Not really everyone. In such a ventilation scheme during the works most personnel is near or in the immediate vicinity of the excavation front, they have "non-

Not really everyone. In such a ventilation scheme during the works most personnel is near or in the immediate vicinity of the excavation front, they have "non-contaminated" "fresh air" at all times. You get some "contaminated" air only during the time it takes from the tunnel entrance to the excavation front, and in some circumstances people making maintenance or other works far from the excavation front. As said the quantity of fresh air fanned in is much more than what actually "needed", there are sensors for the contamination and all in all the air you breath in any modern tunnel during construction is much, much better than what anyone breaths everyday in a trafficked city.