If you do a quick straw poll on this questions, the results will be mixed. Some point to vehicle exhaust and factories and say the indoor air is better. Others point to volatile organic compounds (VOCs) and airborne microorganisms indoors, and say the outdoor air is better.
This morning I was reading the August edition of the journal, Indoor Air. The editorial by William Nazaroff raised this very question about indoor vs. outdoor air.
The answer to this question has wide implications.