Bushfire smoke events in Victoria have become more frequent and more severe. The 2019–20 Black Summer produced smoke that blanketed metropolitan Melbourne for weeks, with air quality index readings reaching hazardous levels on multiple days. Many Melbourne residents closed their windows and ran their ducted heating or cooling systems during these events — unknowingly pulling contaminated air into their ductwork.
This guide explains how bushfire smoke enters ductwork, what PM2.5 particles do once inside the system, how to assess whether your system was affected, and when to book post-smoke cleaning.
How Bushfire Smoke Enters Melbourne Ductwork
The mechanism by which smoke contaminates duct systems is specific to how ducted heating and cooling systems operate — and explains why closing windows is not sufficient protection during a smoke event.
The return air pathway
Ducted systems operate in a continuous cycle: they draw air from inside the home through return air grilles, condition it, and redistribute it through supply registers. The system does not distinguish between clean air and smoke-contaminated air. During a smoke event, even with windows closed, PM2.5 particles infiltrate the home through gaps in the building envelope — around doors, through floorboards, at cable penetrations, and at plumbing entries. The concentration builds slowly, but over hours of indoor air recirculation with the system running, the ductwork accumulates significant contamination.
Why standard filters fail
The filters in residential ducted heating and cooling systems are rated to capture particles typically 10 microns and larger. PM2.5 refers to particles 2.5 microns and smaller — the fraction that poses the greatest health risk and is most efficiently captured by the lungs. Standard 50mm and 100mm media filters in Melbourne homes have MERV ratings of 4 to 8, which capture very little PM2.5. Most smoke particles pass straight through into the duct system. High-efficiency HEPA filters can capture PM2.5, but residential ducted systems are not designed for the static pressure required by HEPA filtration. See our guide on air duct filter types and ratings.
How long smoke persists in ductwork
Smoke particles and their associated volatile organic compounds (VOCs — the chemicals that produce the characteristic smoke smell) adsorb onto duct surfaces. Unlike dust, which can be partially dislodged by system vibration, these compounds chemically bond to duct wall materials. Without mechanical cleaning, they persist in the system for months — released in small quantities every time the system runs. This explains why Melbourne homeowners sometimes report a faint smoke smell from vents months after a major fire event.
Health Risks of PM2.5 in Ductwork
PM2.5 particles are the most health-relevant fraction of air pollution. Understanding what they do helps explain why post-smoke duct cleaning is a genuine health measure rather than precautionary over-reaction.
Respiratory penetration
Particles below 2.5 microns bypass the upper respiratory tract’s natural filtration system and penetrate directly to the alveoli — the gas exchange surface of the lungs. Here they cause inflammation, and in the case of bushfire smoke particles containing carbon, heavy metals, and combustion byproducts, they trigger immune responses that can persist for weeks to months after exposure ends.
Particular risk for Melbourne’s most vulnerable
Melbourne has one of Australia’s highest rates of asthma and other respiratory conditions. For the roughly one in nine Australians with asthma, bushfire smoke PM2.5 is a known trigger for serious attacks. Children and elderly people are also at elevated risk. Allowing a contaminated duct system to continue operating after a smoke event means the most vulnerable household members receive the highest ongoing exposure.
Cardiovascular effects
Research links PM2.5 exposure to cardiovascular events including heart attacks and strokes, particularly in people with pre-existing cardiovascular conditions. Melbourne’s ageing population means a significant proportion of householders face elevated cardiovascular risk from chronic PM2.5 exposure at home.
Was Your System Running During the Smoke Event?
The level of duct contamination depends significantly on whether the ducted system was running during the smoke event, and for how long.
System was running continuously during smoke
The highest contamination scenario. If your system ran throughout a multi-day smoke event, the entire duct network has processed large volumes of smoke-laden air. Professional cleaning is strongly recommended before the next heating or cooling season.
System ran briefly or was off most of the time
Lower but not negligible contamination. Smoke particles still infiltrate the home over time even without the system running, and even brief system operation during a smoke event introduces contamination. Have the system inspected and assess based on the intensity of the event and the inspection findings.
How to assess without professional help
Run the system for 10 minutes on fan-only mode with no heating or cooling. Stand near a supply register and smell the air output. If there is any detectable smoke odour, the system has retained contamination. Also check the filter — a heavily discoloured filter (dark grey or black) after a smoke event indicates significant particle loading.
When to Book Post-Smoke Duct Cleaning in Melbourne
The general principle: the sooner after a significant smoke event, the better. Allowing weeks or months to pass while the system operates distributes smoke contamination continuously.
Immediately after the smoke event clears
If the smoke event was severe (multiple days of hazardous AQI) and your system was running, book professional cleaning as soon as the smoke has cleared and the technician can safely work. Do not wait until the standard scheduled service.
Before the next heating season
If the smoke event occurred in late summer or autumn and was followed immediately by the May to August heating season, cleaning before that season starts is the priority. See our guide on how often to clean air ducts in Melbourne for seasonal scheduling context.
What Post-Smoke Duct Cleaning Involves
Post-bushfire-smoke duct cleaning follows the same fundamental process as a standard clean — negative pressure extraction, rotary brush agitation, compressed air, and sanitisation — with emphasis on the sanitisation phase to address volatile organic compounds and residual odour.
A tea tree oil sanitisation fog is applied after mechanical cleaning. For systems with heavy smoke exposure, multiple sanitisation passes may be required across a 48-hour period. The system filter should be replaced at the time of cleaning — a filter that has processed smoke-laden air for days or weeks cannot be restored by cleaning.
The full post-smoke service is documented in a written report. For Melbourne homeowners who need to demonstrate to an insurer that remediation action was taken following a smoke event, this report provides the necessary evidence. See our guide on what to expect from a professional duct clean for the full process.