Ok, now I see, yes on the street a filter is essential. As a minimum a fine mesh gauze will provide the least restriction, but still filter out anything that would cause damage.
I feel for you with lack of facilities & lack of cash, I'm in the same boat I was laid off a few weeks back, so busy looking for a new job pronto. I guess looking at where Iowa City is, there must be some long straight rarely used roads you could use as ahem test strips
I also lack a van or trailer, so that will be rectified next year.