Breathe Clean Air Air &  Environment

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Car Exhaust - Air Pollutants

In cities across the globe, the personal automobile is the single greatest polluter, as emissions from more than a billion vehicles on the road add up to a planet-wide problem. Driving a private car is a typical citizen's most air polluting activity. The negative effects of automotive emissions are maximum when you sit in traffic surrounded by cars trucks and buses, their engines idling. Everyone sitting in a traffic jam is getting poisoned.

The Combustion Process

Gasoline and diesel fuels are mixtures of hydrocarbons (made of hydrogen, oxygen and carbon atoms.) Hydrocarbons are burned by combining with oxygen. Nitrogen and sulphur atoms are also present and combine with oxygen when burned to produce gases. Attempts to reduce exhaust emissions from gasoline and diesel engines have been compromised by limitations of testing, inherent flaws in the design and inadequate maintenance of emission control devices.

Diesel engines a pose different emission control problems than gasoline engines. Diesels require more sophisticated and expensive components than the catalytic converters fitted to gasoline engines. Diesel emissions contain nitrogen oxide gases and carbon particles, the smallest of which contribute to lung and heart disease. Increases in airborne fine particulate matter increases the risk for myocardial infarctions, strokes and heart failure. Particle deposition in the lungs activates the sympathetic nervous system and triggers the release of systemic pro-inflammatory responses. Brook and Rajagopalanb stated: "Higher circulating levels of inflammatory cytokines cause vascular endothelial dysfunction and activation of vasoconstrictive pathways while blunting vasodilator capacity. At the molecular level, the generation of oxidative stress with the consequent up-regulation of redox sensitive pathways appears to be a common mechanism of these pro-hypertensive responses. Due to the ubiquitous, continuous and often involuntary nature of exposure, airborne fine particles may be an important and under-appreciated worldwide environmental risk factor for increased arterial BP.

Typical Engine Combustion Emissions

Fuel + Air => Hydrocarbons + Nitrogen Oxides + Carbon Dioxide + Carbon Monoxide + Sulphur Dioxide+ water

Hydrocarbon emissions are fragments of fuel molecules, only partially burned. See Toxicity Hydrocarbons in exhaust.

Hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground-level ozone, a major component of smog. Ozone irritates the eyes, nose, throat and damages the lungs. A number of exhaust hydrocarbons are also toxic, some with the potential to cause cancer.

Nitrogen Oxides Under high pressure and temperature conditions in an engine, nitrogen and oxygen atoms react to form nitrogen oxides. Nitrogen dioxide, NO2, is a brownish toxic gas, an important air pollutant. The air of cities with high levels of car ownership has a distinctly brown tinge. NO2 combines with water in the air to form nitric acid - acid rain. A complex chemistry involves NO2 combining with hydrocarbons to form the photochemical smog that poisons city dwellers. Sunlight converts unburned hydrocarbons to more reactive molecules such as aldehydes and ketones which generate peroxyacyl radicals that react with NO2 forming peroxyacyl nitrates(PANs). Catalytic converters in car exhaust systems reduce air pollution in the best case by breaking down NO2 and N20 to nitrogen (N2) and oxygen(O2).

Nitrous oxide (N2O) also known as "laughing gas" has medical uses, but is a pollutant in the air. N2O, for example gives rise to nitric oxide (NO) which reacts with  and depletes ozone. N2O is a combustion product but also originates from forest fires, lightning storms nitrogen-based fertilizers and manure from farm animals. According to the US EPA industrial sources make up only about 20% of all anthropogenic sources of N2O, including the burning of fossil fuel in internal combustion engines. Indoor gas burning appliances generate N2O.  It is also a major greenhouse gas and air pollutant with about 300 times more global-warming potential than carbon dioxide.

Carbon Monoxide Carbon monoxide (CO) is a colorless, odorless, poisonous gas, a product of incomplete burning of hydrocarbon-based fuels. Carbon monoxide consists of a single carbon atom and a single oxygen atom linked together (CO), the product of incomplete combustion of fuel. Most CO is produced when air-to-fuel ratios are too low in the engine during vehicle starting, when cars are not tuned properly, and at higher altitudes, where thin air reduces the amount of oxygen available for combustion. Two-thirds of the carbon monoxide emissions come from transportation sources, with the largest contribution coming from cars. In urban areas, the passenger vehicle contribution to carbon monoxide pollution can exceed 90%.Read more about Carbon Monoxide

Carbon Dioxide U.S. Environmental Protection Agency (EPA) originally viewed carbon dioxide as a product of "perfect" combustion, but now views CO2 as a pollution concern. Carbon dioxide is a greenhouse gas that traps the earth's heat and contributes to Climate Change

