Every year, sometime between July and October, the skies over Singapore turn grey. The familiar acrid smell drifts in. PSI numbers climb. You seal your windows, crank up the aircon, and wait it out.
Then the haze clears. The skies turn blue again. Life returns to normal.
Except your aircon doesn’t smell quite right anymore.
It’s not the usual musty aircon smell of a unit that needs servicing. It’s something else. A slight burnt quality. A heaviness in the air that wasn’t there before. Sometimes it triggers sneezing or a scratchy throat that you didn’t have during haze season itself.
That smell is telling you something important. Your aircon absorbed weeks of haze pollution, and now it’s releasing it back into your home, one cooling cycle at a time.
This guide explains exactly what accumulated in your aircon during haze season, why it smells different from normal dust and mould, and what it takes to properly clean it out.
What Actually Happens to Your Aircon During Haze
Most people assume their aircon protects them from haze. Seal the windows, turn on the aircon, stay indoors. Safe, right?
Partially. But there’s a critical detail most people miss.
Your aircon doesn’t pull air from outside. Split-system aircons, which are what virtually every Singapore home uses, recirculate indoor air. The indoor unit draws air from your room, cools it, and pushes it back out. The outdoor unit handles heat exchange but doesn’t pump outdoor air inside.
So where does the haze come from?
It seeps in. Through gaps around windows. Under doors. Through the gap where your aircon pipes enter the wall. Through the building’s ventilation system. Every time you open your front door, haze particles rush in.
Once inside, those particles circulate. Your aircon draws them in, passes them through the filter, and most of the larger particles get trapped. But here’s the problem: the particles that make haze dangerous are the smallest ones, and standard aircon filters don’t catch them effectively.
What your filter catches: Dust, hair, larger debris, some pollen
What passes through: PM2.5 particles, the fine particulate matter that defines haze pollution
Those PM2.5 particles, smaller than 2.5 micrometres, pass through your mesh filter like sand through a tennis racket. They settle on the evaporator coils, accumulate in the drainage tray, and coat the interior surfaces of your aircon unit.
Over a typical haze season lasting 4-8 weeks, your aircon accumulates a significant layer of this material. And unlike normal dust, this residue doesn’t just sit there passively.
What’s Actually in Haze (And Now in Your Aircon)
Singapore’s haze comes primarily from peat and forest fires in Indonesia’s Sumatra and Kalimantan regions. When peat burns, it releases a cocktail of pollutants far more complex and toxic than ordinary wood smoke.
The primary component: PM2.5
Fine particulate matter makes up approximately 90% of the particle mass in haze smoke. These particles are small enough to penetrate deep into human lungs and even enter the bloodstream. They’re what makes haze a health hazard, not just a visibility problem.
Research on Indonesian peat fire smoke has identified the following components:
Gases:
- Carbon monoxide
- Nitrogen oxides
- Formaldehyde (a known carcinogen)
- Benzene (a known carcinogen)
- Acrolein (a severe respiratory irritant)
- Over 90 different gaseous compounds
Particulate components:
- Organic carbon (the largest component)
- Elemental carbon (soot)
- Potassium (a marker of biomass burning)
- Sugar-like compounds from burned vegetation
- Tar balls (spherical particles formed during combustion)
Heavy metals:
- Cadmium
- Chromium
- Nickel
- Cobalt
- Copper
- Zinc
- Lead
The heavy metals are particularly concerning. Research published in Environmental Science & Technology found that when fires burn through areas with structures, vehicles, or infrastructure, the smoke contains elevated levels of copper, lead, zinc, and nickel. Indonesian fires frequently burn through agricultural areas with equipment, buildings, and treated materials.
Why peat smoke is worse than regular smoke:
Peat fires are fundamentally different from forest fires. Peat is partially decomposed organic matter that has accumulated over thousands of years. When it burns, it smoulders rather than flames, producing incomplete combustion that releases far more particulate matter and toxic compounds.
A study published in Nature Communications found that PM2.5 from wildfire smoke causes 1.3 to 10% increases in respiratory hospitalisations per 10 μg/m³ increase, compared to only 0.67 to 1.3% for the same concentration of non-wildfire PM2.5. The smoke is measurably more toxic than equivalent concentrations of urban air pollution.
This is what’s coating the inside of your aircon.
Why It Smells Different From Normal Dirt
Normal aircon smell, the musty odour you get from a neglected unit, comes from mould and bacteria growing in the damp environment of the evaporator coils and drainage tray. It’s biological. It smells like mildew or a damp basement.
Post-haze smell is different. It has a slight burnt quality, sometimes described as smoky, acrid, or chemical. That’s because the residue contains actual combustion products, not just biological growth.
The smell comes from:
Organic compounds trapped in the residue that slowly off-gas when the aircon runs. These include volatile organic compounds (VOCs) from the original smoke that were absorbed into the particulate matter and are now releasing over time.
