1 >75 2 55-75 3 45-55 4 35-45 5 10-35 6 <10 µg SO2/ m3That is, zone 1 lichens are most tolerant and zone 6 lichens are most sensitive.
The actual presence of a species in an area will depend not only on its sensitivity to sulfur dioxide, but its sensitivity to other pollutants, to the nature of other environmental variables such as moisture, salt spray, etc., and to whether reproductive structures of that species have made it to the site.
These zone ratings were estimated by reference to known sensitivities
to SO2 in other areas of the world and general knowledge of
their distribution in the pacific northwestern United States and adjacent
Canada. Monitoring of SO2 levels critical
for these species has not been done here.
| Name | Zone | Brief description |
| Desmococcus viridis | 1 | "crustose" free-living algal scum |
| Parmelia sulcata | 1 | foliose, mineral gray green, bottle-brush rhizines, soredia |
| Evernia prunastri | 2 | fruticose, flattened, "staghorn lichen" |
| Hypogymnia physodes | 2 | foliose, tubular, mineral gray green, lip-shaped, sorediate |
| Physcia adscendens | 2 | foliose, tiny, mineral gray green, no apothecia |
| Xanthoria polycarpa | 2 | foliose, orange |
| Melanelia spp. | 3 | foliose, dark greenish brown, closely pressed to bark |
| Parmelia hygrophila | 3 | foliose, mineral gray green, simple rhizines, isidia |
| Physcia aipolia | 3 | foliose, tiny, mineral gray green, apothecia |
| tufted Usnea | 3 | fruticose, yellow-green, central cord, no apothecia |
| Cetraria chlorophylla | 4 | foliose, tufted, brownish, sorediate |
| Graphis scripta | 4 | crustose, "hieroglyphics lichen" |
| Menegazzia terebrata | 5 | foliose, hollow, bull's eye shape, pores in top |
| Parmotrema arnoldii | 5 | foliose, large, mineral gray green |
| Usnea rigida | 5 | fruticose, yellow-green, central cord, apothecia |
| Lobaria pulmonaria | 6 | foliose, large, olive green |