Electric dryers do not pose the same carbon monoxide risks as appliances fired by natural or propane gas because combustion is not present with electric dryers.
An average load of laundry can contain one to two gallons of water after finishing the last wash cycle. This water is removed by the dryer and expelled as water vapor, which can condense on joists, framing, wires and metal under the house; cause rot and mold; and encourage the growth of bacteria.
Only a small portion of the lint generated by drying is collected in the dryer's lint trap. The rest is expelled through the exhaust. Lint is a fire hazard and can collect on anything under the house, including the framing.
Many local building codes expressly prohibit venting dryers under houses, in basements or crawl spaces, or into attics.
Dryers always need to be vented outside the home.