Tar balls, which are spherical carbonaceous particles that form during biomass burning. They’re essentially tiny droplets of partially burned organic material that continue to release compounds as they age.
The interaction between haze residue and the mould that grows on it. Haze residue provides nutrients and surface area for biological growth. The combination produces odours that neither component would produce alone.
The timeline matters:
Right after haze season ends, you might not notice much change. The residue is fresh and relatively stable.
Over the following weeks, as the aircon runs through heating and cooling cycles, as humidity fluctuates, and as biological activity begins, the smell develops. This is why people often notice the problem in November or December, weeks after the haze has cleared.
What Happens If You Don’t Clean It
Left untreated, haze residue in your aircon creates several problems:
1. Continuous low-level exposure
Every time your aircon runs, it circulates air over that contaminated residue. Fine particles that settled on coils become resuspended. Trapped compounds off-gas into your room air. You’re breathing diluted haze pollution months after the haze ended.
This isn’t theoretical. Studies have documented that indoor air quality remains compromised long after outdoor haze clears, precisely because HVAC systems retain and re-release trapped pollutants.
2. Accelerated mould growth
Haze residue is rich in organic carbon, essentially food for mould and bacteria. The porous, nutrient-rich layer that coats your evaporator coils after haze season is an ideal growth medium.
Singapore’s humidity already promotes mould growth in aircons. Adding a layer of organic nutrients accelerates the process dramatically. A unit that might normally need deep cleaning once a year could develop significant mould within months after haze season.
3. Reduced efficiency
The layer of residue on evaporator coils acts as insulation, reducing heat transfer efficiency. Your aircon works harder to achieve the same cooling, using more electricity and wearing out components faster.
A heavily contaminated coil can reduce efficiency by 15-25%, adding S$20-40 monthly to electricity bills for a typical bedroom unit.
4. Corrosion
Some haze components are mildly acidic or contain compounds that promote corrosion. The evaporator coil’s aluminium fins are particularly vulnerable. Over time, this can cause pitting and degradation that shortens the unit’s lifespan.
Why Normal Servicing Isn’t Enough
Standard aircon servicing, the kind most homeowners schedule quarterly, typically includes filter cleaning, drainage flush, and a quick wipe of accessible surfaces.
This isn’t sufficient for post-haze cleaning.
The filter problem:
Your mesh filter is the component most obviously affected by haze. It’s visibly darker, possibly with a brownish or greyish tinge rather than the usual dust grey. Washing it helps, but the filter wasn’t catching the most harmful particles anyway.
The coil problem:
The evaporator coil, the component that actually cools your air, is where the real contamination sits. Standard servicing might involve a light spray or brush, but this doesn’t penetrate the layer of sticky, oily haze residue that’s bonded to the coil fins.
Haze residue isn’t like dust. Dust sits loosely on surfaces. Haze particles, especially the tar balls and organic compounds, adhere. They’re sticky. They don’t rinse off with water. They require chemical agents to break down and release.
The drainage problem:
Your aircon’s drainage tray and pipe collect condensate from the cooling process. During haze season, this water carries dissolved compounds from the air. Over time, a biofilm develops in the drainage system that’s enriched with haze compounds. Standard flushing doesn’t remove established biofilm.
What’s actually needed:
After haze season, your aircon needs a chemical wash, not just a basic service.
A proper chemical wash involves:
- Dismantling the indoor unit to access internal components
- Applying chemical cleaning solution to the evaporator coils
- Allowing the solution to penetrate and dissolve residue
- High-pressure flushing to remove dissolved contaminants
- Cleaning and sanitising the drainage tray and pipe
- Treating the blower fan and housing
- Reassembling and testing
The chemicals used are alkaline solutions that break down the organic compounds in haze residue, surfactants that release adhered particles from surfaces, and sometimes sanitisers that address biological contamination.
The Filter Deep-Dive: What You’re Actually Looking At
If you want to understand what accumulated in your aircon, pull out your filter after haze season and examine it closely.
Visual differences:
Normal dust accumulation appears greyish and relatively uniform. It’s mostly skin cells, fabric fibres, hair, and household debris.
Haze-contaminated filters show a brownish or yellowish tinge, especially in the centre where airflow is highest. The discolouration comes from organic carbon and tar compounds. The texture may feel slightly sticky or oily rather than purely dusty.
What you can see:
Under normal lighting, you’ll see the colour difference. Under strong light, you might notice that the material doesn’t brush off easily. It’s adhered to the filter mesh in a way that regular dust isn’t.
What you can’t see:
The PM2.5 particles that passed through are invisible. They’re deposited on the coils behind the filter, in the drainage system, and on the interior housing surfaces. The filter itself only tells part of the story.
