Honda GX120 Owner's Manual page 9

ENGINE SAFETY
Carbon Monoxide Hazard
Exhaust gas contains poisonous carbon monoxide. Avoid inhalation of
exhaust gas. Never run the engine in a closed garage or confined area.
Other Equipment
Review the instructions provided with the equipment powered by this
engine for any additional safety precautions that should be observed
in conjunction with engine startup, shutdown, or operation, or
protective apparel that may be needed to operate the equipment.