What this video covers
The clip explains what a musty smell physically is.
The odour comes from microbial volatile organic compounds, produced by mold only while it is actively growing. Dormant or dead growth does not generate them, which is why the smell carries specific information.
The takeaway
If you can smell it, something is growing right now. The smell is evidence of an active colony rather than a historic one, which makes it worth acting on immediately.
Read the full written guide: Musty Smell in the House.