Cleaning vs. replacing:
Standard mesh filters can be washed after haze season, but they may not return to their original colour. The discolouration is often permanent. If your filter still shows significant staining after thorough cleaning and drying, it’s time to replace it.
For households with respiratory-sensitive members, consider upgrading to a higher-grade filter. Some modern units accept electrostatic or HEPA-style filters that capture finer particles. These cost more and need more frequent replacement, but they provide meaningfully better protection during haze season.
Timeline: When to Service After Haze
During haze season:
Clean your filters weekly. This won’t catch PM2.5, but it reduces the overall particulate load and keeps airflow optimal.
If PSI exceeds 150 for multiple days, consider running your aircon less (to reduce the total volume of contaminated air processed) or sealing your room more effectively.
Immediately after haze clears:
Within one week of PSI returning to normal levels, clean your filters thoroughly. Inspect them for discolouration. Note any immediate odour changes from your aircon.
Two to four weeks after haze:
This is when problems typically become noticeable. The smell develops. Efficiency drops. Respiratory symptoms may appear.
Schedule a chemical wash during this window. Don’t wait for symptoms to worsen.
Why timing matters:
The longer haze residue sits in your aircon, the more it degrades, off-gasses, and supports biological growth. Fresh residue is easier to remove than aged residue that’s been through multiple humidity cycles.
Booking early after haze season also avoids the rush. Every year, aircon servicing companies see a spike in chemical wash requests in October and November. Booking in September, as the haze begins to clear, gets you earlier service and often better availability.
DIY Cleaning: What You Can and Can’t Do
What you can do yourself:
Filter cleaning is straightforward. Remove the filter, vacuum off loose debris, wash under running water, scrub gently with a soft brush if needed, and dry completely before reinstalling.
Exterior wipe-down helps with the smell. Use a damp cloth to wipe the air outlet louvres and the accessible interior surfaces. This removes surface-level residue.
Running the aircon in fan-only mode for 30 minutes with windows open can help ventilate some trapped odours. This is a temporary measure, not a solution.
What you cannot do yourself:
Evaporator coil cleaning requires dismantling the unit and using appropriate chemical solutions. DIY attempts risk damaging the delicate aluminium fins, pushing debris further into the system, or using inappropriate cleaning agents that leave residue or corrode components.
Drainage system deep cleaning requires access to the internal tray and pipe, often necessitating partial unit disassembly.
Blower fan cleaning is particularly tricky. The fan accumulates significant residue but is difficult to access and clean without proper tools.
When DIY makes things worse:
Using household cleaners on aircon components is risky. Many cleaners leave residue, attract more dust, or corrode metal surfaces. Bleach, in particular, can damage seals and gaskets while producing harmful fumes when the unit runs.
Spraying water into the unit without proper drainage preparation can cause leaks and water damage to your wall or ceiling.
High-pressure cleaning without the right technique can bend coil fins, reducing airflow and efficiency permanently.
Special Considerations for Sensitive Households
Some households need to take post-haze aircon cleaning more seriously than others.
Children under 5:
Young children’s lungs are still developing. They breathe faster than adults (30-60 breaths per minute for infants vs. 12-20 for adults), processing more air relative to their body weight. Any contamination in that air has proportionally greater impact.
Research on Indonesian peat fire exposure has documented developmental effects in children exposed during critical periods. While Singapore’s exposure levels are lower than fire-adjacent regions, minimising exposure is prudent.
Elderly family members:
Respiratory function naturally declines with age. The elderly are more susceptible to both the inflammatory effects of particulate matter and respiratory infections that can follow irritation.
Anyone with asthma, COPD, or respiratory conditions:
These individuals often notice post-haze aircon symptoms first. If household members with respiratory conditions report increased symptoms in the weeks after haze clears, contaminated aircon is a likely contributor.
Households where someone is frequently ill:
Repeated respiratory infections, persistent coughs, or recurring congestion that doesn’t respond to normal treatment may indicate ongoing exposure to airborne irritants, including from a contaminated aircon.
For sensitive households, the recommendation is:
- Chemical wash immediately after haze season, not just before
- Consider a second service 3-4 months later to address any biological growth that developed
- Run air purifiers with HEPA filters alongside aircon use
- Replace rather than wash filters after significant haze exposure
The Yearly Haze Cycle: A Maintenance Calendar
Singapore’s haze follows a predictable pattern. Your aircon maintenance should follow it.
May-June:
Pre-haze preparation. This is ideal timing for annual chemical wash or chemical overhaul. Start the haze season with a clean system that can handle the additional load.
Check that your unit is operating efficiently. Any existing problems will worsen during haze season.
Stock spare filters if you use disposable types.
July-September:
Active haze season. Clean filters weekly when PSI exceeds 100 for multiple days. Inspect filters for unusual discolouration.