Particle Pollution and Human Disease The U.S. Environmental Protection Agency (EPA) defines fine-particle air pollution, PM10, particulate matter 10 micrometers or less in diameter. Suspended particles in the air create aerosols that are important to the behavior of whole atmosphere and play a role in determining human disease. Natural sources of atmospheric particles are volcanoes, dust storms, spontaneous forest fires, tornadoes and hurricanes. Clouds are a product of aerosols that seed the formation of water droplets. Human air pollution now dominates aerosol production over cities with negative health effects. Thick aerosols are obvious as haze and contain a complex system of particles with adherent toxic gases such as sulphur dioxide. Air pollution is associated with increased hospital admissions for cardiovascular diseases with increases in acute morbidity and mortality. D'Ippoliti et al studied 6531 patients in Rome who were hospitalized for acute myocardial infarction from January 1995 to June 1997. Air pollution data were taken from 5 city monitors. Positive associations were found for total suspended particulates, NO2 and CO. The strongest and most consistent effect was found for total suspended particulates. See Air Pollution Kills

Evaporative Emissions Hydrocarbon pollutants also escape into the air through fuel evaporation - evaporation causes significant hydrocarbon pollution from cars on hot days when ozone levels are highest. Evaporative emissions occur several ways:

Diurnal: Gasoline evaporation increases as the temperature rises during the day, heating the fuel tank and venting gasoline vapors.

Running Loses: The hot engine and exhaust system can vaporize gasoline when the car is running.

Sitting Evaporation: The engine remains hot for a period of time after the car is turned off, and gasoline evaporation continues when the car is parked.

Adding Fuel: Gasoline vapors are always present in fuel tanks. These vapors are forced out when the tank is filled with liquid fuel.

(See Cars and Pollution US EPA Fact Sheet OMS-5)

Benzene is the main toxin in the hydrocarbon fraction of exhaust. Benzene and other less known hydrocarbons are produced in petroleum refining, and are widely used as solvents and as materials in the production of various industrial products and pesticides. Benzene also is found in gasoline and in cigarette smoke. Other environmental sources of benzene include gasoline (filling) stations, underground storage tanks that leak, wastewater from industries that use benzene, chemical spills, and groundwater next to landfills containing benzene. Exposure to benzene can cause cancer, especially leukemias and lymphomas. Benzene has a suppressive effect on bone marrow and it impairs blood cell maturation and amplification.

Polycyclic aromatic hydrocarbon (PAH)

PAHs are a group of chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances. PAHs can be man-made or occur naturally. A few of the PAHs are used in medicines and to make dyes, plastics, and pesticides. They are found throughout the environment in the air, water and soil. There are more than 100 different PAH compounds. Although the health effects of the individual PAHs vary, the following 15 PAHs are considered as a group with similar toxicity: acenaphthene, acenaphthylene, anthracene, benzanthracene, benzopyrene, benzofluoranthene, benzoperylene, benzofluoranthene, chrysene dibenzanthracene, fluoranthene, fluorene, indenopyrene, phenanthrene, pyrene.

Long term solutions require reduced combustion of all kinds. While vehicles with new energy sources such ethanol, biofuels, propane and natural gas can contribute to reduced air pollution, their benefit is limited if vehicle use continues at current intensities. If you pay more money to buy a hybrid car, but drive it more, you have contributed little to solving air pollution problems. If you buy a gas guzzling clunker and use only one gallon of gas to go 15 miles each week, you have contributed more to the solution.

The problem with all alternative fuels is that the manufacture of fuels requires energy, distribution with a manufacturing infrastructure that consume energy, often derived from burning fossil fuels. No alternative fuel is ideal. See Switch to Biofuels

Hydrogen Ultimately cars might burn hydrogen in fuel cells, but despite working prototypes, a hydrogen economy is a distant fantasy. There are many problems to be solved before hydrogen can replace fossil fuels as a portable energy source. The biggest problem is that producing hydrogen requires a large amount of energy. In Canada, there are opportunities to dam rivers and produce electricity with falling water, a non polluting, renewable energy resource. A more problematic energy source would be be nuclear reactors that "burn" uranium or plutonium.Even if new non-polluting energy sources are developed, hydrogen storage and distribution requires a new, very expensive infrastructure that could replace gasoline and diesel fuels.

With once rich countries such as the USA on the verge of bankruptcy and facing the extensive repairs of already aging, derelict infrastructures, adding a new, unprecedented development costs seems unlikely. Unless, of course the priorities in the US shift dramatically. The US, for example, could adopt a sane, smart strategy, reduce its military budget by 50% and invest the money and skills in rebuilding the country's infrastructure with new sustainable energy sources.

