Surface sampling, usually substantially — it is one sample with no control required. Air sampling means at least two cassettes and often more.

Do I need sampling at all?

Often not. If you can see growth and you know where the water came from, sampling changes nothing about the remediation. It earns its cost for documentation, for clearance, and where something is clearly wrong and nothing visible explains it.

What about a DIY settle plate kit?

A dish left open collects whatever falls into it and will grow something in any building on earth. Without a measured air volume and a control there is nothing to compare the result against.

Side by side

Air sampling vs surface sampling

The difference between air sampling and surface sampling: air sampling counts what was airborne at one spot for a few minutes. Surface sampling identifies what is growing on a specific surface.

Side by side

Axis	air sampling	surface sampling
What it answers	What was airborne here, during these minutes.	What is growing on this spot, definitively.
What it cannot tell you	Where it came from, or how far the growth extends.	Whether any of it is airborne, or whether anywhere else is affected.
Needs a control?	Yes. An outdoor sample the same day, or the result has no scale.	No. It is an identification, not a comparison.
Sensitive to timing	Very. Weather, whether the HVAC was running, whether anyone had just walked through.	Barely. The growth is where it is.
Best used for	Clearance after remediation, and investigating a complaint with no visible source.	Confirming that a visible suspect area is what it appears to be.

Sample the question you actually have

If you can see something and want to know what it is, take a surface sample — it is cheaper, definitive, and needs no control. If you cannot see anything and the building smells wrong, air sampling is one of the few tools available, and it is only interpretable with an outdoor control taken the same day. Neither one finds hidden growth: that is what a moisture survey is for.

Why the outdoor control is not optional

Outdoor spore concentrations vary by orders of magnitude with season, weather and what is happening in the neighbourhood. An indoor count of a few hundred spores per cubic metre might be well below outdoor levels on a summer afternoon and distinctly abnormal on a still winter morning.

A laboratory report showing indoor numbers with no same-day outdoor comparison is not interpretable, and should be treated as such regardless of how definitive the summary language sounds.

What neither of them does

Neither locates hidden growth. A wall cavity with active growth and no air path into the room can produce an unremarkable air sample, and a surface sample requires knowing where to sample.

Finding concealed growth is a moisture problem, not a sampling problem: meters, thermal imaging and an understanding of where water goes in that building. Sampling confirms and characterises what a survey has already found.

This is the most common way money gets spent badly here — several hundred pounds of sampling that produces a laboratory report and no idea where the water is coming from.

Questions people ask

Which is cheaper?: Surface sampling, usually substantially — it is one sample with no control required. Air sampling means at least two cassettes and often more.
Do I need sampling at all?: Often not. If you can see growth and you know where the water came from, sampling changes nothing about the remediation. It earns its cost for documentation, for clearance, and where something is clearly wrong and nothing visible explains it.
What about a DIY settle plate kit?: A dish left open collects whatever falls into it and will grow something in any building on earth. Without a measured air volume and a control there is nothing to compare the result against.

Other pairs that get confused

Both terms are defined in the glossary.