Air pollution is of different kinds; some gases, some particles. It comes in different forms and concentrations at different times and each person’s response can be different. We use Purple Air monitors to measure particles. The instruments are small and don’t cost much and give reasonably well known results. Although more precise results can be had by spending more money, we find that these do 98% of the job for 1% of the cost. Pretty good.
In many places where we have installed these they are the only source of measurements of local conditions and so provide the best information available.