Quick answer
A dryer vent becomes a fire hazard when lint buildup restricts airflow enough to trap heat inside the dryer and duct — the identifiable warning signs are longer drying times, a hotter-than-normal dryer or laundry area during cycles, a burning or hot-dust smell, visible lint at the exterior vent hood, and flexible vinyl or foil venting that's crushed, disconnected, or simply old.
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The short answer
Lint accumulation restricts airflow, and restricted airflow traps heat that’s supposed to exhaust outside — that combination is what fire departments and fire-safety organizations, including the NFPA, have long identified as a real and largely preventable cause of home clothes-dryer fires. The signs below are how that risk shows up before it becomes an emergency.
Performance signs
Clothes taking noticeably longer to dry. This is usually the first sign anyone notices, and it’s often dismissed as the dryer getting old rather than recognized as an airflow problem. If a load that used to take 40 minutes now regularly needs 60 or more, restricted vent airflow is the first thing worth checking.
The dryer or nearby area feeling hotter than usual during a cycle. A properly venting dryer exhausts most of its heat outside. When that exhaust is restricted, more heat stays inside the appliance and the surrounding space — a noticeably hot dryer exterior or a warm laundry closet during a cycle is a real symptom, not just a subjective impression.
A burning or hot-dust smell during drying. This is lint heating up inside a restricted duct. It’s a more urgent sign than slow drying alone and worth addressing immediately rather than waiting for a scheduled cleaning.
Physical signs
Visible lint at the exterior vent hood, or a flap that won’t open and close freely. The exterior termination point is one of the easiest places to check yourself — heavy lint visible around the hood, or a flap that seems stuck or restricted, points to a blockage somewhere in the system.
Flexible vinyl or foil venting. If the visible section of ducting near your dryer is a ribbed plastic or foil material rather than rigid or semi-rigid metal duct, that’s a material-level fire-safety concern independent of how clean it currently is — vinyl is flammable and both materials crush and sag more easily than metal duct, both of which fire-code guidance and manufacturer recommendations now steer away from.
A crushed, kinked, or disconnected section. Furniture, boxes, or the dryer itself being pushed back without enough clearance commonly crushes flexible venting, and disconnected joints can vent hot, lint-laden air into a wall cavity or closet instead of outside.
The honest bottom line
None of these signs individually means a fire is imminent, but each one represents a real, documented mechanism — restricted airflow trapping heat around a combustible material — that fire-safety organizations treat seriously and that professional cleaning or repair directly addresses. If you’re seeing more than one of these signs, or it’s been a year or more since the vent (not just the lint trap) was professionally cleaned, that’s the point to act rather than wait.