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Formulation of Insecticidal Aerosol I. Overview Technically speaking, insecticidal aerosols were first commercialized in 1943. During World War II, heavy \"pest bombs\" were delivered to the islands in the South Pacific for use by U.S. Marines in fighting against Japanese soldiers and mosquitoes. The sprayers, with protruding tops and bottoms, were about 2.25 mm thick and could withstand pressures exceeding 14.9 MPa. Approximately 50 million of these original aerosol cans were produced during that war. Their success sparked the interest of two American canning companies in developing lightweight aerosol cans for consumer use. In early 1946, H.M. Peterson from Continental Canning Company and his research team created the first such lightweight aerosol can. Around the same time, Crown Company’s product killed 100% of the insects that crawled over the treated surface, with a killing rate of 95% or more even after 12 weeks; all deaths occurred within 24 hours of contact. Even at extremely high concentrations of pyrethrin or allethrin, it is not possible to reproduce the aforementioned results, as these substances have much lower toxicity levels against insects and almost no residual activity. Additionally, some common insects have developed significant resistance to these pesticides. For these reasons, the vast majority of modern insecticidal aerosols use alternative pyrethroid compounds. Cypermethrin is a pesticide with exceptional efficacy and long-lasting residual activity. Overall, its lethal effect on houseflies is 9 times that of pyrethrin, and 4 times that on German cockroaches. It has a structure similar to that of pyrethrin, but with two chlorine atoms and one cyano group. Only deltamethrin is more potent; its structure is similar to that of cypermethrin, except that two oxygen atoms are replaced by two bromine atoms, making it the most effective among the eight isomers. Cypermethrin can function effectively without any additional pesticides. Although deltamethrin has not yet been approved by the U.S. Environmental Protection Agency as a registered insecticide, it also does not require any additional pesticides, just like cypermethrin. The concentration used in aerosol formulations containing deltamethrin is as low as 0.02%. Between 1961 and 1963, almost all insecticidal aerosols in the United States were converted to water-based forms, primarily because replacing about 60% of the petroleum distillates with deionized water reduced raw material costs. Another important factor was reducing the flammability of the formulations, so that they could pass the “flame test.” In the United States and Canada, this flame test is used to determine whether an aerosol needs to be labeled as “flammable.” Although water-based formulations typically contain 60% petroleum distillates and 30% isobutane, the large amount of water present prevents the flame from spreading when the mist passes over the tip of a candle flame. In 1996, the California Air Resources Board (CARB) decided that the concentration of volatile organic compounds in insecticide aerosols could not exceed 35%; in 1998, this limit became a national regulation set by the U.S. Environmental Protection Agency. Although oil-based insecticide aerosols are very popular in almost all other countries, regulations imposed by the U.S. Environmental Protection Agency prevent their sale in the United States.