This kind of gets into philosophical questions about what could still be considered a causal link. If the increased chance of depression is caused by significant reduction in quality of life which is caused by pollution, do we still say air pollution has a causal link with depression? I'm leaning towards yes because removing the air pollution will through multiple steps reduce depression/suicide in that case.
About 20 years ago our society gained a deep understanding of the notion of
causality. (To learn more, search the web for "Judea Pearl".) According to the
terminology standardized at that time, yes, if adding particulate pollution to the air makes it more likely that a person breathing that air is depressed, then that is a casual link or a cause-and-effect relationship. It might be the case however that polluting the air reduces the person's quality of life and that if it were possible to effect an identical reduction of quality of life without polluting the air, then polluting the air would have no further effect on the likelihood of depression. if that is the case, we say that quality of life "screens off" the effect of air pollution on depression.
I feel that mere causation is sufficient to promote action. Waiting for causation feels like rationalizing laziness.
People don't wait to determine whether school district performance is caused by the staff or the selection effect of eager parents, they just move to good school districts.
Chase those good effects AND attempt to understand them. Understanding the mechanism is not necessary to select for good results.
(All ethics is utilitarian in effect. Kantians stop pretending.)
