The environmental load and cycle of polycyclic aromatic hydrocarbons

Abstract

Many polycyclic aromatic hydrocarbons (PAH) are known to be carcinogenic to animals and probably to man. Therefore, a better understanding of their formation, behavior and fate in the environment is necessary. Of the PAH carcinogens, 3,4-benzpyrene (BaP), which constitutes only about 1 % – 20 % of the group, is one of the most potent and ubiquitous, and therefore the most widely studied. Although the risk from carcinogenic PAH cannot yet be estimated, synergistic effects with other substances have also to be considered. The formation of PAH in the environment is due to an endogenic synthesis by microorganisms, phytoplankton, algae and highly developed plants, which provide the natural background, and to a second process which is connected with man-controlled high temperature pyrolitic reactions, open burning, and natural volcanic activities. Photooxidation is the major decomposition process by which PAH are reduced, biodegradation by microorganisms being a secondary reduction mechanism. The worldwide production of PAH by biosynthesis cannot be estimated on the basis of the limited data available at present. However, the global emission of BaP derived from exogenous sources has been estimated at about 5000 tons per year, a quantity which appears to exceed that formed endogenically. It may be assumed, that until the beginning of this century there still existed a natural balance between the production and degradation of PAH, which kept the natural background concentration low and fixed. However, with the ever-increasing industrial development in many parts of the world the natural balance is being disturbed and the production rate of PAH is constantly rising, while their rate of decomposition remains more or less constant. In view of the carcinogenic characteristics of many PAH compounds, their concentration in the environment should be considered alarming and efforts should be made to reduce or even eliminate them wherever possible. At the same time, an environmental programme should be developed for monitoring the PAH compounds and the various parameters affecting their formation, retention and destruction.