Risk factors for a fire being started by an electric clothes dryer:
(1) overloaded electrical circuit
(2) defected plug or wiring
Risk factors for a fire being started by a gas clothes dryer:
(1) gas leak from a defective hose
(2) accumulation of gas within the dryer (defective valve, failure of the pilot light, etc)
Risk factors for any kind of clothes drying:
(1) storing combustible material on or near the dryer while it is in operation
(2) placing something combustible within the dryer (Example: synthetic material used to soak up a flammable liquid)
(3) accumulation of lint within the dryer exhaust. This can ignite to cause a fire or smolder to fill the residence with smoke.
(4) failing to perform proper maintenance
(5) leaving the dryer run unattended