The prevalence of nine indicator lichens found on trees in Northern Europe and Western Russia was used for monitoring air quality. The 4200 mile route of the survey went through eight countries. Surveys were carried out in cities, towns, countryside and forests, and along motorways. The author has conducted tree lichen surveys with pupils from schools and orphanages in Western Russia for many years. As expected, the calculated "lichen score" at each site correlated negatively with the published nitrogen dioxide levels in that area, and nitrogen-sensitive lichens were found to be most prevalent in forests and less-industrialised locations close to the Baltic Sea. (Contains 14 figures and 1 table.)