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.
1. Does adding an air filter in peoples home improve health outcomes?
2. Does pumping a suspected noxious element into a room in lab harm some small group of people in some limited testable way?
Question 1 is better than 2. Why?
Because there are more people like those in question 1.
Answers to question 1 inform my future choices, answers to question 2 do not. I prefer answers that improve health to those that harm health.
Always prefer observational studies done in the field (in situ). Demanding experimental studies done in a lab will give you less bang for your buck. Let's all do our part to drive down health care costs, please.
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.
