EMB190 crew reports aircraft exhaust smells in the cabin and cockpit during four short flights that progressively becomes worse; until the aircraft is rejected after the fourth flight.

Narrative

Flight Attendants complained about itchy; burning eyes; scratchy throat and discomfort in the nasal passages. On the first leg; the First Officer brought to my attention a rather strange odor; something that resembled burnt toast; but it dissipated as soon as it started. On the second leg; we smelled exhaust from what I assumed was from an aircraft in front of us; so I increased the distance between aircraft on the taxiway. The smell stayed with us. On the third leg we could smell exhaust in the cockpit and the flight attendants were complaining still of the same symptoms. I asked them to keep me informed of changes. On the fourth leg the First Officer and myself had continuous symptoms and decided; upon arrival; to ground the aircraft. When the Mechanic arrived at the aircraft he could also smell the odor of exhaust. After switching aircraft we no longer had the odor of exhaust and all was normal. The First Officer and myself; now 24 hours after the event; still have symptoms and will be getting blood tests to check toxicity levels. I'm not a mechanic but I think there is a leak in the bleed system or possibly the outflow valve and pressurization system is picking up engine exhaust both in flight and on the ground.