VW, BMW and Mercedes (Daimler) set up a research outfit designed to investigate the impact of this pollution on humans and the environment. As is the case with these sorts of agitprop outlets, they have an impressive name. The European Research Group on Environment and Health in the Transport Sector it was. The cigarette manufacturing industry had a similar research institute, its aim was, of course, to show that cigarettes have not been shown to cause cancer, and to produce citable evidence (ideally published in scientific leading journals) for the industry's lobbyists.
As is usually the case, there was a whole gaggle of university professors more than willing to lend their names to that enterprise, and so were prestigious research institutes, attached to universities. They accepted contracts from the German car industry research outfit. Among their research were two kinds of trials that are widely condemned today, albeit for reasons that are not quite obvious, at least not when it comes to the trial involving human participants.
The first kind of trial include 10 monkeys. They were put in an airtight room where they watched cartoons. Meanwhile exhausts from a manipulated VW Beetle with a diesel engine were pumped into the room. That was then compared against the missions of a 1999 Ford engine. The finding propagated at the time by the industry research outfit: Diesel engine exhausts, even in high concentrations, do not cause lasting damage to monkeys.
The main criticism mounted against this research (apart from the obvious question whether it is ethical to subject monkeys to that kind of research in the first place - a very reasonable criticism that I think is persuasive) seems to be that we knew already that these exhausts are dangerous, so the companies should have better focused on reducing emissions rather than on trying to show that they're not a health risk.
In any case, the research was based on a fraudulent set-up, the monkeys were subjected to nitrogen oxide concentrations far below what VW diesel engines emitted at the time in the real world. It is unlikely that the researchers who were contracted to undertake this research were aware of the manipulation. The research was undertaken at the Lovelace Respiratory Research Institute in the United States.
The main criticism here should be that highly evolved non-human animals were utilised for research purposes that were not even research purposes. The health impact of a 1999 Ford exhaust was compared against that of a non-existent (aka manipulated) 2012 VW diesel engine. These monkeys were subjected to the risk of bodily harm for no scientific reason at all.
Well, it does not end there. The European Research Group on Environment and Health in the Transport Sector commissioned another experiment, this time involving 25 or so health volunteers at a German university hospital. The study received ethics approval, no VW diesel engine was thankfully involved on this occasion, manipulated or otherwise. The researchers aimed to determine what the health implications of different levels of nitrogen oxide concentrations in the workplace environment/air would be on those trial participants. The trial participants were subjected to those nitrogen oxide concentrations for 3 hours. The study concluded that there were no significant health implications. However, the authors of this study were also quite explicit about the limitations of their findings. Their summary contains a longish list of caveats, including the warning that a 3 hour test tells us nothing about the effects of chronic exposure (ie the real world).
This study received the required ethics approval, the trial participants were healthy volunteers who gave first person informed consent to trial participation. It is unclear to me here why German politicians and board members of VW, BMW and Mercedes are falling over one another to condemn this research. It seems to me that no fraud was committed, and the question seems scientifically sound.