Phoenix Journal · Ductwork
When staff complaints outpaced every open window, an air quality test finally put numbers to the problem - and pointed straight at the ventilation ductwork nobody had thought to check.
The complaints arrived one at a time, then all at once - a heavy head by mid-afternoon, a stuffy meeting room nobody wanted to book, a slow creep of tiredness that coffee could not shift. When the facilities manager finally brought in an air quality test, the results told a story that no amount of open windows had managed to explain.
This is the kind of case we see more often than you might think. An office looks clean. The desks are wiped, the carpets hoovered, the kitchenette spotless. Yet the air moving through the building tells a different tale, because most of it never touches a cloth or a mop - it travels through ductwork, filters and air handling units that sit out of sight and out of mind for years at a time. An air quality test simply gives you the numbers to see what your staff have been feeling all along.
Below, we walk through what a proper indoor air quality assessment measures, what the readings in this office revealed, and why the answer so often lies inside the ventilation system rather than the room itself.
What the test measures
A workplace air quality test is not one single reading. It is a small suite of measurements taken over a working day, each pointing at a different aspect of how well the space breathes. Carbon dioxide is the headline figure, because it is a reliable proxy for ventilation - people exhale CO₂ constantly, so if levels climb, fresh air is not being brought in fast enough. The Health & Safety Executive uses a simple banding: readings at or below 800 ppm suggest a well ventilated room, 800 to 1,500 ppm is a grey zone worth reviewing, and anything consistently at or above 1,500 ppm signals poor ventilation that needs action.
Alongside CO₂, a good assessment logs fine particulate matter - the PM2.5 and PM10 dust so small it slips deep into the lungs - along with volatile organic compounds (VOCs), relative humidity and temperature. There is no single legal limit for VOCs in an office; the guidance is simply to keep them as low as reasonably practicable. For particulates, the World Health Organization tightened its guidelines in 2021, and its annual figure for PM2.5 now sits at just 5 µg/m³. Temperature and humidity matter too, because air that sits at the wrong end of the comfort range - too warm, too dry, or hovering near saturation - makes every other problem feel worse.
What the readings revealed
In this office, the CO₂ told the first half of the story. During quiet early mornings the room sat comfortably around 600 ppm. By late morning, with the space filling up, it pushed past 1,500 ppm and stayed there through the afternoon - squarely in the band the HSE flags as poor. On paper the building had a mechanical ventilation system rated to deliver well above the 10 litres per second, per person that CIBSE and the Workplace (Health, Safety and Welfare) Regulations point to for offices. In practice, it was delivering a fraction of that.
The second half of the story came from the particulate readings, which spiked every time the ventilation fans ramped up. That is the tell-tale sign engineers look for. If dust counts rise when the system runs harder, the system itself is the source. Instead of scrubbing the air, the ductwork was seeding it - pushing years of settled debris, skin cells, fibres and spores back out through the supply grilles and straight into people's breathing zone.
Two things had gone wrong at once. The supply ductwork and the air handling unit were heavily fouled, choked with a felt of accumulated dust that both restricted airflow and acted as a reservoir of contaminants. And the filters, long past their service life, had stopped catching anything meaningful. The result was a system working hard, drawing energy, generating noise, and delivering tired, recirculated air that never quite refreshed the room. The staff had not imagined the afternoon fog. They had been breathing it.
Why the fix lives inside the ductwork
It is tempting to treat poor office air as a comfort issue - open a window, wheel in a portable purifier, turn the fans up a notch. Those are plasters, and they rarely hold. A portable unit cleans the air in its immediate corner while the central system keeps quietly dosing the whole floor with dust. Cranking the fans harder simply disturbs more of the debris sitting in the ducts. The only durable fix is to deal with the system that moves the air, and that means cleaning the ductwork and air handling unit back to a genuinely hygienic condition, then restoring the filtration and airflow the design intended.
The payoff is not only comfort. Cleaner supply air, properly delivered, lifts concentration, reduces the low-grade headaches and irritation that sap an afternoon, and cuts the sickness absence that stale, particulate-laden air quietly drives. The connection between the air a building serves and how well people work in it is well established - we explore it in more depth in our piece on the link between air quality and workshop productivity, and the same logic applies to any office floor. A test gives you the evidence; cleaning the system is how you act on it.
A follow-up test after the ductwork and unit were cleaned told the closing chapter. Daytime CO₂ settled back into the 700 to 900 ppm range, particulate spikes disappeared when the fans ran, and the temperature held a steadier few degrees° through the day without the stuffy build-up. Same building, same people, same system - just clean. If your own office has that late-afternoon heaviness nobody can quite explain, an air quality test is where to start, and the ductwork is very often where the answer ends.
Questions
The Health & Safety Executive uses a banding based on carbon dioxide as a proxy for ventilation. Readings at or below 800 ppm indicate a well ventilated space, 800 to 1,500 ppm is a grey zone worth reviewing, and levels consistently at or above 1,500 ppm signal poor ventilation that needs action. Because people exhale CO2 constantly, a rising figure is a reliable sign that not enough fresh air is being brought in.
Yes, and it is one of the most common hidden causes. When supply ducts and air handling units accumulate dust, skin cells and spores, the system stops cleaning the air and starts seeding it, pushing debris back out through the grilles. A clear sign is particulate readings that spike whenever the ventilation fans run harder, which points to the system itself as the source rather than the room.
A proper assessment logs several things over a working day rather than a single number. Carbon dioxide gauges ventilation, while fine particulate matter such as PM2.5 and PM10, volatile organic compounds, relative humidity and temperature build the fuller picture. Taken together, these readings show not just whether the air feels stuffy but why, which is what lets you target the right fix.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.