Here's the law of the land (U.S. Code of Federal Regulations):
TITLE 9--ANIMALS AND ANIMAL PRODUCTS
CHAPTER III--FOOD SAFETY AND INSPECTION SERVICE, DEPARTMENT OF
AGRICULTURE
PART 310_POST-MORTEM INSPECTION--Table of Contents
Sec. 310.16 Disposition of lungs.
(a) Livestock lungs shall not be saved for use as human food.
(b) Lungs found to be affected with disease or pathology and lungs
found to be adulterated with chemical or biological residue shall be
condemned and identified as ``U.S. Inspected and Condemned.'' Condemned
lungs may not be saved for pet food or other nonhuman food purposes.
They shall be maintained under inspectional control and disposed of in
accordance with Sec. Sec. 314.1 and 314.3 of this subchapter.
(c) Lungs not condemned under paragraph (b) of this section may be
used in the preparation of pet food or for other nonhuman food purposes
at the official establishment, provided they are handled in the manner
prescribed in Sec. 318.12 of this subchapter, or they may be
distributed from the establishment in commerce, or otherwise, in
accordance with the conditions prescribed in Sec. 325.8 of this
subchapter for nonhuman food purposes or they may be so distributed to
pharmaceutical manufacturers for pharmaceutical use in accordance with
Sec. Sec. 314.9 and 325.19(b) of this subchapter, if they are labeled
as ``Inedible [SPECIES] Lungs--for Pharmaceutical Use Only.'' Otherwise,
they shall be disposed of at the official establishment, in accordance
with Sec. Sec. 314.1 and 314.3 of this subchapter.
[36 FR 11639, June 17, 1971]
Notice the date is 1971 but I'm not familiar enough with the CFR to know if 1971 is the original date of the regulation. Is there a lawyer in the house?!?
I still haven't found out exactly why lungs were and are deemed unsafe for human cuisine in this country though!!