The short answer
Mostly not for decision-making. A settle plate left open in a room will almost always grow mould, because spores are present in all indoor and outdoor air. It can't tell you how many spores are in the air, how that compares with outside, or where they came from — which are the questions that matter.
How a settle plate works
A settle plate is a petri dish of growth medium left open in a room for a set time. Spores that happen to fall onto it grow into colonies over the following days.
Why the result is hard to use
- No air volume. The plate catches whatever falls on it, not a measured volume of air, so results can’t be expressed as spores per cubic metre.
- No comparison. Without an outdoor sample taken the same way at the same time, there’s no baseline to compare with.
- Bias towards certain moulds. Heavier spores fall faster; some moulds grow better on the medium than others. Some important indoor moulds grow poorly or slowly on standard plates.
- Spores are everywhere. The World Health Organization notes there is no health-based numeric threshold for mould anyway, so “it grew mould” says very little.
Better first steps
- Look for the moisture. Leaks, condensation on windows, a weak exhaust fan, a wardrobe pushed against a cold external wall.
- Use a humidity monitor. Indoor relative humidity that stays high for long periods tells you more than a petri dish. The Queensland Government’s advice is to keep the home dry and well aired.
- Use a moisture meter as a comparison tool. A wall that reads much higher than the others deserves a closer look.
- Fix the obvious. The Australian Centre for Disease Control’s prevention advice is to ventilate, keep spaces dry and fix leaks and plumbing faults.
When to get a professional inspection
If mould keeps coming back after the obvious causes are fixed, if there’s a musty smell you can’t trace, if someone in the household is more vulnerable, or if you need documented results for a purchase, claim or tenancy, a measured inspection with moisture mapping and properly controlled air sampling is the better spend.
Questions people ask
My DIY plate grew lots of mould. Is my house contaminated?
Not necessarily. Spores are always present, so plates nearly always grow something. Without a measured air volume and an outdoor comparison, the result can't show whether indoor air is worse than outside.
Is a cheap moisture meter useful?
As a guide, yes. Comparing a suspect wall with a dry wall in the same house can show where damp is. Readings vary between materials, so treat them as relative, not absolute.
References
- [1]WHO guidelines for indoor air quality: dampness and mould (2009) — World Health Organization Regional Office for Europe
- [2]Preventing mould exposure — Australian Centre for Disease Control
- [3]Mould and damp — Queensland Government
Checked against these references on 2 November 2026.