Three German carmakers conducted or supported a study which exposed humans and monkeys to car exhaust fumes, German media reported Monday.
The study, commissioned by Daimler, BMW and Volkswagen, was never published.
In one test, monkeys were forced to breathe fumes for hours in an attempt to prove how clean the Volkswagen Beetle's emissions were. Human subjects were exposed to nitrogen dioxide, a major byproduct of burning fuel.
The New York Times reported last week that some of the research was meant to counter a World Health Organization decision classifying diesel exhaust as a carcinogen.
The Environmental Protection Agency says breathing air with high levels of nitrogen dioxide can cause respiratory distress.