Chemical substances play an important role in daily life. The list of chemical substances is huge, their groups are many, and their contribution to economic growth and progressive societal changes are equally diverse. A cursory glance at elementary classes/groups of chemical substances include, but are not limited to, alcohols, aliphatic hydrocarbons, aliphatic nitriles and cyanates, aliphatic nitrosamines, aromatic nitro compounds, aromatic organophosphorous compounds, azo compounds, chlorine compounds, chemical fumes, dust, toxic gases and vapor of metals, organic solvents, halogenated aromatic compounds, halogenated cresols, metals and metal compounds, phthalates, pesticides, poly-cyclic aromatic hydrocarbons, and polychlorinated biphenyls. Occupational workers and the public can be exposed to groups of chemicals, such as paint thinners, thin oil-based paints, and cleaning agents. This group of chemical substances includes acetone, mineral spirits, mineral turpentine, wood turpentine, naphtha, toluene, white spirit, xylene, and many more. Unstable/reactive chemicals undergo vigorous polymerization and decomposition; water-reactive chemical substances react with water and release flammable and toxic gas; corrosive chemicals cause visible destruction of living tissue; chemicals as irritants; sensitizers/allergens produce their own kind of adverse health effects on occupational workers. Each group includes individual chemical substances as well. (Readers will find further details on the classification of chemical substances in textbooks of organic chemistry.)