IAQ vs IEQ: What’s the Difference?

IAQ vs IEQ looks like a pedantic distinction until a client asks why a clean air quality report hasn’t stopped the complaints from residents, workers, or visitors.

Indoor air quality (IAQ) describes the air itself.

Indoor environmental quality (IEQ) covers everything that shapes how a space feels to be in, air included.

This guide explains how the two relate, who actually gets to judge “quality”, and where ASHRAE, CIBSE and BESA currently stand. If what you need first is a clear picture of what’s in your air, start with our indoor air quality testing service.

IAQ vs IEQ: the short answer

IAQ is one part of IEQ. IAQ covers the composition of indoor air: CO₂, particulates (PM2.5 and PM10), VOCs (volatile organic compounds, the gases released by paints, cleaning products and furnishings), mould spores, and the ventilation that dilutes them. IEQ covers IAQ plus thermal comfort, lighting and acoustics, and — increasingly — water quality.

That last row is where the interesting argument starts because it marks a shift in the relationship between buildings and people. More on that later.

What does IEQ include?

ASHRAE Guideline 10-2023, Interactions Affecting the Achievement of Acceptable Indoor Environments, names four principal factors:

• Indoor air quality

• Thermal environment

• Acoustical environment

• Visual environment

Each has its own sub-aspects. Lighting alone spans illuminance, colour temperature, glare and luminance ratios.

The guideline’s real contribution is its focus on interactions. A space can hit its target in every category and still be unacceptable because fixing one factor upsets another: more ventilation can mean more noise or colder draughts, for example. IAQ alone can’t see that, but IEQ is purposefully designed to.

Who does “quality” belong to?

In a paper presented at ASHRAE’s 2025 IEQ conference,IEQ Confluence: A Pathway for Implementing ASHRAE Guideline 10, Carl Grimes argues that the “quality” of IEQ is “an occupant characteristic”. A building has properties: temperatures, airflow rates, lux levels, pollutant concentrations. These can be measured objectively. Quality only exists once a person experiences them.

The paper’s second point is subtler. Objective measurements and subjective reports shouldn’t be blended into a single score, because they behave differently and the result confuses both. Keep them as two separate systems, then study how one relates to the other. Guideline 10 does this well on the physical side. What it doesn’t do, the paper notes, is cover the occupant side.

Guideline 10 is candid about this gap. It acknowledges that ergonomics, occupant density, privacy, views to the outdoors and the aesthetics of a space can dominate how people react, however good the measured factors are. None of those appear on a sensor dashboard.

In practice, this means continuous data is only half the picture. IAQ monitoring tells you what the building is doing. The other half is asking the people in it.

The Rincón case study: measuring the building and the person

The same paper uses a published study as its worked example: Rincón et al. (2024), Environmental house calls can reduce symptoms of chemical intolerance, in Primary Health Care Research & Development. Grimes is one of its co-authors.

The wider programme visited 37 homes of people reporting chemical intolerance, a condition where low-level exposure to everyday chemicals triggers multi-system symptoms. A team led by a physician/industrial hygienist and a certified indoor air quality specialist made structured visits (five per home). They interviewed occupants, inventoried exposure sources, and took air samples analysed by an accredited lab. Occupants then received verbal and written guidance and removed sources themselves. The team retested the air afterwards.

The subjective side was tracked with the QEESI Symptom Star, a chart built from the Quick Environmental Exposure Sensitivity Inventory. Occupants rate ten symptom categories (head, cognitive, skin, gastrointestinal and so on) from 0 to 10, before and after, and the two plots are laid over each other. The paper points out that the symptoms that didn’t change are as informative as those that did, because they show what the environment wasn’t responsible for.

The results in the featured home were striking. Gasoline-related VOCs fell from 530 to 19 ng/L (a 96% decrease) and personal care product VOCs from 1,200 to 560 ng/L (53%), with fragrance, mothball and paint-related compounds also dropping. The Symptom Star showed improvements across the symptom categories. The published study reports that symptom improvement corresponded with reduced indoor VOCs, most notably fragrances.

Two cautions. This was a small, specific group, so it illustrates a method rather than proving a general rule. And the paper itself says the Symptom Star isn’t proof or an objective measure on its own. Its value is in the side-by-side: measured exposure on one chart, lived experience on the other, and occupants who could see for themselves that changing the environment changed how they felt.

Nobody in an office will be filling in a QEESI, but the principle transfers directly. Pair the sensor data with a structured occupant survey, run both consistently, and compare.

What do ASHRAE, CIBSE and BESA say about IEQ?

ASHRAE's Center of Excellence for Indoor Environmental Quality lists acoustics, IAQ, lighting, thermal comfort and water as its areas of focus. ASHRAE is also developing BSR/ASHRAE Standard 235P, Standard for Indoor Environmental Quality (IEQ) in Buildings, which would treat IEQ as a whole rather than through separate standards for air, thermal comfort and energy. It went out to tender for its first draft in mid-2026, so it is not yet something to cite as a requirement. ASHRAE’s stated scope excludes medical criteria and regulatory compliance thresholds.

CIBSE's TM40 Health and wellbeing in building services (2020 revision) already covers thermal conditions, humidity, air, light, acoustics, electromagnetic fields and water, so water as part of the wider picture isn’t new in the UK. It also recommends designing “for the people in a space and not for the space itself”, which is close to the occupant-centred argument above. TM68 (2022) goes on to cover monitoring IEQ across thermal comfort, air, light and sound, and CIBSE held a dedicated Health, Wellbeing and IEQ conference in September 2026.

BESA welcomed BS 40102-1:2023, the first British Standard giving recommendations for measuring, monitoring and reporting IEQ in non-domestic buildings. It rates air quality, lighting, thermal comfort and acoustics to give facilities managers a benchmark score. BESA’s own IAQ guide takes the air-specific route, setting health-led targets aligned with the WHO’s 2021 Air Quality Guidelines. Its IAQ group has also argued that UK government targets lag behind WHO guidance.

So the UK picture is not “IAQ or IEQ”. IAQ guidance serves its specific purpose of ensuring the air is safe and healthy to breathe, while IEQ frameworks are developing and becoming broader, at the same time.

Why the shift from IAQ to IEQ?

Because a building can have excellent IAQ and still fail as a place to work or live. People don’t experience temperature, noise, light and smell separately. They experience them all at once, and stress from one lowers tolerance for the others. Grouping them under IEQ puts the pressure on to fix the whole experience rather than one dial.

There is also a practical driver. Newer buildings are more airtight and energy efficient, yet some occupants report poorer temperature and humidity control, lighting they dislike, and stale air. Energy performance targets matter, but a building that meets them while its occupants struggle hasn’t succeeded.

IAQ or IEQ: which should you use?

Both, for different jobs. Use IAQ when you need a specific, testable answer: pollutant targets, ventilation validation, compliance evidence, or air credits under BREEAM and WELL. Use IEQ when you’re setting strategy, reporting on wellbeing or investigating complaints that the air data doesn’t explain.

For facilities managers, a sensible order of play is:

1. Get IAQ right and measured. It is still the factor with the clearest health evidence and the most regulation behind it.
2. Look at thermal comfort, lighting and acoustics alongside it, and note where they interact.
3. Ask occupants, consistently, using the same method each time so you can compare.

Expect wider legislative and guidance change over the next few years, including from ASHRAE’s 235P.

Get in touch

If you want hard data on the air side of IEQ, talk to our team about indoor air quality testing and monitoring for your building.

Standards and guidance referenced here are revised periodically, and ASHRAE 235P is still in development. Please ensure you verify any important information against the current published versions before relying on them.