I just had this silly idea for a population group. They believe that their world is blessed and sacred, and that the touch of any outsider will defile it and them. When traveling offworld, they wear containment suits and breathe air from the homeworld (recycled, though), eat food from the homeworld, collect... no, let's not go there... 
They've established enclaves on several neighboring worlds. Fully contained arcologies with soil, plants, building material, etc. imported from the homeworld. When outsiders visit the enclaves, they get to wear the containment suits.
I'm not sure if they control the homeworld politically. If they do, visitors to the entire world would have to wear containment suits. If not, they're liable to riot if offworlders get close enough to breathe their air!
Hans

They've established enclaves on several neighboring worlds. Fully contained arcologies with soil, plants, building material, etc. imported from the homeworld. When outsiders visit the enclaves, they get to wear the containment suits.
I'm not sure if they control the homeworld politically. If they do, visitors to the entire world would have to wear containment suits. If not, they're liable to riot if offworlders get close enough to breathe their air!
Hans