Here is a somewhat misleading video essay in The New York Times about air pollution in Houston, TX:
In a former life I worked for the TNRCC / TCEQ and reviewed air pollution permits for such plants in this area and other parts of Houston. As far as I know, the permits complied with all federal, state, and local laws and regulations; and when they did not an investigation was conducted.
Yes, they stink. Are they toxic? I dunno.
But I will say that I would not live in those neighborhoods.