Originally Posted by
Ozymandias
You should not use GTC or external pneumatic air to run aircraft air conditioning packs because the air supply can be contaminated with exhaust or oil fumes from whatever engine drives the compressor, and this can contaminate the cabin air.
99% of the turbine-powered external air carts are the same as any aircraft APU or engine. Air is bled off just like an engine. In the common Garret GTCP series, the lubrication system is a pretty simple two bearing system with little risk of oil contamination, especially because of the design of the annular compressor. The hot bleed air is really no different than bleed air off the aircraft's power plant.
In the "other" kind of pneumatic start carts, yes - there is a risk of contamination from the compressor. But, as explained above, those units will not provide sufficient air long enough to run the packs anyways.
There's a greater risk of contamination from a bung AC pack or exhaust from nearby ground support equipment being drawn into the buffet cart intake.