Proponents of Nishi have made much of the promised mitigation features: the tree screen and the air filters. One has to ask, of course, why mitigation is even necessary, and the EIR for the project makes that clear: the location between I-80 and the train tracks brings with it poor air quality and "significant and unavoidable" health impacts. There is no controversy on that point, although some "merchants of doubt" have tried to turn it into one.
Questions have also been raised about whether the promised mitigation will do what it is supposed to do; for example, Dr. Thomas Cahill has pointed out that the tree screen will be much less effective because the freeway is elevated adjacent to Nishi, and the supposed 95% efficiency of the air filters has never actually been demonstrated in a real-life situation (with filters operating at a much lower efficiency in real-life situations).
But the situation is even worse than that. The promised mitigation measures might not even be implemented.