A research team used moss bio-indicators and mathematical spatial regression modelling to develop fine-scale maps of heavy metals and polycyclic aromatic hydrocarbons (PAHs) in Portland, Oregon. Moss (Orthotrichum lyellii) samples were collected from 346 site across Portland in December 2013 and tested for 18 PAHs and 22 elements, including several toxic metals such as lead, cadmium, and chromium. PAHs levels were especially high along roads and highways, unsurprising since PAHs are by-products of fossil fuel and biomass combustion. Of particular interest was how vegetation cover (trees, grass-and-shrubs) appeared to substantially reduce PAH levels, suggesting that planting vegetation buffers may be a viable strategy for reducing human exposure. Heavy metals did not follow roads, occurring instead in “hotspots” associated with relatively localized sources of pollution. A detailed spatial analysis of cadmium levels in the moss found hotspots of extremely high concentrations around two stained-glass manufacturers, neither of which were previously known as cadmium emitters. Regulators measured cadmium levels near the larger hotspot and found concentrations exceeded health benchmarks by 49 times. “Hotspot” maps were developed for all 22 elements measured in moss and the raw data released to the public for continuing investigation of possible pollution sources. A map showing hotspots with elevated levels of two or more of the 6 most toxic metals identified seven locations for further study. The Oregon Department of Environmental Quality verified the moss cadmium results using air quality monitors and both stained-glass manufacturers voluntarily stopped using cadmium after the results were made public. Atmospheric cadmium levels dropped precipitously in both locations.