Indoor air quality is easier to improve when you measure the right things in the right rooms. This guide explains how to track humidity, carbon dioxide and particulate matter at home, how to interpret changing readings, and how to check whether ventilation and everyday habits are making a lasting difference.
Overview
Indoor air quality is not a single number. It is the result of moisture, ventilation, pollutants, building condition and household activities interacting over time. A home can feel comfortable while still having high humidity in a bathroom, elevated carbon dioxide in a bedroom or fine particles after cooking.
For most households, three measurements provide a useful starting point:
- Relative humidity: helps identify conditions that may contribute to condensation, dust mites and mould growth.
- Carbon dioxide (CO2): acts as a practical indicator of how well a room is being ventilated while people are using it.
- Particulate matter (PM): can rise during cooking, candle burning, smoking, DIY and when outdoor pollution enters the home.
These readings are prompts for investigation, not a diagnosis. A domestic monitor cannot identify every pollutant, prove that mould is harmless or replace professional advice where there is persistent damp, a suspected gas or combustion problem, or health concern. Use readings alongside visible condensation, odours, draughts, occupant symptoms and the condition of the building.
If poor air quality appears to be linked to inadequate airflow, compare the options in our guide to which home ventilation system you need. A suitable extractor fan, positive input ventilation system or whole-house mechanical ventilation system should address the cause rather than simply masking symptoms.
What to track
Relative humidity
Record humidity as a percentage, preferably with temperature. As a general household guide, readings around 40–60% are often a reasonable comfort range, but the important question is whether high humidity is sustained and where it occurs. Repeated readings above 60% deserve investigation, particularly in bedrooms, bathrooms, kitchens and cold corners. Short-lived peaks after a shower or while boiling food are expected; the concern is moisture that remains high because it is not being removed.
Place a hygrometer away from radiators, windows, exterior walls, direct sunlight and the outlet of an extractor fan. Take readings at roughly the same height each time. If two monitors disagree, compare them side by side for several hours before deciding that one is faulty.
Carbon dioxide
CO2 is produced by people and builds up when fresh air is not entering quickly enough. It is therefore most useful in occupied rooms, especially bedrooms, home offices and living rooms. A rising reading during normal use can indicate that windows, background ventilators or mechanical systems are not providing enough air movement for the space.
Use broad bands as investigation prompts rather than pass-or-fail limits. Readings closer to outdoor background generally suggest more effective air exchange, while persistently elevated readings during occupation suggest that ventilation should be reviewed. A sudden rise in a closed bedroom overnight is more informative than a single high result immediately after several people enter a room.
Choose a monitor with a true CO2 sensor rather than one that estimates CO2 from volatile organic compounds. Check whether the device explains its sensor type, calibration method and recommended operating conditions.
Particulate matter
PM monitors estimate tiny airborne particles, commonly reported as PM2.5 or PM10. Indoor levels can increase when frying, grilling, burning candles or solid fuels, smoking, sanding or using sprays. Particles can also enter from outside through open windows, gaps and ventilation inlets.
Look for patterns rather than treating one spike as a permanent condition. A sharp increase during cooking that falls after the hob hood or extractor is used suggests a source-control opportunity. If readings remain high after the activity has stopped, check filtration, cleaning, ventilation and possible outdoor sources. Avoid placing a monitor directly beside a cooker, open window or air purifier outlet, as this can distort comparisons.
Room-by-room checklist
| Room | What to measure | What to observe |
|---|---|---|
| Bedroom | Overnight humidity and CO2 | Morning condensation, blocked vents, closed doors and comfort during sleep |
| Bathroom | Humidity before, during and after bathing | Extractor run-on time, steam clearance and mould around joints |
| Kitchen | Humidity and PM during cooking | Whether the cooker hood extracts outdoors, filter condition and lingering odours |
| Living room | CO2 while occupied; PM after candles or fires | Airflow, open-fire use, furnishings blocking vents and window condensation |
| Home office | CO2 during working hours | Occupancy, closed windows and whether background ventilation is available |
| Utility or laundry area | Humidity during drying | Drying clothes indoors, door position and local extraction |
Cadence and checkpoints
Start with a seven-day baseline if possible. Place the monitor in one location for several days rather than moving it repeatedly, and note occupancy, cooking, bathing, window opening and unusual activities. A simple log can include the date, time, room, temperature, humidity, CO2, PM reading and relevant household activity.
