My criteria was a minimum 50
Since it did not take much additional effort, I also included PM2.5 concentrations (μg/m3) at each marathon location. The WHO determined breathing values higher than 25 μg/m3 are 'less than ideal'. Perhaps factor that into your next 'destination' marathon (and if you'd like to see more discussion and a ranked list of clean-air marathons, check out the piece I wrote for Canadian Running).
Without further ado, here's the map. Click on individual races to get more info.
In case the above map does not survive, here's a static image: