But the embarrassment deepened for the group today as German newspaper Sueddeutsche Zeitung reported tests on the effects of inhaling toxic nitrogen oxides (NOx) were also carried out on some 25 healthy human beings.
VW's "dieselgate" scandal saw the group admit in 2015 to manipulating some 11 million cars worldwide to fool regulatory tests, making it appear as though they met NOx emissions limits when in fact they exceeded them by many times in real on-road driving.
On its website the World Health Organisation points to "growing evidence" that nitrogen dioxide exposure "can increase symptoms of bronchitis and asthma, as well as lead to respiratory infections and reduced lung function and growth."
Exposure is "linked to premature mortality... from cardiovascular and respiratory diseases," it continues.
The studies were commissioned by an organisation known as the European Research Group on Environment and Health in the Transport Sector (EUGT), financed by VW alongside fellow German auto sector stalwarts Daimler and BMW.
Hoping to defend diesel's environmentally-friendly reputation -- and the valuable tax breaks that go with it -- the EUGT commissioned the tests from the US-based Lovelace Respiratory Research Institute.
According to the NYT, 10 monkeys were locked in airtight chambers and left to watch cartoons as they breathed in diesel fumes from a VW Beetle.
The car companies decided in late 2016 to dissolve the EUGT, which finally shut its doors last year.
"We expressly distance ourselves from the study and the EUGT," a Daimler spokesman told AFP Monday.
"We are appalled by the extent of the studies and their implementation," he added, saying the Mercedes-Benz parent "condemns the experiments in the strongest terms."
BMW did not immediately respond to a request for comment.
Politicians from across Germany's party spectrum scrambled to repudiate the studies.
The diesel tests were "absurd and inexcusable" said Bernd Althusmann, economy minister of Germany's Lower Saxony state -- home to VW's Wolfsburg headquarters and one of the car group's biggest shareholders.
There should be "tough personal consequences" for the people responsible, added Althusmann, who sits on the firm's supervisory board.
The auto industry was "a sector that seems to have lost all scruples in its mania for cheating," Greens party transport spokesman Oliver Krischer tweeted.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)