Car Emissions Testing - The Volkswagen Scandal

in Sept. 2015 a scandal erupted when Volkswagen, the world's largest car manufacturer, was caught cheating on emission tests of their diesel engines. In testing lab conditions, VW could show conformity with emission standards. Subsequent independent testing of VW diesel vehicles in road tests revealed high levels of nitrogen oxides emitted in real operating conditions. Errors in media reports proliferated with talk of defeat devices and software that would fool emission tests. Relevant engineering data was not available but likely the cause of the problem was the Nitrogen Oxide converter (aka NOx storage catalytic converter ) that required injections of unburned fuel to keep the converter clean and functional. The exhaust output was supposed be free of nitrogen oxides. The computer that controlled fuel injection was programmed to inject more fuel than was needed for combustion for about 2 seconds per minute. The fuel reaching the converter would burn increasing the temperature in the converter. Burning the diesel fuel in the converter would likely increase the emission of other air pollutants. The software functioned optimally for emissions testing and was turned down or off when the engine was in service. The NO converter was a poor design that would increase fuel consumption and decrease engine performance if the converter was operated for full emissions control.

Jack Baruth advocated the end of diesel cars and pickup trucks. He stated: "Western democracies encouraged diesel even though they were perfectly aware of the health hazards posed by diesel particulate exhaust. Those risks are far better documented than even the most "settled" climate science, and they are very real. Eurocrats chose diesel in order to be seen to be doing something about global warming, and the manufacturers had to abide by their choice. The result? Paris has had to ban cars for hours or even days at a time because of smog. According to The Guardian, "diesel-related health problems cost (the British National Health Service) more than 10 times as much as comparable problems caused by petrol fumes. Last year the UN's World Health Organization declared that diesel exhaust caused cancer and was comparable in its effects to secondary cigarette smoking. And that was when people thought that these diesels were meeting pollution standards! Now, of course, we know that many of them were not, and that even the diesel cars that weren't designed to cheat the tests are not performing in the real world the way they do in the test labs. In other words, diesel-powered automobiles are killing people, and in not inconsiderable numbers. The jury is in and the evidence is clear." (Jack Baruth. Road & Track. Oct 2015)

A review in the Science journal, Nature, questioned the relationships between auto manufacturers and regulatory bodies: "Among the questions raised by the scandal that allowed the German car maker Volkswagen to sell 11 million vehicles containing software that cheats emissions tests, many will ask why the regulators failed to notice and halt the practice. The answer is not complicated. Regulated industries exert massive, discreet pressure on regulators such as the US Environmental Protection Agency (EPA), to stop them doing their jobs properly."

To put the VW scam in perspective, the company's diesel cars are uncommon in in the USA and Canada and only contributed a small amount to air pollution, compared with other sources, even with high nitrogen oxide emissions. The big problems were corporate cheating and deliberate violation of public trust. It appears that this deception will cost VW several billion euros and is an embarrassment for all of Germany. Regulatory agencies have been alerted to their limitations and will be changing testing procedures for all new engines that include monitoring emissions during real driving tests in real driving conditions.

Selective Catalytic Reduction

The best method of cleaning diesel exhaust is Selective Catalytic Reduction (SCR) which converts oxides of nitrogen (NOx) emissions into benign nitrogen gas and water. SCR can deliver near-zero emissions of NOx. The NOx reduction process starts with an efficient engine design that burns clean Ultra Low Sulfur Diesel (ULSD) and produces inherently lower exhaust emissions—exhaust that is already much cleaner due to leaner and more complete combustion. The vehicle’s onboard computer regulates the addition Diesel Exhaust Fluid (UREA) in precisely metered spray patterns into the exhaust stream just ahead of the SCR converter where the conversion happens. Together with the catalyst inside the converter, the mixture undergoes a chemical reaction that produces nitrogen gas and water vapor.


  • Discussions of Environmental Science and Human Ecology were developed by Environmed Research Inc. Sechelt, B.C. Canada. Online Topics were developed from the book, Air and Breathing. This book helps you understand air quality issues, normal breathing and the causes of breathing disorders. You will find detailed information about the atmosphere, air pollution, climate change, airborne infection, air quality and airborne hazards at home. Air and Breathing is available as a Printed book or as an eBook Edition for Download

    The Author is Stephen J. Gislason MD. Not all respiratory diseases are caused by airborne pathogens. If asthma, bronchitis and/or nose sinus congestion is chronic or attacks occur frequently in all seasons and are not related to airborne exposure, then consider delayed pattern food allergy as the cause and do diet revision using the Alpha Nutrition Program.

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