Can dirty ductwork cause health problems for my family in our Toronto home?

Dirty ductwork can contribute to indoor air quality problems that affect your family's health, particularly for household members with asthma, allergies, or compromised immune systems. Your duct system circulates air through every room in your home hundreds of times per day, and whatever is inside those ducts — dust, pet dander, pollen, mould spores, bacteria, insect debris — gets distributed throughout your living spaces every time the furnace or air conditioner runs.

The health impact depends on what is actually in your ducts. Normal household dust accumulation over a few years is unlikely to cause serious problems for healthy adults, though it can aggravate allergy and asthma symptoms. The real health concerns arise when ducts contain mould growth, rodent or insect contamination, or construction debris. Mould inside ductwork is particularly common in GTA homes because of our high-humidity summers — when cold air conditioning supply ducts pass through warm, humid basements or attics, condensation forms on the duct exterior and can migrate to the interior through unsealed joints. Once mould establishes inside ductwork, spores are blown into every room whenever the system runs. Symptoms of mould exposure include persistent coughing, sneezing, watery eyes, headaches, and worsening asthma. In older Toronto homes — especially post-war bungalows and split-levels across Scarborough, North York, and Etobicoke — decades of accumulated dust combined with moisture intrusion creates an environment where mould thrives undetected inside ducts hidden in walls and ceilings.

Rodent and insect contamination is another serious health concern. Mice commonly nest inside ductwork in GTA homes, particularly in basement duct runs and panned joist return plenums. Mouse droppings and urine inside ducts become airborne particles that circulate through the home, and exposure to mouse allergens is a documented trigger for asthma attacks. If you hear scratching inside your ducts or notice a musty, ammonia-like smell from certain registers, have the system inspected and cleaned.

It is important to understand that duct cleaning alone does not solve mould problems. If mould is growing inside your ducts, cleaning removes the existing growth, but the moisture source that caused the mould must be identified and fixed — whether that is condensation from uninsulated ducts, a leaky humidifier connection, or water intrusion from a foundation crack. Without addressing the root cause, mould will return within months.

Practical steps to protect your family's health include changing your furnace filter every one to three months with a MERV 11 or MERV 13 filter, which captures fine particles including mould spores and pet dander. Have your ducts professionally cleaned every 3 to 5 years, or sooner if you notice visible mould, rodent evidence, excessive dust, or musty odours from registers. If family members are experiencing unexplained respiratory symptoms that improve when they leave the house and worsen when they return, your duct system should be one of the first things investigated. A professional duct cleaning with inspection costs $300 to $700 in the GTA and can provide peace of mind about what your family is breathing.