Mold spores are in all indoor and outdoor air. That is why the most common home kit, a settle plate you leave open in a room, almost always grows something. A plate with colonies does not mean you have a mold problem, and EPA notes there are no federal limits for mold or mold spores to compare a result against.
When you do not need a test
If you can see mold or smell it, EPA's guidance is that sampling is usually unnecessary: fix the moisture and clean it up. CDC likewise says you do not need to identify the type of mold, because any mold growth should be removed.
When testing helps
- A persistent musty smell with no visible mold, which may mean it is inside walls, under floors or in ductwork.
- After a leak or flood, to check that cleanup worked.
- Before buying a home with past water damage, or in a landlord dispute where you need a written record.
In those cases a professional assessment, with outdoor comparison samples and a lab report, is worth more than a plate kit. A DIY kit is a cheap first look, not an answer.
Cleaning it yourself
EPA's rule of thumb: an area under about 10 square feet (roughly 3 feet by 3 feet) you can usually handle yourself. Fix the water source first, scrub hard surfaces with detergent and water, dry everything within 24 to 48 hours, and throw out soaked porous materials like ceiling tiles and carpet. Larger areas, or mold in HVAC systems, call for a professional.
Keeping it from coming back
- Keep indoor humidity below 60%, ideally 30-50%. A cheap hygrometer or air quality monitor shows it.
- Run bathroom fans during and after showers, and vent dryers outside.
- Fix leaks fast. Mold can start growing within 24 to 48 hours on wet materials.
Hiring an inspector
Only a few states license mold assessors, among them Texas, Florida, New York and Louisiana, plus Washington DC. Your state page under air quality by state shows the rule where you live. Anywhere, ask for a written report, and be wary of a company that both tests and sells the cleanup.