PPRuNe Forums - High Flight Deck Temperature

High Temperature in the Flight Deck

I am seeking information / studying into the following.
At my company we are allowed to despatch the aircraft under MEL with one air conditioning pack inoperative.
We are operating in an area of high OAT 30 – 40 degrees.
With the aircraft in this state it is impossible for us to reduce the flight deck temperature below 40 after an hour of so of flight we are lucky if you can get it to 35 degrees.

So here is my question for our resident PPRUNE doctors, how long is it safe (if at all) for flight crew to work in this heat? Is there any studies documenting this?
Many thanks!

PT6A