We spend around 90% of our time indoors, and the air inside a home is often more polluted than the air outside it. The tricky part is that poor indoor air quality rarely announces itself.

It shows up as tired staff, a stuffy meeting room, or a headache that clears the moment you step outside.

This guide covers the warning signs UK building owners, landlords and facilities managers should look for, what each one usually means, and when it’s worth getting the air properly tested.

The Quick answer: what are the signs of poor indoor air quality?

The most common signs are stuffy or stale air, musty smells, condensation on windows, visible mould, unusual dust build up, and occupants reporting headaches, tiredness, itchy eyes or worsening asthma that improve once they leave the building.

The clearest single indicator is symptoms that follow the building if people feel better at the weekend and worse on Monday, the building is very likely the cause.

First: some signs need immediate action

Before the everyday warning signs, a few situations are urgent rather than routine. Act straight away if you notice:

• Sudden headaches, dizziness, nausea or drowsiness affecting several people at once this can indicate carbon monoxide. Get everyone out, ventilate, and call the Gas Emergency Service on 0800 111 999.
• A smell of gas
• Widespread breathing difficulty
• Sewage ingress or flooding into porous materials like carpet and plasterboard

Everything below is about the slower, less obvious problems.

1. The air feels stuffy or stale

Stuffy air almost always means not enough fresh air is getting in. As people breathe, carbon dioxide builds up, and with it a general heaviness that makes rooms feel tired.

CO2 is the easiest indicator to measure and the best proxy for ventilation. Outdoor air sits at roughly 420 ppm.

Indoors, below 800 ppm suggests good ventilation, 800–1,500 ppm is acceptable but worth watching, and consistently above 1,500 ppm means ventilation is inadequate. In UK schools, BB101 sets a daily average limit of 1,500 ppm for mechanically ventilated spaces for exactly this reason.

2. Symptoms improve when people leave the building

This is the single most useful diagnostic. If headaches, tiredness, or itchy eyes ease within an hour or two of leaving and return the next morning, the building is the common factor.

When several people across the same floor or zone report it, you’re looking at a building-level problem rather than individual sensitivity. Ask people to note which rooms and what time of day the pattern usually points straight at the cause.

3. There’s a musty or earthy smell

A musty smell means moisture and microbial growth somewhere, even when you can’t see it. The source is often hidden: behind plasterboard, under flooring, inside wall cavities, or in ductwork.

Air fresheners make it worse, not better they mask the smell while the moisture keeps working on the building fabric. If the smell returns after cleaning, the source is still active.

4. Condensation on windows and cold surfaces

Regular condensation means indoor humidity is too high for the surface temperatures in the building. It’s most visible on windows in the morning, but it also happens on cold external walls, around window reveals, and on pipework.

Relative humidity should sit between 40% and 60%. Above 60% and you’re into the range where mould, dust mites and bacteria thrive. Below 40% and you get dry eyes, irritated throats and static.

5. Visible mould or dark patches

Mould is not a cosmetic issue it’s evidence that moisture has been present long enough for growth to establish. Check corners of external walls, behind furniture pushed against cold walls, around windows, under sinks, and in poorly ventilated bathrooms.

For rented homes in England, this matters legally. Under the Homes (Fitness for Human Habitation) Act 2018, a property with serious damp and mould can be considered unfit for habitation, and Awaab’s Law now sets strict timescales for social landlords to investigate and fix damp and mould hazards.

6. Dust builds up faster than it should

If surfaces need dusting again within days, or dust collects heavily around supply grilles and return vents, filtration or air distribution isn’t working properly. Fine particles are recirculating instead of being captured.

Visible dust is the part you can see. PM2.5 particles small enough to reach deep into the lungs is invisible and matters more for health. The WHO recommends an annual average below 5 µg/m³.

7. Occupants report headaches and tiredness

Persistent afternoon headaches and heavy tiredness across a team usually trace back to raised CO2 and poor ventilation. Research consistently links elevated indoor CO2 with reduced concentration and slower decision making.

If your meeting rooms are the worst offenders, that’s a classic sign small sealed rooms with several people fill with CO2 quickly.

8. Asthma and allergy symptoms get worse indoors

More inhaler use, more sneezing, more itchy eyes inside the building points to allergens or irritants in the air: dust mites thriving in high humidity, mould spores, pet dander, or fine particles from cooking and traffic outside.

This one deserves attention rather than tolerance. Children and older adults feel it first.

9. A chemical or “new” smell that lingers

That smell after new carpet, fresh paint, or new furniture is VOCs volatile organic compounds off gassing from the materials. Sources include paints, adhesives, cleaning products, printers, and MDF furniture.

