The safest thing is not to use the dryer at all. A clothesline either outdoors or inside your home is cheaper and safer.

Dryers use electrical energy, gas or propane to create the high temperatures necessary to properly dry clothes. Warm air is expelled from the dryer during the cycle, and this exhaust can contain some elements that are not desirable to have in the home.
Facts
Properly installed and vented dryers take the exhausted air used throughout the drying cycle and channel it away from the home.
Carbon Monoxide
Carbon monoxide, which is a natural byproduct of combustion, is present in the exhaust of natural gas and propane powered dryers.
Lint
While lint traps remove a portion of what is generated in the drying cycle, most of it is taken away from the dryer through the exhaust. Lint can collect in the dryer vents, creating a fire hazard, and it can block the elimination of carbon monoxide, which can enter the living spaces of homes.
Moisture
While not inherently dangerous, moisture can encourage the growth of mildew, mold and bacteria if not properly vented out of the home. One to two gallons of water remain in clothes after the final washer rinse cycle, and this water is present in dryer exhaust as vapor.
Safety
With all dryers, and particularly with those that are gas fired, it is critical to vent the exhaust to the outside and away from air intakes like air conditioning systems and furnaces.