Three top German automakers are under fire after reports that they financed studies in which humans and monkeys inhaled harmful gases, including nitrogen dioxide a component of car exhaust.

The New York Times reported on the monkey study last week, citing a lawsuit Volkswagen owners filed against the automaker. On Monday, German media described that study and another involving human subjects. Volkswagen , BMW and Daimler, the owner of Mercedes, financed the group that commissioned the studies, according to the reports.

The since-dissolved European Research Group on Environment and Health in the Transport Sector commissioned the tests, which took place in Germany and the United States. According to the Times, the EUGT received all its funding from the auto companies.

In one of the studies, at Germanys Aachen University in 2013 and 2014, 25 human subjects were exposed to nitrogen dioxide for three straight hours once a week, over four weeks.

The study noted that among all nitrogen oxides, nitrogen dioxide (NO2) is the most toxic one, but that it had obtained written consent from its test subjects and was approved by the universitys ethics committee.

Like the study undertaken at Aachen University, the EUGT also commissioned an experiment that forced monkeys to inhale car exhaust in 2014, according to the Times report.