The revelations show the rocky road for Volkswagen as it emerges from its biggest crisis after the 2015 bombshell that the company installed emissions-cheating software in some 11 million diesel vehicles to dupe official tests.
After reports on Friday of U.S. tests on macaque monkeys, politicians in Berlin reacted with horror to news on Monday that tests to examine the health effects of diesel exhaust fumes were also carried out in the western city of Aachen on human test subjects.
The study reportedly compared exhaust fumes and particulates with a then-new diesel-powered VW Beetle with those of an old Ford ute.
An environmental campaign group is calling on Volkswagen to end diesel sales, following allegations that the carmaker tested its emissions on monkeys.
The disclosures sparked widespread outrage, led by Chancellor Angela Merkel who strongly condemned the latest controversy to engulf the nation's powerful but scandal-tainted auto industry.
"We are convinced that the scientific methods chosen at the time were wrong".
All three manufacturing groups are distancing themselves from the study, although the head of the EUGT, Michael Spallek, said that it was Volkswagen's lawyers that suggested "using primates instead of humans" for the test.
The carmakers said the research group was independent and overseen by reputable scientists.
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The research was carried out by an organisation funded by Volkswagen, Daimler and BMW. The European Research Group on Environment and Health in the Transportation Sector (EUGT) is the group financed by the major German automakers to conduct studies into the environmental and health impact of diesel engines.
The tests "raise many critical questions about who was behind them", said German government spokesman Steffen Seibert. Though the company said it didn't have any influence on the studies' design, it has launched a "comprehensive investigation into the matter".
In the human testing, 19 men and six women were exposed to different concentrations of nitrogen oxides for several hours at a time, according to details released in the German press.
Daimler AG said it was "appalled by the nature and extent of the studies". All were interested in selling or already selling diesels in the USA, but in 2007 when the group was founded, according to The New York Times, VW was making a big push to sell more TDIs here.
The human study, carried out by Aachen University, involved studying the effects of exposing 25 subjects, mostly students, to low levels of nitrogen dioxide like those that could be found in the environment - from a 40-litre bottle, not a diesel engine.
The European Commission is aware of reports of third-party testing and "we hope that the Minister of the respective country will be able to explain what has happened" at a ministerial air-quality summit that will take place in Brussels on Tuesday, a spokeswoman for the European Commission said. "We condemn the experiments in the strongest terms", Daimler said in a statement.
Although VW posted record revenues a year ago, totalling the largest sales volume of any auto brand, the spectre of its American emissions scandal still looms.
"The outrage felt by many people is completely understandable".