Is there any science behind the Air Quality Indexes besides cost benefit analysis? Usually when an index goes above, let's say 100 or 200, it is said it is harmful to general population. Any scientific reason behind that statement?
I would like to suggest you use the WHO Air Quality Guidelines, that I attach here for your ease reference. The AQG are based on the best evidence available. Recently, IARC considered air pollution carcinogenic type I based on new evidence on the matter.
Look at the publications of the Health Effects Institute as well.
Researchers from Chile, Brazil, and Mexico (among others) have produced a large body of evidence of health effects of air pollution in the conditions of Latin America as well, and those findings fully support the implementation of WHO AQG.
This paper may help you to find a good answer to your question.
J.C Fensterstock et al., " The Development and Utilization of an Air Quality Index," Paper No. 69-73, presented at the 62nd Annual Meeting of the Air Pollution Control Administration, June 1969.