The study solved this problem by looking at changes in the same areas, not between areas. Slow traffic would increase pollution. Noise could be an interesting

The study solved this problem by looking at changes in the same areas, not between areas. Slow traffic would increase pollution. Noise could be an interesting one, I'm not sure whether there are statistics that measure noise pollution over a country, and how would noise pollution be coincident with air pollution? Overpopulation wouldn't count as they are measuring the same areas with the same population. Daytime population could be an issue (e.g. weekends vs weekdays) but I can't access the study.