Moss, that ubiquitous tree hugger of the Pacific Northwest, can be an inexpensive, effective tool for identifying pollutants in cities where it commonly grows, according to a Drexel study.

The study of naturally growing tree moss at more than 300 sites across Portland, Oregon, was a joint project between the U.S. Department of Agriculture’s Forest Service and researchers from Drexel’s Dornsife School of Public Health.

Moss has been established as a bio-indicator for chemicals in the air for some time, but this study used the moss in a new way.

Moss samples indicated two previously unknown hot spots (in red) for high cadmium levels. The black dots on the map show areas where samples were taken.

“What’s unique about this study is that we used moss to track down previously unknown pollution sources in a complex urban environment with many possible sources,” says Sarah Jovan, a research lichenologist with the Forest Service.

Jovan was joined by Geoffrey Donovan, Demetrios Gatziolis, Michael Amacher and Vicente Monleon from the Forest Service, and Igor Burstyn and Yvonne Michael, both associate professors in the Dornsife School of Public Health, as co-authors for the study, which was published in Science of the Total Environment.

The research team collected Orthotrichum lyelli moss in late 2013 using a modified, randomized, grid-based sampling strategy.

When tested, the samples indicated two distinct hotspots for cadmium. These fell outside of the area where cadmium was expected to be higher, where companies with permits for cadmium emission were located. Within each hotspot, there was a stained glass manufacturer that used cadmium (sometimes used in pigments) in production.

Neither company was required to have cadmium emissions permits and both voluntarily stopped using cadmium after the monitoring results were made public.