Since this is clearly not a hypothetical based on your other comments…

• what is the nominal purpose of the tube? Drain pipe? Irrigation?
• Is the tube open at both ends?
• Is there any sort of airflow through it? (Take a handful of dirt/dust, let it fall directly in front of the tube opening, watch dust carefully for signs of air movement.)

If the tube is open, at that length I would expect the natural conditions (temperature and pressure differentials) to create a natural airflow, in which case it’s almost certainly breathable.