What actually gets measured
Four quantities, each of which points somewhere different
Indoor air quality is a broad phrase covering several unrelated problems, and treating it as one thing is why so much money gets spent on equipment that addresses the wrong one. A useful assessment separates it into quantities that can be measured, each of which has published guidance to compare against and each of which implies a different remedy.
Particulate is the first and the one most people mean. Fine airborne material is counted by size fraction, with readings taken in several locations rather than averaged over the building, because the source is often local. Elevated particulate in one room with normal readings elsewhere points at that room. Elevated readings everywhere that fall when the system is off points at the duct system distributing something, which is where the return leakage question becomes relevant.
Relative humidity is second and is routinely the actual culprit behind complaints attributed to something else. Air that is too dry produces the irritation, static and respiratory discomfort people associate with poor air. Air that is too humid supports dust mites and mould growth and makes a building feel warmer than it is. Both are measurable in seconds, both vary substantially by season, and both have straightforward remedies that do not involve air cleaning equipment at all.
Carbon dioxide is third and is the proxy for ventilation. It accumulates from occupancy, so a rising level indicates that fresh air exchange is not keeping pace with the people in the building. High readings in bedrooms overnight or in a home office through the working day explain a great deal of reported stuffiness and afternoon fatigue, and the remedy is ventilation rather than filtration. Filtering air that is already in the building does nothing for its carbon dioxide content.
| Measured | Typical complaint it explains | Where the remedy usually lies |
|---|---|---|
| Fine particulate | Visible dust, surfaces needing frequent cleaning | Filtration, duct sealing, or a local source in one room |
| Relative humidity, low | Dry skin, static, respiratory irritation in winter | Humidification, sized against the building rather than the room |
| Relative humidity, high | Musty smell, condensation, feels warmer than the reading | Dehumidification, ventilation, or correcting equipment oversizing |
| Carbon dioxide | Stuffiness, afternoon fatigue, poor sleep quality | Ventilation, since filtration does nothing for this |
| Flue gas composition | None. It is a safety check, not a comfort one | Appliance service, venting correction, combustion air |
| Spillage under depressurisation | None, until a detector activates | Venting, make up air, or exhaust fan capacity |
One practical note on interpreting any of these numbers: a single reading taken on one afternoon describes that afternoon. Particulate rises with cooking, vacuuming and outdoor conditions. Carbon dioxide rises through the evening and peaks in the early hours in an occupied bedroom. Humidity swings with the weather and with showering and laundry. Where a complaint is clearly time dependent, the useful approach is to leave logging instruments in place across a full day and read the curve rather than the point, because the shape of that curve usually identifies the source on its own.
Combustion safety is fourth and it is the one that is not negotiable. Every fuel burning appliance in the building is checked for flue gas composition and tested for spillage under worst case depressurisation, meaning with exhaust fans running and doors positioned to create the least favourable condition. That test is performed whatever the reason for the visit, because an appliance that spills combustion products is a different order of problem from a dusty room.
Assessment sequence
Six stages of an instrumented assessment
The interview matters as much as the instruments, because it decides where and when to take readings.
Complaint interview
Which rooms, which times of day, which seasons, and who is affected. That shapes the entire measurement plan.
Building walkthrough
Construction, ventilation provision, obvious sources and the condition of the mechanical system.
Room by room readings
Particulate, humidity, temperature and carbon dioxide taken in several locations rather than averaged.
System on and off comparison
The same readings with the equipment running and stopped, which identifies the duct system as a source.
Combustion safety testing
Flue gas analysis on every fuel appliance and a spillage test under worst case depressurisation.
Reported findings
Numbers against published guidance, with recommendations only where a reading supports one.
In scope
What an air quality assessment covers
All of it happens regardless of what you think the problem is, because the complaint frequently points at the wrong quantity.
Particulate by size fractionMeasured in multiple locations so a local source is not hidden in a building average.
Humidity mappingRoom by room, since a single reading at a thermostat tells you very little about a whole building.
Carbon dioxideThe ventilation proxy, measured where people actually spend time rather than in a hallway.
Combustion safetyFlue gas analysis and a spillage test under worst case conditions, on every fuel appliance.
System comparisonReadings with the equipment running and off, which identifies distribution as a source.
Written reportEvery figure, the guidance it is compared against, and what each one does and does not imply.
Customer testimonies
Feedback on air quality assessments
The conclusion was that my particulate readings were fine and my bedroom carbon dioxide was high. That was a ventilation problem and no equipment was sold to me.
Readings with the system on and off were completely different. The ducts were the source and no filter would have fixed that.
They found a water heater spilling under worst case conditions during what I thought was a dust survey. That was not what I called about and it mattered far more.
Air quality questions
What gets measured, what it means, and what it does not
Will you test for mould?
We measure the conditions that allow mould to establish, which are relative humidity, surface temperatures where condensation is likely, and moisture sources such as unvented appliances or a failing condensate path. Those measurements are what let you act.
Identifying and quantifying mould species is laboratory work performed by a specialist, and where the findings warrant it we will say so plainly rather than offering an opinion outside our competence. Correcting the conditions is the mechanical part, and that is squarely ours.
Is an air purifier the answer to poor air quality?
It is the answer to a particulate problem, and only to a particulate problem. If the readings show elevated fine particulate and no local source that could be removed instead, filtration is a sensible response.
It does nothing for carbon dioxide, which needs ventilation, and nothing for humidity at either extreme, which needs humidification or dehumidification. It also does nothing about combustion products, which need the appliance fixing. Selling filtration for all four is common and is why the measurement comes first.
What humidity should a building sit at?
Broadly in the middle of the range, avoiding both extremes, with the winter target lower than the summer one. Push humidity too high in cold weather and it condenses on the coldest surfaces in the building, typically window frames and exterior wall corners, which creates exactly the mould problem the humidity was meant to relieve.
The right target for your building depends on its construction and window performance, which is why it is a measurement exercise rather than a number to copy from an article. We will give you a range for your property and explain what happens at each end of it.
Should I test before or after making changes?
Before, and ideally again afterwards. Testing first establishes which of the four quantities is actually out of range, which frequently turns out not to be the one the complaint pointed at. Buying equipment before that measurement is how people end up with a filtration system addressing a ventilation problem.
Testing again after any change is what tells you whether the money worked. It also catches the cases where solving one thing worsened another, which is more common than it sounds: tightening a building reduces particulate ingress and raises carbon dioxide, and adding humidity in winter can create condensation. Both are manageable once measured and invisible otherwise.
Why check combustion appliances during an air quality visit?
Because it is the only quantity measured that is a safety matter rather than a comfort one, and because the test is quick relative to its consequence. Carbon monoxide has no smell and its symptoms resemble ordinary illness, so people routinely fail to connect them.
The spillage test under worst case depressurisation is the part that finds problems a normal check misses. Running the exhaust fans and closing the right doors can pull a building negative enough to reverse the flow in a flue, and an appliance that vents perfectly under ordinary conditions can spill under those. Testing for it is the only way to know.
Related services
Where an air quality assessment usually leads
Air quality findings usually point toward the distribution system or toward specific equipment rather than a general purchase.
Duct Sealing
Dryer Vent Cleaning
Air Cleaning & Humidifiers
Bathroom Plumbing Repair
#link-bathroom-plumbing-repair#
Get numbers before buying equipment
An instrumented assessment across Airmont, NY frequently concludes that the complaint is ventilation or humidity, neither of which a filter will fix.
Fixture work is quoted per job once the cause is confirmed, so scope creep is visible.