Humans were exposed to a gas found in diesel fumes during an experiment requested by a group funded by German automakers, local media revealed late Sunday.
The reports in the Stuttgarter and Sueddeutsche newspapers follow recent revelations of an experiment where monkeys inhaled the fumes of a diesel Volkswagen Beetle in 2014.
The tests were requested by the European Research Group on Environment and Health in the Transport Sector (EUGT) - a now defunct organization founded by Volkswagen, Daimler and BMW.
They requested a study on "Short-term nitrogen dioxide inhalation by healthy people" sometime between 2012 and 2015, according to the newspapers, which claimed to have seen the relevant document.
An institute at Aachen's University Hospital then examined 25 people after they had inhaled varying amounts of nitrogen dioxide over several hours, but apparently found "no reaction" to the inhaled gas.
On Tuesday, Volkswagen suspended its head of external relations and sustainability in response to the controversy, stating Thomas Steg was stepping away from his duties at his own request.
The statement said the company was "drawing the first consequences" as it investigates the activities of EUGT, the entity backed by Volkswagen and other automakers that commissioned the monkey experiment.
In 2015, it was found that Volkswagen had manipulated the readings of nitrogen dioxide in its cars for years in the U.S., in order for the vehicles to pass diesel emission regulations. The practice's public exposure toppled then-CEO Martin Winterkorn.
Daimler, responsible for automobile brands such as Mercedes-Benz and Smart, said Sunday it wanted to "expressly distance" itself from the study on humans and from the EUGT.
"We are shocked by the extent of these studies and the way they were carried out," Daimler said, in a statement quoted by the Sueddeutsche newspaper.
"We strongly condemn the tests," the company said, adding it had no say in the testing and that the steps by the EUGT went against Daimler's "values and ethical principles."
Last week, The New York Times reported on a U.S. research facility where 10 monkeys were locked in an airtight container, watching cartoons to distract them, as they breathed in fumes from a Volkswagen vehicle.
On Saturday, Volkswagen and Daimler issued responses to their complicity in the monkey tests.
"We are convinced that the scientific methods chosen at the time were wrong," Volkswagen said in a statement. "It would have been better to forgo such a test from the very beginning."
Stuttgart-headquartered Daimler, meanwhile, was less apologetic, saying in a statement that it had launched an investigation into the incident and that it considered the test "superfluous and repulsive."