Why an Air Quality Monitor Can Show Good PM But Bad Odours

If your air quality monitor says the particulate matter looks fine but the room still smells bad, the monitor is probably telling the truth and missing part of the story. Particle readings and odour problems are related in some settings, but they are not the same thing, and they do not always move together.

For homeowners, facility managers and industrial buyers, that distinction matters. A space can have good PM readings while volatile gases, fumes, cleaning chemicals, damp materials or process odours are still present and noticeable to occupants.

PM and odour are measured differently

Most everyday air quality monitors are designed to track particles in the air. That usually means dust, smoke, pollen, soot and other small solid or liquid particles suspended in the air.

Odours are different. They are often caused by gases or vapours, not particles, so a monitor focused on PM may have no direct way to detect them.

That is why a room can look “clean” on a particle monitor while people still notice:

• •food odours drifting from a kitchen or lunchroom
• •solvent or cleaning chemical smells
• •damp, mouldy or musty odours
• •fumes from printing, coating, welding or manufacturing processes
• •sewer or drainage odours

In other words, the monitor may be accurately showing low particle levels, but the source of the smell may be a gas-phase contaminant.

Why one sensor can look fine while people still smell something

A single sensor cannot tell the whole story of indoor air quality. The result depends on what the device is built to measure.

Particle sensors only see part of the problem

Many consumer and building monitors focus on PM2.5 or PM10. These are useful indicators when the concern is dust, smoke or other airborne particles. They do not directly measure every smell-causing compound.

So if the odour is coming from a gas, the PM reading may stay low. If the odour source is intermittent, the monitor may also miss the brief peak that people notice when they walk through a doorway or past a vent.

Occupants notice odours before a monitor does

Smell is subjective, but it is also a practical warning sign. People may notice an odour because:

• •the source is close to them, even if the overall room air is fairly clean
• •the smell is stronger at floor level, in corners or near returns
• •airflow is carrying contaminants from another area
• •the issue is a small gas leak, chemical spill or poorly ventilated process

That means a “good” particle reading should not be used as proof that the air is acceptable in every respect.

Common reasons for odours with low PM readings

If the odour is strong but particulate readings stay low, it is worth looking beyond the filter and the particle monitor.

Gas-phase contaminants

Some odours are caused by vapours rather than solids. Examples include cleaning products, solvents, fuel vapours, adhesives and certain process emissions. These may need source control, ventilation changes or gas-phase filtration, not just particle filtration.

Moisture and hidden mould

Musty odours often point to moisture problems. Wet insulation, carpet underlay, wall cavities, ceiling spaces and HVAC components can all hold smells long after a spill or leak has been fixed.

A particle monitor will not diagnose moisture damage. The next step may be inspection of the source, not more airflow alone.

Ventilation imbalance

If stale air is not being removed effectively, odours can linger even when dust levels are low. This is especially common in rooms with:

• •poor extract ventilation
• •doors that stay closed
• •low outdoor air supply
• •pressure differences between rooms
• •localised process emissions

AS 1668.2:2024 provides guidance for ventilation design and performance, but site requirements should be confirmed with the mechanical services engineer or building owner.

Filter selection that suits particles, not gases

A standard particulate filter can capture dust and similar particles, but it will not remove every odour. If the problem is gas-based, a different approach may be needed, such as activated carbon or another gas-phase solution where appropriate.

If you are unsure what is suitable, our products page is a good starting point, and custom air filters may be relevant where existing sizes or system requirements are not standard.

What to check when the smell remains

A practical response is to treat the monitor as one tool, not the whole diagnosis.

Start by asking:

• •What exactly does the monitor measure?
• •Is the odour constant or only present at certain times?
• •Does the smell appear near a return grille, doorway, work area or plant room?
• •Has anything changed recently, such as cleaning chemicals, process materials or occupancy patterns?
• •Is there any evidence of moisture, leaks or soiling in ductwork, ceiling spaces or filters?

From there, inspect the system from the source to the room. That may include air intakes, exhaust points, ducting, filters and nearby activities that could be creating the odour.

If your setup uses pressure gauges or differential pressure indicators, monitor pressure drop where fitted. A rising pressure drop can indicate loading in the filter bank, but it does not tell you whether an odour is particle-related or gas-related.

Choosing the right filtration approach

The right filter depends on what you are trying to remove.

For particles

If the issue is dust, smoke or other particulate matter, look at the filter grade and how it suits the system. In Australian projects, you may still hear EN 779 terms such as G3, G4 or F5 to F9, even though ISO 16890 is now more widely used. These classes do not convert directly, so avoid assuming one rating is equivalent to another without checking the product data.

If a HEPA filter is required for a specific application, note that H13 is at least 99.95% efficient and H14 is at least 99.995% efficient at the most penetrating particle size under EN 1822.

For odours

If the issue is smell rather than dust, a particulate filter alone may not be enough. Depending on the application, you may need:

• •activated carbon or another gas-phase media
• •better source capture at the point of emission
• •improved exhaust or make-up air design
• •maintenance of ductwork, coils and drain trays

Some filters are designed to be washable - check the product instructions before cleaning or reusing them.

Don’t rely on one reading alone

An air quality monitor is useful, but only if you understand what it is measuring and what it is not. Low PM readings can be reassuring, yet they do not rule out odours, vapours or ventilation issues.

If people can smell something, investigate the source rather than assuming the monitor has cleared the air. The best outcome often comes from combining the right filter media, proper ventilation, system inspection and a clear understanding of the contaminant type.

For sites with recurring odour issues, it may also help to review maintenance access and servicing arrangements. filter servicing and commercial filter servicing can be useful if you are trying to keep a system performing as intended.

Need the right filter for the job?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established in 2009. If you need help matching filters to your application, or you are unsure whether the issue is particles, odours or both, we can help supply or make the right filters for your system.

Call 07 3800 3448 or request a quote. You can also contact us for practical advice on the next step.