Core topic · Issue 01
Indoor Air Quality: a complete guide for modern buildings
The air inside our buildings shapes how we feel, think and recover. This long-form guide unpacks the science of IAQ — from pollutants and ventilation to monitoring and the standards driving healthier indoor environments.
~90%
Time spent indoors
UK adults across home, work and transit
30+
Pollutants tracked
Particles, gases, biologicals, semi-VOCs
<1000
ppm CO₂ target
Cognitive performance threshold
<10
µg/m³ PM2.5
WHO healthy guideline
Indoor environments shape most of our daily air exposure
Definition
What is indoor air quality?
Indoor air quality describes the chemical, particulate and biological composition of the air inside buildings — and how that composition interacts with occupant health, comfort and cognition. It is shaped by outdoor pollution, building materials, occupant activities, HVAC operation and unventilated combustion sources.
Because UK adults spend roughly 90% of their time indoors, indoor concentrations of common pollutants typically have a far greater health impact than ambient outdoor levels.
The framing has shifted over the past decade. IAQ used to be a niche concern raised after complaints. It is now a continuously measured operational metric in well-run offices, schools and healthcare estates — closer to how energy or water use are tracked than to a one-off survey.
Background
How IAQ became a measurable discipline
The modern interest in indoor air quality begins with the oil crises of the 1970s. As buildings were sealed for energy efficiency, complaints rose and a recognisable cluster of symptoms emerged — what the WHO labelled sick building syndrome in 1983. The first generation of investigations established that the engineering response to one problem (heat loss) had created another (poor dilution of indoor sources).
Through the 1990s and 2000s, the science matured. Epidemiological work tied long-term PM2.5 exposure to cardiovascular outcomes. Chamber studies quantified the cognitive cost of elevated CO₂ and VOCs. The Harvard COGfx programme produced the first defensible dose-response curves for office workers in 2015 and 2016.
The pandemic finished what those decades had started. Continuous CO₂ monitoring became normal. Ventilation rate moved from a building services parameter to an HR conversation. And the regulatory framework that had been in place since the 1992 Workplace Regulations was, for the first time, being read carefully by people outside the engineering profession.
Doubling outdoor air ventilation rates can produce double-digit improvements in cognitive function scores — at a cost of pennies per person per day.
Why it matters
Health, cognition and the cost of poor air
Poor IAQ is linked to respiratory irritation, headaches, asthma exacerbation, reduced productivity and long-term cardiovascular and neurocognitive risk.
Cognitive performance
Decision-making scores rise with lower CO₂ and VOC exposure.
Respiratory health
Asthma and allergy triggers reduced via filtration and source control.
Sleep and recovery
Bedroom CO₂ above 1500 ppm degrades restorative sleep.
Workplace wellbeing
Lower sick leave and higher engagement in well-ventilated offices.
Long-term risk
Chronic PM2.5 exposure linked to cardiovascular and neuro outcomes.
Pollutant intelligence
The pollutants we measure
A workable IAQ programme focuses on a small set of indicators that, together, capture most of the variance in real-world exposure.
Particulates
PM2.5 — the invisible threat
Fine particulates below 2.5 µm penetrate deep into the alveoli and cross into the bloodstream. Indoor sources include cooking, combustion, candles, printers and infiltration of outdoor traffic-related PM. In urban offices, outdoor PM2.5 sets the indoor baseline before any indoor activity is added.
The WHO 2021 annual guideline of 5 µg/m³ is a public-health target rather than a UK regulatory limit. Most UK buildings sit comfortably above it without intervention. Filtration to ISO ePM1 50% — broadly equivalent to MERV 13 — brings indoor levels well under the guideline in normally occupied offices.
Gases
CO₂ as a ventilation proxy
Carbon dioxide is not toxic at concentrations seen indoors, but it is the most useful single indicator of how well a space is being ventilated. Because occupants are the source, CO₂ tracks the dilution rate of every other occupant-generated pollutant — bioeffluents, exhaled droplets, body odour, and the moisture load that drives humidity.
Levels above 1000 ppm correlate with measurable cognitive decline in controlled studies. Sustained levels above 1500 ppm warrant intervention regardless of whether occupants report symptoms.
Chemistry
VOCs and the everyday materials problem
Volatile organic compounds are emitted by paints, adhesives, MDF, soft furnishings, cleaning products and electronic equipment. The mixture is dominated by formaldehyde, benzene, toluene, xylenes and a long tail of semi-volatile compounds that linger for months after a refurbishment.
Total VOC levels below 300 µg/m³ are a working comfort target. Specific compounds — formaldehyde, benzene — have their own health-based thresholds that may bind before TVOC does. VOCs in buildings →
Biology
Humidity, mould and the moisture balance
Relative humidity sits at the centre of a surprising amount of IAQ practice. Below 30% RH, mucous membranes dry, dust becomes more irritant and static charge increases particle resuspension. Above 60%, dust mite and mould growth accelerates, and the perception of indoor temperature shifts upward.
The 40–60% band is the working target for occupied spaces. Hitting it consistently in a UK winter usually requires controlled humidification at the AHU; in summer, dehumidification is the harder problem. Humidity and health →
Measurement
How indoor air quality is actually measured
A credible IAQ assessment combines continuous monitoring with targeted laboratory sampling. The instrument is rarely the variable that matters most — sensor placement and sampling strategy decide whether the data is useful.
Continuous monitoring
CO₂, PM, temperature and humidity captured minute-by-minute in the breathing zone. Twelve months of trend data underpins any serious operational claim.
Laboratory sampling
Sorbent tubes for VOC speciation, DNPH cartridges for formaldehyde, viable air samples for mould. Lab analysis is the only way to attribute a TVOC reading to a source.
Ventilation assessment
CO₂ tracer-gas decay and direct airflow measurement at terminals confirm whether designed ventilation is being delivered in practice. Method →
Remediation
The hierarchy of improvement
Source control beats ventilation, ventilation beats filtration, and verification closes the loop. Skipping a stage rarely saves money.
- 1
Stage 01
Source control
Substitute low-emission materials, vent point sources, switch cleaning chemistry, remove damp at origin. The only intervention that addresses chronic emitters at their cause.
- 2
Stage 02
Ventilation
Bring outdoor air rates up to BS EN 16798-1 Category II. Use demand-controlled ventilation tied to CO₂ where occupancy varies. Commission to measured airflows, not specified ones.
- 3
Stage 03
Filtration
Upgrade to ISO ePM1 50% as a baseline; add activated carbon stages where VOC or odour sources cannot be eliminated. Maintain to a documented filter-change regime.
- 4
Stage 04
Verification
Re-measure after intervention. Without it, remediation is faith-based engineering. Include the verification report in the building log.
Standards
The references that frame UK IAQ practice
UK practice draws on a layered set of references. The Workplace (Health, Safety and Welfare) Regulations 1992 and ACOP L24 establish the legal duty to provide effective ventilation. CIBSE Guide A and TM40 give detailed operational and design guidance. BS EN 16798-1 defines indoor environmental categories for non-residential buildings.
WHO global air quality guidelines provide the health-based reference points for outdoor pollutants that also apply indoors. Voluntary frameworks — WELL, BREEAM, RESET Air — reward credible monitoring and material selection. Workplace compliance →
FAQ
Common questions about indoor air quality
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