A HS125-600A PIC EXERCISES HIS EMER AUTHORITY DURING A DESCENT OUT OF FL300 WHEN THE ACFT'S PRESSURIZATION SYTEM IS ACCIDENTALLY TURNED OFF 30 SOUTH OF YEG; AB.

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CLBING THRU FL300 FOR FL330; I GOT OUT OF THE LEFT SEAT TO RECOVER OUR ENROUTE CHART WHICH HAD FALLEN INTO THE AISLE; UPON REENTERING THE COCKPIT; MY HEAD BRUSHED THE OVERHEAD PANEL AND TURNED OFF THE AIR VALVES. I GOT BACK IN THE SEAT AND BUCKLED IN BEFORE THE LOSS OF PRESSURE BECAME NOTICEABLE. AT THE TIME WE NOTICED THE PRESSURE LOSS THE CABIN WAS CLBING THRU 1200 FT. AT THE TIME I ASSUMED CTL OF THE ACFT AND INSTRUCTED THE FO TO SQUAWK 7700. THE REASON FOR ME TAKING CTL IS THAT THE TRANSPONDER IS LOCATED ON THE CO-PLTS SIDE PANEL AND I NEEDED HIM TO MAKE THE CHANGE AS THE FREQ; WAS VERY CONGESTED. WHEN WE GOT BELOW 10000 FT AND LEVELED OUT; I HAD A CHANCE TO TROUBLESHOOT; AND NOTICED BOTH AIR VALVES IN THE OFF POSITION. I TURNED THEM ON AND THE ACFT RE-PRESSURIZED. LATER ON THE GND; WE TRIED TO DUPLICATE THE PROBLEM AND FOUND THAT WHEN I ENTER THE COCKPIT; MY HEAD IS JUST BELOW THE AIR VALVES. AS I DON'T GET OUT OF THE COCKPIT THAT MUCH; I HAVE NEVER HAD THIS PROBLEM BEFORE IN THIS SIX YEARS WE HAVE HAD THIS ACFT. WHEN THE ACFT RE-PRESSURIZED; I CANCELLED THE EMER AND CONTINUED ON OUR FLT TO CYOJ.