How polluted is the habitat if the trees have Pleurococcus and crusty yellow lichens on them?

The presence of Pleurococcus and crusty yellow lichens on trees typically indicates a certain level of air pollution, particularly sulfur dioxide and nitrogen compounds. While Pleurococcus can thrive in various environments, the crusty yellow lichens often suggest that the habitat has experienced some stress due to pollutants. However, these organisms can also tolerate a range of conditions, so while pollution is likely present, the exact level of habitat degradation would require further ecological assessment.