WHILE UAL FLT 1 WAS CRUISING AT FL 310, APRX 1.8 HR AFTER TAKEOFF, A FLT ATTENDANT (F/A) RPRTD A HEADACHE & LIGHTHEADED-NESS. THE FLT CLIMBED TO FL 350, THEN ANOTHER F/A & SVRL PAX RPRTD SIMILAR SYMPTOMS. AT THAT TIME, THE TROPOPAUSE WAS AT40,000'. AS THE FLT CONTD, THE TROPOPAUSE LOWERED SOME, THEN INCREASED IN ALT. SUBSEQUENTLY, SVRL OTHER OCCUPANTS HAD SIMILAR SYMPTOMS. THE CAPT TURNED ON THE 'NO SMOKING' SIGN AS A PSBL SOLUTION, BUT THIS DID NOT SEEM TO HELP. NO HAZARD-OUS CARGO LEAK WAS FND & NO PAX ADMITTED CARRYING BUTANE OR OTHER SUCH SUBSTANCE. THOSE AFFECTED BY THE PSBL IRRITATING OR GASEOUS SUBSTANCE WERE IN ZONES B, C, D, E OR THE UPPER DECK OF THE ACFT. NO OCCUPANTS OF ZONE A OR THE FLT DECK WEREAFFECTED, EXCEPT THE 2ND OFFICER RPRTD A SLIGHT PAIN BETWEEN HIS EYEBROWS AFTER SPENDING 10 TO 20 MIN IN THE PAX CABIN. AN INVESTIGATION FOR A POSSIBLE OZONE ENCOUNTER WAS MADE, BUT NO SPECIFIC EVIDENCE WAS FOUND. THE CATALYTIC CONVERTERS IN THE AIR CONDITIONING SYS WERE TESTED & FOUND TO BE 51.2% EFFICIENT.