When German car maker Volkswagen acknowledged last year that it was guilty of cheating on diesel emissions tests, the fallout cost the company over $US25 billion ($31 billion). But the full extent of the company's willingness to sideline ethics concerns to increase profits is still emerging. Three German car makers appear to have commissioned or supported a study in 2014 which exposed monkeys and humans to exhaust fumes and nitrogen dioxide, according to German media reports on Monday.

The study by the European Research Group on Environment and Health in the Transport Sector (EUGT) was never published and the research institute overseeing it has since been dissolved. All three carmakers involved in the study - Daimler, BMW and Volkswagen - distanced themselves from the research over the weekend.

"We are shocked by the extent and application of the studies ... We condemn the experiments in the strongest terms," car maker Daimler wrote. The statement, released at a time when only experiments with monkeys but not humans were publicly known, accused the researchers of having violated ethics rules and company values, even though an ethics commission had approved the study. Daimler and BMW said they had no knowledge of the Volkswagen-led study.

Meanwhile, Volkswagen blamed the "mistakes and misjudgments of individuals."