After the baseline, review readings:
- Daily: check for overnight humidity and CO2 peaks, bathroom recovery after bathing, and PM spikes during cooking.
- Weekly: compare similar rooms and identify whether problems occur at particular times or with particular activities.
- Monthly: inspect and clean accessible grilles, filters and fan covers according to the equipment instructions. Record any changes to heating, occupancy or room use.
- Quarterly: compare the current pattern with the previous season. Cold weather, drying clothes indoors and less window opening can all alter results.
Make one change at a time where practical. For example, use the bathroom fan correctly for a week before changing several other habits. This makes it easier to see whether the intervention helped.
How to interpret changes
| Pattern or symptom | Possible explanation | Useful next step |
|---|---|---|
| High humidity mainly in the bathroom | Steam is not being extracted or the fan is undersized, poorly positioned or not running long enough | Check the fan, grille, duct route and controls; use the fan during bathing and for its intended run-on period |
| High humidity across several rooms | Moisture production exceeds whole-home air exchange, or there may be a building or plumbing issue | Review cooking, drying and occupancy; inspect for leaks and seek a ventilation assessment if the pattern persists |
| CO2 rises overnight in a bedroom | Limited fresh-air supply in a closed room | Check background ventilators and mechanical ventilation; improve airflow without creating unsafe draughts |
| PM spikes during cooking | Particles from frying, grilling or poorly captured cooking emissions | Use effective source extraction, keep filters clean and reduce high-emission cooking methods where practical |
| Condensation and mould on cold surfaces | High moisture combined with cold surfaces, poor circulation or thermal bridging | Reduce moisture at source, improve air movement and investigate the fabric of the building rather than painting over mould |
| Readings change sharply between devices | Different sensor technologies, placement, calibration or response times | Place devices together temporarily, compare trends and follow the manufacturer’s calibration guidance |
Ventilation should not be the only response. Use lids and cooker extraction when preparing food, vent moisture from bathrooms, avoid drying large loads in unventilated rooms and keep air paths clear. Do not permanently seal designed ventilation openings or disable a system because it feels draughty without first checking whether it is correctly commissioned.
For recurring condensation and mould, our room-by-room ventilation guide provides a useful follow-up. If you are comparing systems, see MVHR versus PIV, remembering that suitability depends on the property, airtightness, layout, existing ventilation and installation quality.
When to revisit
Return to your records after any change that could affect air quality: replacing an extractor fan, installing PIV or MVHR, altering windows, renovating, adding insulation, changing heating, moving home or changing household occupancy. Recheck after a week for immediate effects and again after a month to see whether the improvement remains consistent.
Review the baseline at least seasonally. A home that performs well in mild weather may show different humidity and CO2 patterns during a cold winter when windows stay closed. Likewise, summer outdoor pollution, increased occupancy and different cooking or drying habits can change PM readings.
Use this practical review routine:
- Choose one room with the clearest problem and confirm the monitor is positioned correctly.
- Record a normal week, including activities that create moisture or particles.
- Address the source first, then check local and whole-home ventilation.
- Inspect filters, grilles, ducts and fan operation in line with manufacturer guidance.
- Repeat the same measurements under similar conditions and compare trends, not isolated readings.
- Arrange professional advice if high readings persist, mould returns, there is evidence of damp or leaks, or the ventilation system cannot maintain acceptable conditions.
Keep the log with appliance manuals and maintenance records. Over time, it becomes a practical record of how your home responds to seasonal changes and which indoor air quality measures genuinely work.
