Why Some Indoor Smells Stick Around: The Role of Ventilation and Absorption

Indoor odours can be frustrating because they do not always behave the same way. Some smells clear quickly when a window opens, while others seem to cling to rooms, furniture and ductwork long after the source is gone. The reason often comes down to two things: how air moves through a space, and how surfaces inside the building absorb and later release odour compounds.

For homeowners and facility managers, the key is understanding that odour control is not just about masking a smell. It is about removing the source where possible, moving stale air out effectively, and using filtration to capture airborne particles that may carry or contribute to the odour.

Why some odours linger longer than others

Not all smells are made up of the same kind of particles. Some are carried by gases and volatile compounds, while others are attached to dust, smoke, grease or moisture. That means two rooms with the same source can smell very different depending on airflow, humidity, temperature and the materials in the space.

Odours tend to stick around when:

• •fresh air is not replacing stale air often enough
• •the space has dead zones where air movement is weak
• •odour compounds are trapped in soft furnishings, porous linings or duct surfaces
• •moisture is present, allowing smells to build up or re-release later
• •filters are not matched to the dust load or the system they are fitted to

In homes, this might show up after cooking, pet activity, smoking, water damage or a renovation. In commercial and industrial settings, odours can also come from cleaning chemicals, stored materials, food handling, waste areas or process equipment.

A smell can fade for a short time and then return because it was not fully removed from the space. It may have settled into carpet, insulation, ceiling materials or even dust on internal surfaces.

How building materials absorb and release smells

Many building materials act a bit like a sponge for odour compounds. Porous surfaces can absorb airborne molecules, then release them again when temperature, humidity or airflow changes. This is why a room can smell fine in the morning and stronger later in the day, or why an odour seems to return after the air conditioner starts running.

Materials that commonly hold odours include:

• •carpet and underlay
• •curtains and upholstery
• •acoustic insulation
• •painted plasterboard and ceiling tiles
• •stored cardboard, fabric or paper products
• •dust inside returns, diffusers and ductwork

The issue is not always the material itself. Dust on the material can hold odour compounds, and once that dust is disturbed, the smell can re-enter the air. In some situations, lingering moisture can make the problem worse because damp surfaces trap odours more readily.

This is why cleaning alone sometimes gives only a short-term result. If the source has soaked into absorbent surfaces, the smell may keep coming back until airflow improves, contaminated materials are cleaned or replaced, and the underlying issue is addressed.

Ventilation: the first line of defence

Good ventilation dilutes stale air and carries odours out of the building rather than letting them accumulate. In many spaces, the issue is not that ventilation is absent, but that it is poorly balanced, poorly distributed or no longer suited to how the space is being used.

For commercial and larger residential buildings, ventilation design should be checked against AS 1668.2:2024, and site requirements should be confirmed with the mechanical services engineer or building owner.

Useful ventilation tactics include:

• •increasing outdoor air where the system allows and the space calls for it
• •checking that supply and return air paths are not blocked
• •making sure exhaust systems actually remove air from the source area
• •avoiding short-circuiting, where supply air goes straight back to the return without properly washing the room
• •using local exhaust near odour sources, such as kitchens, waste areas, printing zones or workshops

If a smell is strongest in one area, local extraction is often more effective than trying to ventilate the whole building harder. The aim is to capture the odour close to its source before it spreads through the air handling system.

In some buildings, poor pressurisation can also pull odours from bins, plant rooms or adjacent tenancies into occupied areas. That is why investigating the airflow path matters as much as identifying the smell itself.

Where filtration fits in

Ventilation removes and replaces air, while filtration captures particles moving through the system. Filters do not remove every odour on their own, especially if the smell is mostly a gas rather than a particle. But they can make a meaningful difference when odours are being carried on dust, smoke residue, lint, pollen, soot or grease particles.

The best filtration approach depends on the system and the problem. In many Australian installations, common classes still discussed are G3/G4 and F5-F9 under EN 779, though this standard has been superseded by ISO 16890. These ratings do not convert directly to MERV or ISO classes, so it is better to choose filters based on the actual air-handling system and the contaminant type rather than trying to match labels.

For fine particle control in suitable systems, HEPA filters may be used where the application calls for them. H13 filters are at least 99.95% efficient and H14 filters are at least 99.995% efficient at the most penetrating particle size under EN 1822. They are only appropriate where the system is designed for that level of resistance and installation detail.

Good filtration practice includes:

• •selecting the right filter type for the equipment and air quality goal
• •checking seals so air does not bypass the filter
• •monitoring pressure drop where gauges are fitted
• •inspecting based on system type, site conditions and dust load
• •following the filter and equipment manufacturer instructions

If a filter is overloaded, damaged or fitted incorrectly, air can move around it instead of through it. That can reduce its effectiveness and allow odour-carrying particles to keep circulating.

Cleaning, source control and material choices

The most effective odour strategy usually combines several actions rather than relying on one fix. Start by locating the source, because no amount of ventilation will fully solve a smell that keeps being produced.

Useful practical steps include:

• •removing waste, spoiled materials or contamination promptly
• •cleaning surfaces that collect dust and residue
• •drying out any damp areas and addressing the cause of moisture
• •reviewing storage practices for strong-smelling products
• •replacing materials that have become heavily contaminated or absorbed odour over time

If filters are part of the system, they should be inspected and serviced as needed. Some filters are designed to be washable - check the product instructions before cleaning them. For many systems, replacement is the better option, but the right approach depends on the product and the application.

Where a building has recurring odour issues, it can also help to review whether the current filter arrangement is suited to the actual use of the space. A workshop, commercial kitchen, manufacturing area or busy office may need a different approach from a lightly used home system.

If you need help working out what is suitable, our products and filter servicing pages are a good place to start.

When to get tailored help

If odours keep returning after cleaning and basic ventilation checks, the problem may be in the airflow design, the filter selection, the ductwork, or the materials inside the space. In that case, it is worth stepping back and reviewing the whole system rather than trying to treat the smell alone.

This is especially useful in buildings with:

• •recurring cooking or grease odours
• •smoke or combustion residue
• •damp or mould-related smells
• •strong process or cleaning chemical odours
• •a mix of uses across different rooms or tenancies

A site-specific review can help identify whether you need improved airflow, better source extraction, a different filter grade or a custom size to suit the equipment. If your system needs a non-standard fit, see custom air filters or how to measure an air filter.

Total Air Filtration can help

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. We can help supply or make the right filters for your system and application, and we can also discuss servicing needs for homes and commercial sites.

For practical help, call 07 3800 3448 or request a quote. You can also contact us or view the industries we supply.