CO₂ · PPM
Is there enough fresh air for the people in the room?
Outdoor air is around 420 ppm. Indoors, CO₂ rises as people breathe, so it's a good proxy for ventilation per person. Sustained levels above roughly 1,000 ppm are widely treated as a sign that ventilation isn't keeping up; UK guidance for school classrooms, for example, sets a daily average below 1,500 ppm.
It matters beyond stuffiness. A Harvard study found people scored markedly better on decision-making tests in well-ventilated offices than in typical ones.
PM2.5 · µg/m³
Fine particles
Tiny particles from traffic, cooking and outdoor pollution that reach deep into the lungs. The World Health Organization's 2021 guidelines set 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours. Good filtration in the air handler, and purifiers where needed, bring indoor levels down.
TEMPERATURE · HUMIDITY
Comfort
Most people are comfortable around 20 to 24 °C, depending on season and clothing. Relative humidity of about 40 to 60% is commonly recommended: drier air irritates eyes and throats, damper air encourages mould.
VOCs
Chemicals in the air
Volatile organic compounds come from furniture, paints and cleaning products. Affordable sensors show trends rather than precise amounts, which is enough to spot a problem room or time of day.
How to measure well
- Measure in occupied rooms, at breathing height, away from windows, doors and vents.
- Run for at least one to two weeks to see daily and weekly patterns.
- Use proper CO₂ sensors (NDIR type), not cheap estimates.
- Start with the busiest spaces: meeting rooms, classrooms, open-plan floors.
What usually fixes it
Often it's the controls, not new equipment: ventilation on a fixed timer instead of following occupancy, outdoor air dampers closed too far to save energy, or filters overdue. Demand-based ventilation gives busy rooms more fresh air and empty ones less, which can improve air and save energy at the same time.
Want to know what your building is breathing?
We measure air quality zone by zone for two weeks and show you where it matters and what it takes to fix.