"This review focuses on the toxicological aspects of the products and byproducts produced during the gasification of coal underground, or in situ. This report is based in part and builds upon a TRW report prepared for the ERDA Office of Program Planning and Analysis, Fossil Energy - July 1976, entitled ""Carcinogens Relating to Coal Conversion Processes, Final Report."" ""Toxic"" here includes ""carcinogenic"", although the terms are often separated. Chemical carcinogenesis can be regarded as ""a very special aspect of an adverse toxicological reaction."" (WeiSburger 1975). The range of toxicants and their possible health hazards is extremely wide, from simple chemicals like carbon monoxide to complex mixtures of organic materials including carcinogens. In any coal gasification plant, whether it be a surface gasifier or an underground coal gasification process, process streams and effluents contain complex mixtures of chemicals. If the chemicals are similar in chemical constitution and toxicological mechanism, then the total effect would be additive. Synergistic effects (i .e., the combined effect is greater than the sum of the individual effects) may also occur with certain combinations of chemicals. Synergism is of particular significance with carcinogens. Other chemicals, such as phenol, SO2 and N02, which are not carcinogenic of themselves, can enhance the potency of carcinogens or promote tumor formation. Carcinogenic potency can also be decreased, as in the application of weak carcinogens such as dibenz (e,g) fluorene and chrysene together with strong carcinogens such as benzo (a ) pyrene (BaP). Finally, if components act independently, each can be considered as though the others were not present. This is seen with some carcinogens and with other toxicants; e.g., in concurrent exposure to an azo dye that induces liver tumors and 4-dimethylaminostilbene, which affects the ear duct (Weisbulger 1975). The known and suspected carcinogens present a toxic hazard in coal gasification because they are hazardous at low levels of exposure, have delayed action with no immediate warning, and have grave consequences. As for routes of entry, potentially carcinogenic solids and liquids may reach the body directly by inhalation or deposition of particles, or indirectly by contact with dirty surfaces; there are also unproven possibilities of cancer induction by vapor inhalation. Other toxicants are most likely to enter the body by inhalation."