For many years humanity has been affected by the levels of air pollution that we breathe every day, this is why pollution levels are monitored by government agencies. Carbon Dioxide (CO₂) and Sulfur Dioxide (SO₂) are among the best known contaminants, toxics and harmful to the environmental. In Puerto Rico, the Junta de Calidad Ambiental established monitoring stations to register the levels of these toxics, but these systems are too expensive, and it is for this reason that it would be helpful to develop alternate monitoring methods. In the case of environmental pollution with SO₂, lichens can be an excellent tool to monitor the environmental pollution levels of aerial SO₂. Lichen is a mutuality relation, between a fungus (mycobiont) and an alga (photobiont). Due to the susceptibility of these organisms to air pollution, they are considered excellent environmental indicators. It is my interest to establish a possible relation between the lichens presence and the air pollution levels in the town of Guayama, Puerto Rico. The town was divided in 12 parcels; twelve trees at least were studied per parcel. The kind of lichens found were observed and tabulated by tree and the pollution level was based in the kind of lichens found. The recorded SO₂ levels in this zone were obtained from the Junta de Calidad Ambiental. Fifty three percent (53%) of the lichens found were Crustose, seventeen percent (17%) Foliose, one percent (1%) Fruticose and four percent (4%) Filamentous. Absence of lichens was observed in twenty five percent (25%) of the trees studied. The northern area of Guayama is the area where the most diversity of lichens is observed. The presence of Crustose lichens or the absence of lichens was observed in the southern, central, eastern and western areas of the town. For this reason the air quality in the town of Guayama can be categorized as regular.