It's funny because homelessness and the perceived/visible "dirtiness" or trash it causes is not on the methodology of this list; for pollution, it's:
Median Air Quality Index
Presence of Water Quality Violations
Greenhouse-Gas Emissions
Annual Excess Fuel Consumption (Gallons per Auto Commuter)
Share of Residents Exposed to Near-Roadway Pollution
RSEI (Risk-Screening Environmental Indicators) Score
Percentage of Smokers
I have yet to see a completely clean American metro area.
The cleanest big city/metro I've experienced was Tokyo and Vienna... clean public transportation, clean public restrooms... some naughty...
Oh my, did I say that?