The first picture is one i took at a house..a dirty house with 2 toddlers toddling around the house and a big black dog laying on the return register. The filter was not in the furnace..hadn't been in it for years. I tore the furnace top off and saw the 'dog sweater' wrapped around the inside of the A-coil, I peeled it off in one section, laid it on the floor and took this picture. This is what happens when you don't install a filter in your furnace.
The second small picture is a return air in a bar. Always filthy, the bars air..
Makes you want to get up and change the filter, doesn't it??