Admittedly I did not follow scientific procedure like you suggested. I have no control.
I just looked - I made an observation. That's all. I did not even make a conclusion. This is the first time I have looked at the dust in my or any air filter.
Thanks for the information and counsel. I inquired with NOAA (National Oceanographic and Atmospheric Administration) in Boulder and they told me there was nothing different about the condensation trails. Maybe more airplanes but nothing different.
People who look up know different and know there is something different. The condensation trails remain for minutes instead of seconds. People who have had an interest in airplanes have noticed how much longer contrails linger.
So I began doing photograph searches "vacation 1990"; "vacation 1995"; "vacation 2000"... People take a lot of photographs outside during vacations. I started searching the sky for planes over the years. Take a look for yourself:
http://www.alltheweb.com/search?cat=...s=0&itag= crv
That is not all that scientific either but it helps to get a look.
Regards,
David Merrill.