Know what's in the air at home.
CO2, humidity, gas and radiation checks, and what to do with the readings. Tap a problem to jump to it.
Stuffy rooms (CO2)
Why it happens
People breathe out CO2. If a room isn't ventilated enough, it builds up.
How to tell it apart
High CO2 tells you ventilation is poor. It doesn't measure other pollutants.
How to test
- Place the monitor at head height, away from windows, doors and vents, and more than 50 cm from people.
- Log readings while the room is in normal use.
- Use the average over the occupied period, not a snapshot.
A monitor such as the ZN-MT13 logs CO2 over time.
Good and bad readings
| CO2 in an occupied room | What it means |
|---|---|
| About 1000 ppm | Roughly 10 litres of fresh air per second per person |
| Consistently above 1500 ppm | Poor ventilation: take action |
Source: HSE, Using CO2 monitors (workplace guidance; HSE notes CO2 is a broad guide, not a "safe level").
What next
Open trickle vents or windows, use extractor fans and re-measure.
Common mistakes
Placing the monitor next to people or a window.
Smell after painting or new furniture (VOCs/HCHO)
Why it happens
New paint, flooring, furniture and cleaning products release volatile organic compounds (VOCs), including formaldehyde (HCHO).
How to tell it apart
A smell that fades over days or weeks and is strongest near the new item points to off-gassing. A smell of gas is different, so see the gas section.
How to test
- Log VOC/HCHO readings in the room before and after airing it.
- Compare with another room.
- Watch the trend over several days.
What next
Ventilate well, especially in the first weeks. Low-cost sensors show trends, not exact concentrations, so we don't quote a limit here. If you're worried about health effects, ask your GP or local environmental health team.
Common mistakes
Treating a sensor value as a lab-grade measurement.
Damp air and mould
Why it happens
Cooking, washing and drying clothes add moisture; poor ventilation keeps it in.
How to tell it apart
Steamed-up windows every morning and mould in corners point to condensation.
How to test
- Log humidity in each main room for several days.
- Note peaks after showers, cooking or overnight.
- Compare rooms.
Good and bad readings
| Indoor relative humidity | What it means |
|---|---|
| 40%–65% average | The recommended range |
| 70%–80% | Mould spores multiply |
Source: Westmorland and Furness Council, Damp and mould guide.
What next
Ventilate, heat evenly and cut indoor moisture. See our building and damp page for wall checks.
Common mistakes
Single readings; drying clothes on radiators.
Smell of gas
Why it happens
Leaking fittings or faulty appliances.
How to tell it apart
A detector can help find a small leak at a joint. It never replaces the emergency procedure.
How to test
- If you can smell gas, follow the emergency steps first.
- For checks, warm up and zero the detector in clean air.
- Move it slowly along joints.
Detectors such as the FNIRSI GD-02 or Benetech GM8800 detect combustible gas.
What next
HSE advises calling the Gas Emergency number 0800 111 999, opening doors and windows, and shutting off the gas at the meter if you know where it is (HSE domestic gas FAQs). A gas detector does not detect carbon monoxide, so fit a CO alarm too.
Common mistakes
Using a detector instead of calling the emergency line.
Radiation / EMF concerns
Why it happens
Background radiation varies by place. Radon is a natural radioactive gas that can build up indoors.
How to tell it apart
A Geiger counter does not measure EMF from wiring or Wi-Fi. It measures gamma/beta radiation in real time. It is not a radon test: radon is assessed as an average over months with passive detectors.
How to test
- Note a background reading outdoors.
- Compare indoor readings in the same units.
- For radon, use a validated three-month test from UKHSA.
A Geiger counter such as the GC-03 shows dose rate and counts.
Good and bad readings
| Radon (annual average in a home) | What it means |
|---|---|
| Above 200 Bq/m³ | UK Action Level: reduce radon |
| 100 Bq/m³ | Target level for remediation |
Source: UK National Radon Action Plan (GOV.UK). This figure applies to a radon test, not a Geiger counter reading.
What next
If a radon test is high, follow UKHSA advice on reducing radon.
Common mistakes
Treating a Geiger counter reading as a radon result.
Instrument for this step
GC-03 3-in-1 Geiger Counter, Nuclear Radiation and EMF Detector£107.99
FAQs
Is CO2 the same as carbon monoxide?
No. CO2 shows how well a room is ventilated. Carbon monoxide is a poisonous gas and needs a CO alarm.
Where should I place an air monitor?
At head height, away from windows, doors and vents.
Can I get a VAT invoice?
Yes. All prices include VAT and VAT invoices are available.
Need help choosing?
Contact us and tell us what you need to measure.