Volatile Organic Compounds

Transportation creates emissions that can have deleterious impacts on public health. Emissions that emanate from motor vehicles or any other device that moves from one location to another are typically known as mobile sources of emissions. These consist not only of on-road vehicles such as cars and trucks but also off-road vehicles such as all-terrain vehicles (ATVs) or snowmobiles, as well as non-road vehicles such as airplanes, locomotives, and certain types of lawn and garden equipment.

One of the most common pollutants is volatile organic compounds, also known as VOCs. According to the U.S. Environmental Protection Agency (EPA), a VOC is any compound of carbon—excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate—that participates in atmospheric photochemical reactions.

In more basic terms, these are all substances that contain carbon and evaporate easily. There are numerous VOCs also emanating from non-mobile sources, or stationary sources, such as oil and gas operations, paints, and cement factories, to name just a few. Some of the more common VOCs include benzene, typically emanating from exhaust fumes from cars in attached garages; formaldehyde, typically used as a disinfectant for biological specimens; and chlorofluorocarbons (CFCs), which were widely used as refrigerants, propellants in aerosol applications, and solvents before being strictly regulated in the late 1970s and beyond.

The emissions just described are known as anthropogenic emissions, because they are basically manmade. There are also VOCs generated from biogenic sources, such as plants and animals. Not counting methane, biological sources emit an estimated 1,150 teragrams (approximately 1.27 billion short tons), of carbon per year in the form of VOCs.

The majority of VOCs are produced by plants, the main compound being isoprene. The remainder are produced by animals, microbes, and fungi such as molds. Of the total VOC concentration in the atmosphere, approximately 72 percent is from biogenic sources and 28 percent from anthropogenic sources. The major anthropogenic VOC emissions are from on-road vehicles and industrial processes. Although biogenic and stationary sources comprise the majority of sectors producing VOC emissions, mobile-source VOC emissions are the focus of this article. Some VOCs are also considered mobile source air toxics (MSATs), another set of hazardous air pollutants.

VOCs combine with nitrogen oxides (NOx) under sunlight to create ozone (O3), one of six criteria pollutants regulated by the EPA under the Clean Air Act. Ozone contributes to what most people experience as “smog” or haze, which occurs most frequently in the summertime but can occur throughout the year in some southern and mountain regions. The highest ozone concentrations usually occur between 2 P.M. and 8 p.m. from May through September.

Most operating vehicles are powered by an internal combustion engine, in which burning fuel creates heat that ultimately propels the wheels supporting the vehicle. VOCs represent one type of emissions that are by-products of this combustion process. These emissions, which occur when the vehicle is operating, are known as tailpipe emissions. VOCs can also escape into the air through fuel evaporation, which can occur even when the vehicle is idling or parked. These emissions are known as evaporative emissions.

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