THE HELICOPTER WAS BEING OPERD FM A TEMPORARY LNDG ZONE (LZ) IN A PARKING LOT, WHILE THE HOSPITAL HELIPORT WAS BEING RESURFACED. AS THE PLT WAS PREPARING TO TAKEOFF AT NGT TO GET A PATIENT AT ANOTHER LCTN, HE NOTED PSNL IN THE AREA OF THE LZ & ADZD THE DISPATCHER OF THE LACK OF SECURITY. AT THAT TIME, THE WX (10 MI EAST AT HOUSTON HOBBY ARPT) WAS IN PART: 900' OVC, VIS 8 MI WITH LIGHT DRIZZLE, WIND FM 040 DEG AT 14 KTS. THE PLT BGN A VERTICAL TAKEOFF TO CLR OBSTACLES.HE RPRTD THAT AFTER LIFT-OFF, THE HELICOPTER ENCTRD TURBC & A VENTURI EFFECT FM WIND BLOWING AROUND THE BUILDINGS. ALSO,HE INDCD THAT HIS VISUAL QUES WERE REDUCED AS HE WAS WAS WATCHING FOR PEOPLE IN THE AREA OF THE LZ. AT ABOUT THAT TIME, WITNESSES OBSERVED THE HELICOPTER DRIFT BACKWARD. SUBSEQUENTLY, THE TAIL ROTOR CONTACTED THE TOP OF A GARAGE, THEN THE HELICOPTER BGN AN UNCTLD SPIN & CRASHED. THE PLT & BOTH MEDICAL CREW MEMBERS WERE SERIOUSLY INJURED.