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Asphalt can be a naturally occurring substance (think of the LaBrea Tarpits), but it is generally created by <br />people in a number of ways. Asphalt production includes numerous ingredients which, when combined, <br />result in a particular performance specification. Ninety-five percent of asphalt is aggregate material from <br />gravel mines and five percent is petroleum distillate products. Within Elk River, Hot Mix Asphalt Plants <br />create different "recipes" of bituminous asphalt for clients who use the asphalt for a variety of operations <br />(not least of which is road paving). Gravel and sand are main constituents of asphalt, so the hot mix plants <br />are located as close to the source of these raw materials as possible. The other main ingredient in asphalt <br />is crude oil derivatives which act as a glue or binder of the other materials. The ingredients are mixed and <br />heated to facilitate application on roofs or paved surfaces. Heating also maintains fluidity during <br />transportation and storage (see storage silos above). <br />Hoppers for hot mix ingredients <br />Unfortunately, it is the crude oil derivatives which contribute to poor air quality by releasing harmful <br />chemicals into the air when heated. Some of the chemicals are cancer causing, including arsenic, benzene, <br />formaldehyde, and cadmium, while others have been labeled as "probable carcinogens" (such as volatile <br />organic compounds [VOCs] and polycyclic aromatic hydrocarbons [PAHs or PACs]). The amount and <br />proportion of individual chemicals released increases as the temperature increases. Fumes generated at <br />high temperatures are more likely to have cancer causing chemicals in them. For example, 80 times more <br />2 <br />~, .+ <br />.T1 ~ ~y~ <br />