Why do tank barrels have a bore evacutator?

The evacuator pulls the gases and smoke from the fired shell out the barrel instead of letting it be sucked back into the turret.

Kind of hard on the lungs breathing the fumes from the hazardous propellants.

Quoted: The evacuator pulls the gases and smoke from the fired shell out the barrel instead of letting it be sucked back into the turret. Kind of hard on the lungs breathing the fumes from the hazardous propellants.

Ah. What did they do before bore evacuators? Just inhale the gasses that came back into the turrent and live with it?

Yep, but the older tanks weren't sealed up as good, modern AFVs are tighter sealed for NBC protection.  

Sometimes.

If you take a look at the AMX-30 or Leclerc, you will note that there is no fume extractor. (English term for Bore Evacuator). Those tanks (and the M551 Sheridan as well, if memory serves) use compressed air to blast the gasses out the end.

If it's a tank with neither fume extractor nor compressed air blowout system, the next best thing was a ventilator fan in the turret. Failing that, open the hatches....

A fume extractor works by having a place to form a 'resevoir' of high-pressure air which is pressured up by the propellant gasses. After the round leaves the tube, these gasses now have a route out, out the end of the gun tupe. The little holes between the barrel and the extractor are angled down the tube, so the air is directed that way. When the breech opens, there is now a clear way of sucking all the air in the barrel out the end of the tube.

NTM