Exhaust fumes inside car?

What generally causes exhaust-type fumes to enter the vehicle? Particularly at a stop light will I smell fumes coming through my vents. When I open the doors, the exhaust smell rises making me gag. I will steam clean under the car soon and see if that is the culprit. Do you think it is an exhaust leak or the engine burning all the dried up oil from many past oil leaks?

Breslin header bolts backed out. Thanks for the help. :P

The bolts need to be checked a lot at first. I would check them every few months. Be careful of the opti when ever you steam clean the engine.