Running engines indoors
Exhaust fumes are toxic
Sometimes it is necessary to run an internal combustion engine indoors, or in a confined space. If you do this without following the correct safety precautions, concentrations of fumes from exhaust gases can quickly build to toxic levels.
Exhaust extraction equipment
The best solution is to directly couple the vehicle's exhaust pipe to an exhaust extraction equipment hose so that the fumes will be ventilated away from the enclosed space and into the open air. These should be vented to where they will not be drawn back into the same workshop, to a place well away from other people and other premises.
Catalytic converters are not enough
Do not assume that an engine fitted with a catalytic converter can be run safely indoors, they can't. Catalytic converters are less effective when the exhaust gases are relatively cool - such as when the engine is only idling or being run intermittently.