01
Define the decision
Write down what will be decided from the result, which rooms matter and what change is expected.
Practical evidence guide
How to measure whether an indoor-air intervention worked.
A before-and-after result is useful only when the decision, baseline, intervention and comparison are defined clearly. This is a practical method for ventilation, filtration, retrofit and remedial work.
Start with the decision
Measurement should answer a question, not simply produce a dashboard.
“Did it work?” is too broad. Decide first whether the work is expected to reduce particles, improve occupied-room ventilation, stabilise moisture conditions or change another defined outcome.
Write down who will use the result and what they may do next. That choice determines the rooms, measures, monitoring window and level of certainty the study needs.
A useful brief can be one sentence: “After the extractor repair, did repeated bathroom moisture events clear more quickly under comparable use?”
The method
Six stages from intervention to decision.
02
Establish the baseline
Capture enough representative use to see the existing pattern, not just one convenient day.
03
Record the intervention
Log the date, settings, rooms and operating changes precisely enough for the comparison to mean something.
04
Monitor the follow-up
Use consistent sensor placement and collect the same outcome measures after the work.
05
Compare like with like
Review similar occupied periods, weather context, room use and any untreated comparison rooms available.
06
Make the next decision
Report the observed change, the uncertainty and whether to retain, adjust, investigate or monitor again.
There is no universal monitoring duration. The window should be long enough to capture representative use and the pattern the intervention is expected to change. Plan it before the work wherever possible.
Choose the right evidence
Different interventions require different measures.
| Intervention | Primary evidence | Context to record | Claim boundary |
|---|---|---|---|
| Ventilation or operating routine | Occupied-period CO₂ pattern, duration and room-clearing behaviour | Occupancy, doors and windows, system operation, temperature and outdoor conditions | CO₂ is ventilation and occupancy context; it is not a complete air-quality or safety measure. |
| Filtration or air purification | Particle levels, event frequency, duration and recovery | Equipment setting, room volume, indoor sources, outdoor particles and placement | Particle filtration should not be judged from CO₂, which most portable filters do not remove. |
| Damp, moisture or remedial work | Humidity and temperature recurrence, duration and recovery | Inspection findings, heating, ventilation, drying, weather and room use | Environmental monitoring can add context but does not diagnose the source of damp or mould by itself. |
| Retrofit or heating change | Room-level temperature, humidity and ventilation-context patterns | System schedule, occupancy, weather, room differences and the intended building outcome | A simple before-and-after comparison cannot automatically rule out seasonal or behavioural explanations. |
Compare responsibly
A change after the work is not automatically proof of cause.
Weather, occupancy, source activity and equipment operation may all change between periods. Compare similar conditions, retain an untreated room where practical and document alternative explanations.
HM Treasury’s evaluation guidance recommends planning evaluation early, defining how an intervention is expected to work and using comparison evidence where feasible. A simple before-and-after design can show association, but it cannot by itself rule out another cause.
Observed change
The measurements changed after the work.
This is the minimum defensible finding when the baseline, follow-up and data quality are clear.
Supported interpretation
The change is consistent with the intervention working.
Confidence improves when the timing, expected signal and comparison evidence align and alternatives are considered.
Causal conclusion
The intervention caused the change.
This needs a stronger design than a simple pre/post comparison. A comparison group or other robust method may be required.
Common weaknesses
Six signs the result will be difficult to defend.
- 01The baseline begins after the work has already started.
- 02The sensor moves between the baseline and follow-up periods.
- 03Several interventions or operating routines change at once.
- 04Occupied and unoccupied periods are averaged together without context.
- 05Weather, outdoor pollution or seasonal conditions shift substantially.
- 06Only the best-looking day or headline average is reported.
A useful report
The output should support one of four honest conclusions.
Clear improvement
The expected measure improved repeatedly and the wider context supports the interpretation.
Mixed result
Some rooms, periods or measures improved while others still need attention.
No clear change
The selected outcome did not materially improve, or natural variation was larger than the observed difference.
Insufficient evidence
Coverage, comparability or context is too weak to make a responsible call.
Primary guidance
Sources behind the method.
HM Treasury: Magenta Book evaluation guidanceDefining the intervention, planning evaluation early, comparison approaches and proportionate claims.HSE: Using CO₂ monitorsRepresentative placement, repeated monitoring and the limits of snapshot readings.GOV.UK: Existing home ventilation guideThe relationship between ventilation, indoor moisture and changes to the building.GOV.UK: Damp and mould guidance for housing providersFollow-up after treatment, property-condition monitoring and the need to address underlying causes.US EPA: Air cleaners and air filtersHow portable air cleaning relates to particles, sources and ventilation.
This guide explains a proportionate monitoring approach. It is not a substitute for competent ventilation, building, damp, occupational-hygiene, legal or clinical assessment where those are required.
What intervention needs measured follow-up?
Define the rooms, expected outcome, baseline, intervention date and decision audience. Aeroshi will suggest a proportionate starting scope.