THE PLT WAS ON A 21 MI FLT FM HIS HOME ARPT TO THERMAL, CA, TO PRACTICE TAKEOFFS & LNDGS. BFR DEPARTING, HE SUCCESSFULLYREAD BACK A VFR CLNC TO FLY OUT OF THE PALM SPRINGS TRSA. SHORTLY AFTER TAKEOFF, WHILE EN ROUTE TWD THE SE (TWD THE SUN,WHICH WAS 22 DEG ABV THE HRZN), HE TOLD THE CTLR THAT HAZE & SMOG WERE MAKING HIM DIZZY; HE SAID HE WOULD USE OXYGEN. SOON THEREAFTER, HE REQD VECTORS TO THE DESTN ARPT. VECTORS WERE PROVIDED, BUT THE ACFT CONTD EASTERLY TWD MOUNTAINOUS TERRAIN (TRRN). THE PLT'S SPEECH SEEMED TO DETERIORATE AS HIS REPLIES BCM SHORTER & GARBLED. RADIO & RADAR CONTACT WERE LOST APRX 8 MI NORTH OF THERMAL, WHERE THE ACFT IMPACTED RISING TRRN ABOUT 500' ABV THE VALLEY FLOOR. THERE WAS EVIDENCETHE ACFT CRASHED IN A NOSE LOW, LEFT WING LOW ATTITUDE. THE WX AT THERMAL WAS, IN PART: 25,000' SCATTERED, VIS 20 MI. ANOTHER PLT RPRTD THE TOP OF THE HAZE LAYER WAS AT 3000'. NO SPECIFIC MEDICAL CAUSE OR FACTOR WAS VERIFIED. THE PLT'S WIFE SAID HE WAS SENSITIVE TO CERTAIN SMELLS, SMOG & BRIGHT SUNLIGHT & THAT HE RECENTLY ACQUIRED A VIRUS & SEEMED TIRED.