https://www.medscape.com/viewarticle/581344_7
http://www.mdedge.com/ecardiologynews/dsm/5224/hospital-medi...
https://www.ncbi.nlm.nih.gov/pubmed/2199002
To my mind, this is actually related:
https://www.energyvanguard.com/blog/54084/Why-I-m-Not-a-Fan-...
One of the problems you run into with any lung related mechanical devices is the need for proper sterilization. From what I gather, this is much less of an issue with negative pressure ventilation.
People in iron lungs are people with fundamentally compromised immune systems. Exposing them to germs forced into their lungs by positive pressure while potentially damaging their lung tissues in the process strikes me as a great way to give them a deadly infection, sooner or later.