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In determining the specific gravity of vapors, air is commonly used as the standard reference substance. Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance at a specified temperature and pressure. For gases and vapors, air serves as a useful standard because it is readily available and provides a consistent point of comparison for measuring other gases or vapors.

Using air as the reference allows for an effective comparison because it closely approximates the average behavior and density of many gases present in the atmosphere. This choice is particularly relevant when considering various applications, such as environmental monitoring or industrial processes, where the properties of vapors relative to air can have practical implications.

Other substances like water, gasoline, or hydrogen do not provide the same level of relevance for vapors since they differ significantly in terms of density and flow characteristics compared to air. Water, for instance, is often used for liquids, while gasoline is a liquid that does not accurately reflect the behavior of vapors. Hydrogen, while a gas, is lighter and not commonly used as a reference for measuring specific gravity in vapors. By using air, the understanding of how a vapor's behavior compares to the natural environment is more applicable.