If you notice sudden odour changes or efficiency drops during this period, don’t wait. Schedule service promptly.
Keep a record of particularly bad haze days. Multi-day periods with PSI above 150 are especially hard on aircon systems.
October:
Haze typically clears. Within 2 weeks of sustained clear air, schedule chemical wash for all units that ran frequently during haze season.
This is peak demand for aircon servicing. Book early.
November-December:
Post-haze monitoring. Note any developing odours, efficiency changes, or respiratory symptoms. These indicate cleaning may have been incomplete or additional service is needed.
January-April:
Standard maintenance period. Quarterly servicing as normal. Address any issues that emerged during the post-haze period.
Signs Your Aircon Needs Post-Haze Attention
Immediate red flags (service within 1 week):
- Visible mould or black spots on vents or interior surfaces
- Strong musty or chemical smell when the unit starts
- Family members experiencing new respiratory symptoms
- Visible discolouration on the evaporator coils (if you can see them)
Warning signs (service within 1 month):
- Subtle but persistent unusual odour
- Unit taking longer to cool the room
- Higher electricity bills without usage changes
- Increased condensation or dripping
Monitoring signs (service at next scheduled maintenance):
- Filter discolouration that doesn’t wash out
- Slightly reduced airflow
- Occasional faint odour that comes and goes
What to Tell Your Technician
When booking post-haze service, provide context that helps the technician understand what’s needed:
“My aircon ran throughout haze season. We had PSI above 150 for several weeks. I’m noticing a different smell from the unit now and want a thorough post-haze chemical wash.”
This tells them:
- The unit has significant contamination potential
- You’re experiencing symptoms
- You understand that basic servicing won’t be sufficient
Ask specifically about:
- Chemical wash vs. basic service (confirm they’re doing chemical wash)
- Coil cleaning method (should involve chemical solution, not just water)
- Drainage system cleaning (should be thoroughly flushed)
- Expected post-cleaning improvement
After service, run the unit for 30 minutes before occupying the room. This clears any residual cleaning solution fumes and allows you to verify the smell has improved.
The Cost of Ignoring It
Postponing post-haze cleaning feels like saving money. It isn’t.
Immediate costs:
- Higher electricity bills from reduced efficiency (S$20-40/month per unit)
- Potential medical costs from respiratory issues
- Reduced sleep quality from poor air or unusual odours
Medium-term costs:
- More extensive cleaning needed as contamination worsens
- Possible coil replacement if corrosion develops
- Mould remediation if growth becomes severe
Long-term costs:
- Shortened aircon lifespan (potentially 2-3 years less)
- Complete system replacement earlier than necessary
A post-haze chemical wash costs S$80-150 depending on the unit size and service provider. Replacing an aircon system costs S$1,500-3,000+.
The math is straightforward.
Prevention for Next Year
While you can’t prevent haze, you can minimise its impact on your aircon:
Improve room sealing:
Install weatherstripping around doors and windows. Seal the gap where aircon pipes penetrate the wall with proper foam or caulk. These measures reduce how much haze enters your home in the first place.
Strategic aircon use during haze:
When PSI is very high (200+), consider running the aircon less or not at all during peak haze hours. Every cubic metre of air your aircon processes deposits some contamination.
If you must run the aircon during severe haze, use it in specific rooms with better sealing rather than trying to cool the whole home.
Air purifiers:
A HEPA air purifier in frequently occupied rooms captures PM2.5 that your aircon can’t. Running a purifier alongside your aircon during haze season significantly reduces the particulate load reaching your aircon’s interior components.
This doesn’t eliminate the need for post-haze cleaning, but it reduces the severity of contamination.
Pre-haze servicing:
Having your aircon chemically cleaned before haze season means it starts with clean coils that are more resistant to contamination buildup. A clean, efficient system also cycles less frequently, processing less total contaminated air.
That strange smell from your aircon after haze season is your system telling you it absorbed weeks of pollution. The residue coating its internal components isn’t ordinary dust. It’s a complex mixture of combustion products, organic compounds, and fine particles that don’t belong in your breathing air.
Normal servicing won’t remove it. Time won’t make it better. In fact, waiting makes things worse as the residue degrades, off-gasses, and feeds biological growth.
The solution is straightforward: chemical wash after every significant haze season. It’s a small investment that protects your family’s respiratory health and extends your aircon’s working life.
Haze season affected your aircon more than you realise. VD Aircon’s post-haze chemical wash removes trapped pollutants, tar residue, and developing mould that basic servicing misses. Call 96540044 or WhatsApp to book before the end-of-haze rush.
We specialize in Aircon installation, repair, and service. We have all type & model of recon compressor, full set & fancoil. Our technicians are highly proficient in their respective field and repairs and fixes aircon of various brands.
