> The results show strong correlations, but research that would prove a causal link is difficult because ethical experiments cannot deliberately expose people to harm. This is kind of my hobby horse: you don't need to subject people to harm to test causuality. You can do the opposite: Remove harm and see what happens. Maybe give people free HVAC upgrades to filter more particles.
That would also remove other things from the air, which ruins the experiment. The only way to measure if a particular type of pollution is responsible is to only filter that one out, which is basically impossible, or to increase the amount of that one, which is unethical because it's potentially harmful.
You're changing the goal. The question being asked is whether polluted air is healthier than filtered air. The health effects of individual pollutants is a separate matter.