Although rare earth elements (REEs) are important in modern industry, few processes use them, so the possibility of these elements being dispersed in the environment is small. For this reason, there are few studies on their presence in urban environments. REEs exhibit similar chemical properties, and Group 3 elements may be used as indicators of the presence of other REEs in the environment. Tree bark is a suitable collector of airborne trace elements and has been successfully used as a bioindicator. In this study, samples of holm oak bark were collected from three sampling sites subjected to different anthropic pressures (one area is far from human influence, while the other two sites are urban areas characterized by different types of human impact). The reference area is significantly different from urban areas where the concentrations measured are up to 5 times higher (Y) than the reference area. The differences between the two urban areas are also significant. The presence of industrial activities causes a 50% higher exposition of the population with respect to an urban area where only residential activities are present. In the case of La, there is no significant difference between the reference and the residential areas, while the industries are responsible for higher La concentrations (0.9 μg g^-1vs. 0.4‒0.6 μg g^-1). The availability of samples, due to the wide distribution of trees in urban environments, makes possible the production of maps indicating the sources of these elements and highlighting areas which are critical for certain atmospheric pollutants.

尽管稀土元素（REE）在现代工业中很重要，但很少有过程使用它们，因此这些元素散布在环境中的可能性很小。因此，关于它们在城市环境中的存在的研究很少。稀土元素表现出相似的化学性质，第3组元素可以用作环境中其他稀土元素存在的指标。树皮是空气中微量元素的合适收集者，已成功用作生物指示剂。在这项研究中，圣栎树皮样品是从三个承受不同人为压力的采样点采集的（一个区域远离人类的影响，而另外两个站点则是具有不同类型人类影响特征的城市地区）。参考区域与市区相比明显不同，在市区中，测得的浓度（Y）比参考区域高出5倍。两个城市地区之间的差异也很大。相对于仅存在居民活动的城市地区，工业活动的存在使人口的曝光率增加了50％。就La而言，参比区和居民区之间没有显着差异，而各行业负责更高的La浓度（0.9μgg^-1对0.4±0.6μgg ^-1）。由于城市环境中树木的广泛分布，样品的提供使制作地图成为可能，这些地图表明了这些元素的来源并突出了对某些大气污染物至关重要的区域。