| Summary: | A review of the literature reveals considerable evidence to support the suggestion that vegetative surfaces remove particulate matter from the atmosphere. Preliminary observations of the leaf surfaces of an important urban tree indicate the presence of numerous particulate contaminants. In view of the medical importance of fine particles in urban atmospheres, it is important to assess the efficiency of tree surfaces in particle retention. Can particulate loads be reduced below biologically significant thresholds by vegetation? Are trees acutely injured or subtly influenced in the process of this removal? A brief assessment of research needs is provided. It is evident that shade trees, of proper kinds, and suitably arranged, supply the conditions necessary to counteract the evils of excessive heat. They protect the paved streets and the buildings largely from the direct rays of the sun; they cool the lower stratum of air by evaporation from their immense surfaces of leaves; they absorb at once the malarious emanations and gases of decomposition, and abstract their poisonous properties for their own consumption; they withdraw from the air the carbonic acid thrown off from the animal system as a poison, and decomposing it, appropriate the element dangerous to man, and give back to the atmosphere the element
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