Three duct types
Lint is ready fuel and a dryer runs hot. As lint restricts airflow the temperature climbs to ignition at the element - and oil-contaminated fabric can catch fire on its own.
The short answer
Lint is highly combustible, and it builds up on every surface of a dryer's exhaust. As it accumulates it restricts airflow, which drives operating temperatures up until the fluff can ignite at the heating element - the single most common cause of dryer fires. Once alight, the fire runs through the exhaust duct into the surrounding structure. On oil-contaminated laundry, there is a second route to ignition entirely.
The mechanism
A tumble dryer works by pushing heated air through the load and out along the exhaust duct. Lint - fine, dry, fibrous and readily combustible - sheds continuously and coats the filter, the duct walls and the fan. As that layer thickens it chokes the airflow the dryer depends on to carry heat away. Temperatures climb, the machine's thermostats work harder, and eventually the packed lint sitting near the hot heating element reaches its ignition point. That is why restricted airflow, not a random spark, is the usual starting gun: the blockage and the fire are the same problem at different stages.
It is also why the first symptom is so mundane. Clothes taking longer to dry is the earliest sign the duct is restricting - the same restriction that is raising the temperature toward ignition.
Why it spreads
A lint fire rarely stays put. The exhaust duct that vents the dryer is lined with exactly the fuel the fire feeds on, so flame travels along it, and where the duct runs through walls, voids or roof spaces it can carry fire into the building's structure and ignite whatever it meets. A significant share of dryer fires - around a third in the available data - trace back to a simple failure to clean and maintain, which means most are preventable by keeping the lint down.
That structural spread is what turns a contained appliance fault into a building fire, and it is the reason lint in the ductwork - not just in the filter - is the part that matters for fire safety.
The second hazard
There is a second, less obvious route to fire that has nothing to do with the element. Fabrics carrying oils - spa towels and robes with massage or emollient oils, or cloths with cooking oil - can undergo an exothermic self-heating reaction after a hot drying cycle. The heat produced has nowhere to escape when the items are folded or packed into a trolley, and they can spontaneously combust well after leaving the machine. Controls are procedural: process oil-contaminated laundry the same day, segregate it, wash hot with an emulsifier, run the full cool-down, and unload promptly rather than leaving hot loads sitting.
Between combustible lint at the element and self-heating oils in the load, laundry ductwork carries a fire risk every bit as serious as grease - which is why it earns the same discipline: keep the lint down, keep the duct clear, and handle contaminated fabric with care.
Questions
Lint is fine, dry and highly combustible, and it builds up throughout a dryer's exhaust. As it restricts airflow, operating temperatures rise until the packed lint ignites at the heating element - the most common cause of dryer fires.
Clothes taking longer to dry. That is the earliest sign the duct is restricting airflow - the same restriction that raises the temperature toward ignition.
Along the exhaust duct, which is lined with the same combustible lint. Where the duct passes through walls, voids or roof spaces, fire can carry into the building's structure.
Fabrics carrying massage, emollient or cooking oils can undergo an exothermic reaction after a hot cycle and spontaneously combust when folded or packed, even after leaving the machine.
Keep lint down by clearing the filter every cycle and cleaning the full duct on a risk-based schedule, and handle oil-contaminated laundry carefully - process same day, cool fully and unload promptly.
Phoenix Duct Clean · by the numbers
We clean the full laundry exhaust - filter, duct and fan - so lint isn't sitting where the heat is. Book a laundry duct clean.