After watching a video from Matthias Wandel (http://www.youtube.com/watch?v=OmehjfmoxPQ) where he made different shots of the dust particles on some rooms of his house and on his workshop before and after sawing some boards, I decided to create this little Python script to count how many dust particles are present on those images.
This script is far from perfect but it can give a ballpark estimate and you should be able to compare two (or more) different pictures from those results.
All images are credit of Matthias.
Here are some screenshots as example:
$ python dust.py -s images/box_bedroom-b.jpg
images/box_bedroom-b.jpg : 144 particles
$ python dust.py -s images/box_tablesawed-b.jpg
images/box_tablesawed-b.jpg : 492 particles