I also worked at an Indoor Range in the early 90's and we were tested every 4-6 months. My average count was 1.5-1.8. If anyone went over 2.5 their range time was moved to sales and not shooting or training. Indoor ranges are not easy to keep clean and clean air is at the top of the list for me. If you do not feel air flowing from behind you down range toward the targets, the range you are shooting at may not be that clean. All in all lots of good info in this thread.