.net - How to set environment variables for external app -

i developing .net 4.0 class, calls external app reads 1 parameter environment.

however, environment class can modified thread. , there's obvious problem when using 2 instances of class need different value in variable.

what correct way set independent environment variables when calling external application?

note: have updated question, heavily changing wording, because asking wrong question. if comment seems out of place, maybe should check edit history.