Most off gassing drops sharply in the first few weeks, but poor ventilation lets it linger far longer. In new or refurbished buildings, this is one of the most common causes of complaints in the first months of occupation.

10. Rooms are uneven hot, cold, draughty or airless

If one end of a floor is stifling and the other is cold, the ventilation system isn’t distributing air properly. Blocked grilles, closed dampers, poorly balanced systems and unsealed ductwork all cause it. Where air doesn’t reach, pollutants accumulate.

11. The ventilation system is neglected

Ask when filters were last changed, when the system was last serviced and balanced, and whether extract fans in kitchens and bathrooms actually work.

A common and easily missed problem: extract fans that run but no longer move any meaningful air, or trickle vents in windows that occupants have taped shut against draughts.

Sign, Likely Cause and What to do next

Sign you notice	Likely cause	What to do next
Stuffy, stale air	High CO2, not enough fresh air	Monitor CO2; increase ventilation rates
Musty smell	Hidden damp or mould growth	Moisture survey; find and fix the source
Condensation on windows	Humidity above 60%	Monitor humidity; improve extract ventilation
Visible mould	Persistent moisture ingress	Fix moisture first, then remediate
Rapid dust build-up	Weak filtration, particles recirculating	Check filter grade and seals; measure PM2.5
Headaches and tiredness	Raised CO2 or VOCs	CO2 and VOC testing across zones
Worsening asthma	Allergens, mould spores, fine particles	Full indoor air quality survey
Chemical or new smell	VOCs off-gassing from materials	VOC and formaldehyde testing; purge ventilate
Uneven temperatures	Unbalanced or blocked ventilation	Airflow survey and system rebalancing

What you can check yourself

Before calling anyone in, a walk round tells you a lot:

• Buy a basic CO2 monitor they’re inexpensive and instantly show which rooms are under-ventilated
• Check relative humidity in a few rooms with a cheap hygrometer
• Look behind furniture on external walls, under sinks, and around window reveals
• Hold a tissue near extract fans and trickle vents to see whether air is actually moving
• Ask occupants to keep a simple log: which room, what time, what symptom

When to bring in a professional

Get a proper survey when symptoms persist after basic fixes, when several people across different areas are affected, when there’s visible mould you can’t trace to an obvious source, or when you need documented evidence for a landlord dispute, an insurance claim, a BREEAM assessment, or a workplace complaint.

A professional indoor air quality assessment uses calibrated instruments to measure CO2, VOCs, formaldehyde, PM2.5 and PM10, temperature and humidity, across different rooms and times of day.

That data tells you what’s actually in the air and where it’s coming from, rather than leaving you guessing.

Typical cost is around £600–£2,500 depending on building size and how many pollutants are measured. Most surveys take a day on site, with results reported within one to two weeks.

A real example

We were called to an office where staff on one floor had complained for months about headaches and tiredness that lifted at weekends. The landlord had already replaced filters and increased cleaning, with no improvement.

CO2 monitoring across the floor showed levels above 2,000 ppm in the meeting rooms by mid-morning and consistently over 1,300 ppm at desks.

The ventilation system was running, but dampers serving that zone had been left almost closed after a refit two years earlier, so the floor was getting a fraction of its design fresh air rate.

Rebalancing the system brought desk CO2 below 800 ppm. The complaints stopped within a fortnight. Nothing needed replacing the problem was simply invisible until it was measured.

Your legal duties in the UK

Employers have a duty under the Workplace (Health, Safety and Welfare) Regulations 1992 to provide sufficient fresh or purified air in enclosed workplaces.

Landlords must keep properties free from serious hazards under the Homes (Fitness for Human Habitation) Act 2018, with damp and mould assessed through the Housing Health and Safety Rating System. Awaab’s Law introduces fixed timescales for social landlords to act on damp and mould.

Schools work to BB101, which sets ventilation and CO2 performance standards.

New and refurbished buildings must meet Approved Document F (Ventilation) of the Building Regulations.

How we can help

We carry out indoor air quality surveys for offices, schools, care homes and residential buildings across London, Birmingham and Manchester. We measure what’s actually in your air, identify where it’s coming from, and set out practical fixes in a clear report.

Learn more about our indoor air quality services, or read about our air quality monitoring surveys if you need longer-term data. For developments at planning stage, see our air quality assessment services.

Conclusion

Poor indoor air quality rarely arrives with a bang it builds quietly until people stop expecting the building to feel any better. The signs are there if you look: stuffy rooms, condensation on the glass, dust returning too quickly, and symptoms that vanish at the weekend.

Most causes come down to ventilation and moisture, and both are fixable once you know what you’re dealing with. Start with a CO2 monitor and a walk round; if the problem persists, measuring the air properly turns guesswork into a clear plan.

Frequently Asked Questions