Indoor Air Quality — The Four Numbers That Matter

Number 1 — CO₂ (ppm)

CO₂ is the proxy for occupant-generated pollutants. As you breathe, you put CO₂ into the air; you also put water vapour, exhaled VOCs and skin particles. CO₂ tracks all of these — and unlike the others it's cheap and easy to measure.

<800 ppm — excellent. Indistinguishable from outside.
800–1000 ppm — good. Normal indoor target.
1000–1500 ppm — noticeable cognitive impact. Mild headache risk.
1500–2000 ppm — measurable drop in concentration, decision-making and sleep quality.
>2000 ppm — concerning. Common in unventilated bedrooms overnight.

Number 2 — Relative humidity (% RH)

The sweet spot is 40–60% RH. Below 30% RH skin dries, eyes itch, respiratory infections rise. Above 65% RH dust mites thrive, condensation forms on cold surfaces and mould risk climbs. The target indoor band is 40–60% year round.

Humidity is also the canary for condensation problems. If you're chronically above 65% RH in winter, condensation on cold walls is almost guaranteed — and remediating the cold surfaces (IWI, thermal-bridge correction) without controlling humidity will just shift the problem.

Number 3 — PM2.5 (µg/m³)

Fine particulate matter — 2.5 microns and smaller. Comes from cooking, candles, biomass burning, traffic, woodburners and (less obviously) from outdoor pollution that enters through windows. PM2.5 enters the bloodstream through the lungs and is linked to cardiovascular and respiratory disease.

<5 µg/m³ — WHO 24-hour guideline. Excellent indoor target.
5–15 µg/m³ — moderate. Worth investigating sources.
15–35 µg/m³ — elevated. Significant exposure if sustained.
>35 µg/m³ — high. Health-impacting if sustained.

Number 4 — VOCs (µg/m³ or ppb)

Volatile organic compounds — a catch-all for emissions from cleaning products, paints, glues, MDF, candles, air fresheners, carpets and personal-care products. Formaldehyde is the highest-impact VOC indoors. VOC levels are typically highest in newly-renovated homes.

Source control first — low-VOC paints, formaldehyde-free MDF, fragrance-free cleaning products
Ventilation second — fresh air dilutes whatever is still being emitted
Filtration third — activated-carbon filters in MVHR units can adsorb a meaningful proportion of VOCs

The RetrofitIQ approach to IAQ

Continuous logging of CO₂, RH, T, PM2.5 and VOCs for 7–14 days
Spot measurements of formaldehyde where new MDF / particleboard is present
Source-by-source inventory (cooking, cleaning, furniture, candles, biomass)
Ventilation rate verification (anemometer + blower-door + tracer-gas where indicated)
Prioritised IAQ improvement plan: source control → ventilation → filtration
Re-measured IAQ data after improvements as verification

Common questions from clients.

Do I need to test for radon?

Only if you live in a designated radon-affected area or have a sub-floor / cellar with poor ventilation. UK Health Security Agency publishes detailed maps — postcode-check before commissioning a test.

Are the £100 IAQ monitors any good?

They're useful as continuous trend monitors but their absolute accuracy on PM2.5 and VOCs is poor. For a proper assessment we use research-grade instruments alongside the home monitors for context.

Can opening windows solve the problem?

Briefly, yes. Continuously, no — heating goes out, outdoor pollutants come in, and most occupants don't actually open them enough in winter. A properly-designed mechanical ventilation strategy is the durable solution.

Indoor air quality — the four numbers that actually matter.