petrol fumes


hi people, nice sunny day, decided to open up sun roofs, while driving along and stationary,the cab seemed to fill up with petrol fumes, closed sun roofs and could breath again???, noted fuel cap slightly woobly however is not leaking, no leaks from under the tank, anyboby any ideas, realy strange, funny how it disappears when all is shut????, cheers bones..... does the fuel cap need replacing????