch smaller than a square meter, or there is one or more isolated patches larger than a square meter but smaller than 3 square metres (size of a 4 x 8 foot sheet of plywood). Assessment by a professional is recommended. You can clean up moderate amounts of mold but you must follow the proper procedures and use the proper protective equipment.

A mold area is considered "extensive" if a single patch of mold is larger in area than a sheet of plywood. Being exposed to this much mold is not a good idea. Do not attempt to clean up large areas of mold yourself. You need professional help to determine why the mold is there in the first place and how to clean it up.