In the third section of Burnt Norton we find these lines
Eructation of unhealthy souls
Into the faded air, the torpid
Eructation have two meanings
So, if we take up the second meaning then "eructation of unhealthy souls" could mean death, that is soul is erupting out of the body.
Taking up the first meaning and considering this chat discussion, we find few things that requires little explanation. First, how belching is related to souls? And after all what does "unhealthy souls" mean? Does this belching have some biblical refernces of what North found and quoted :
So, because you are lukewarm - neither hot nor cold - I am about to spit you out of my mouth
If we take up the first meaning of "Eructation" from above, and refer to Tsundoku's paraphrase
a haunting evocation of commuters traveling by tube to work. The metro belches forth the commuters
Then it becomes strange as suddenly commuters and metros have come up in the poem. As I say, every line of Burnt Norton seems unconnected, Minute by minute they change.