The list of pollutive gasoline emissions is extensive; however, "The Big Three" emissions that receive the most attention from emissions regulators are: 1) Carbon Monoxide, 2) Hydrocarbons, and 3) Nitrogen oxides.
- Carbon Monoxide (CO) is a product of incomplete combustion which occurs when carbon in fuel is partially oxidized rather than fully oxidized into carbon dioxide. CO is a colorless, odorless gas which reduces the flow of oxygen in the bloodstream and can impair mental functions and visual perception (3). At very high levels, carbon monoxide is life threatening. Long term (chronic) exposure to low levels of carbon monoxide may produce heart disease and damage the nervous system. At high levels, carbon monoxide will cause illness to animals, and as in women, exposure to carbon monoxide by pregnant animals may cause low birthrates and nervous system damage to their offspring (4). In addition, carbon monoxide increases the amount of other greenhouse gases (methane, for example), and eventually oxidizes into carbon dioxide, a greenhouse gas which is purported to affect to global warming.
- Hydrocarbons (HC) emissions result when fuel molecules in the engine do not burn, or burn only partially. Hydrocarbons react with nitrogen oxides in the presence of sunlight and elevated temperatures to form ground-level ozone, which is a primary component of smog. Ground-level ozone causes health problems such as breathing difficulty, lung damage, and reduced cardiovascular functioning. A number of hydrocarbons are also considered toxic, meaning they can cause cancer and other serious health problems (5).
- Nitrogen oxides (NOx): "NOx" is a generic term which is used to describe various nitrogen oxides produced during combustion. Some of the most common nitrogen oxides include: Nitrogen oxide (NO), Nitrogen dioxide (NO2), Dinitrogen monoxide (N2O), Dinitrogen trioxide (N2O3), Dinitrogen tetroxide (N2O4), and Dinitrogen pentoxide (N2O5) (6).
Under the high pressures and temperatures of an engine, nitrogen and oxygen atoms in the air react to form various Nitrogen oxides. Nitrogen oxides, like hydrocarbons, are precursors to the formation of ground-level ozone which is a primary component of smog. NOx also contributes to the formation of acid rain which can harm vegetation and change the water chemistry of lakes and rivers, rendering them uninhabitable for wildlife.
Other emissions which receive less attention from regulators than The Big Three emissions, but which are still sources of intense focus include (in alphabetical order):
- Carbon Dioxide (CO2) occurs naturally in the atmosphere and is believed not to directly or adversly affect human health. In recent years however, many environmental regulatory agencies have began to consider carbon dioxide"a product of "perfect" combustion"a major pollution concern. The burning of fossil fuels produces carbon dioxide in enormous quantities. During combustion, carbon and oxygen atoms join to form carbon dioxide. CO2 is an extremely efficient greenhouse gas that traps the earth"s heat and contributes to theory of global warming (7,8). Because of this, CO2 has become a central target of national and supranational emissions regimes such as the United States Environmental Protection Agency (EPA), and the United Nations Framework Convention on Climate Change.
- Hazardous Air Pollutants (HAPS) are chemicals that can cause serious health and environmental effects. Health effects include cancer, birth defects, and nervous system problems. Environmental effects are numerous and be very serious. The United States Environmental Protection Agency publishes a comprehensive list of HAPs entitled The Original List of Hazardous Air Pollutants.
- Lead (Pb) emissions originate from the combustion of leaded gasoline. Thankfully, in most places lead has been phased out of gasoline because lead emissions are highly toxic. Lead emissions can produce a range of severe health effects which include nervous system damage, cancers, and learning behavior problems.
- Ozone (O3) is a gas that consists of three oxygen atoms. Ozone forms naturally and is beneficial in the stratosphere, a layer of atmosphere high above Earth, where it filters harmful ultraviolet (UV) rays. Ozone that is close to the earth"s surface ("ground-level ozone" ((O3 (GL))) however, is a pollutant that can irritate the respiratory tract, cause chest pain and persistent cough, and affect one"s ability to breath deeply. O3 (GL) increases susceptibility to lung infection and pulmonary disease. Gasoline motor vehicles are a major source of O3 (GL), which is formed by the oxidation of VOCs in tailpipe emissions. Ground level ozone is a principal component of smog which damages vegetation and reduces atmospheric visibility.
- Particulate Matter (PM) is an aerosol comprised of complex physical and chemical structures. PM contributes to the greenhouse effect and causes serious environmental and human health consequences. PM is responsible for the majority of the black smoke that people normally associate with exhaust. PM is also a major contributor to urban smog (9).
- Sulfur Dioxide (SO2) is bi-product of the combustion of fuel sulfur. One of the major problems with SO2 is its tendency to oxidize into sulfur trioxide (SO3), and then into sulfuric acid (H2SO4) in the presence of high temperatures and moisture. SO2 is an odorless gas at low concentrations, but at high concentrations it can irritate the lungs and have a very strong smell. H2SO4 is a strong, toxic mineral acid which causes acid rain, acidification of waterways, crop and property damage, and harm to human health.
- Volatile Organic Compounds (VOC's) in combination with nitrogen oxides, are responsible for ground level ozone and smog (10). Volatile Organic Compounds, or VOC's, are organic chemicals that form gases at room temperature. They are called organic because they contain elemental carbon. There are many types of VOC's. Hydrocarbons have both hydrogen and carbon atoms and include benzene and toluene. Oxygenates contain carbon, hydrogen and oxygen and originate in automobile exhaust. In addition to the health problems caused by VOC's, VOC's in the air react with ultraviolet light and nitrogen oxides to form ground level ozone (11). The Ontario Ministry of the Environment publishes a List of Volatile Organic Compounds that provides a comprehensive inventory of VOC"s.