However, while diesel cars release less CO2 than petrol-powered vehi-cles, they emit higher levels of particulate matter, volatile organic compounds (VOCs) and nitrous oxides-toxic pollutants that can cause serious harm to human health and the environment. In fact, some public health agencies consider diesel emissions to be the source of at least 70 per cent of the total toxic risk posed by air pollutants (Environ Health Perspect, 2002; 110: A458-64).
And now, due to the expanding use of diesel equipment, more and more workers-such as truckers, railroad workers and miners-are being exposed to diesel exhaust and its associated health hazards. What's more, adverse health effects are also being seen among the general population-and at emission levels well below those found in occupational settings.
Diesel exhaust is a complex mixture of thousands of different chemical substances, including carbon monoxide, sulphur oxides, nitrogen oxides, VOCs, alkenes, aromatic hydrocarbons and aldehydes.
The health effects of most of the individual substances are often poorly understood, as are the chemical reactions that occur after the exhaust has spread into the atmosphere. However, more than 40 of the chemicals found in these exhaust fumes are known to be carcinogenic and hazardous air pollutants (Environ Health Perspect, 2002; 110: A458-64). Moreover, the various gaseous and solid partic-ulates can act synergistically to cause mutations and cancer (Lancet Oncology, 2002; 3: 581).
Indeed, the US Environmental Protection Agency (EPA) labels diesel exhaust as an air-toxic and something "likely to be carcinogenic to humans". In September 2002, the Agency finally published its long-awaited assessment of the health consequences of diesel-engine emis-sions. The report-which had been redrafted five times-was the cul-mination of 10 years of study, review and public hearings.
The report's main finding was that inhaling diesel exhaust fumes from large vehicles such as trucks and buses, among other sources, can substantially increase the riskof lung cancer. Also, environmental exposure to such fumes can invoke immunological reactions and pose a chronic respiratory hazard, leading to asthma and other lung diseases (Lancet Oncology, 2002; 3: 581).
The EPA's findings mirror other studies of diesel emissions. Partic-ularly worrying was a US study by a coalition of state and local air-pollution-control agencies that calculated that an additional 125,110 people would be expected to develop lung cancer during their lifetime as a result of exposure to the toxic components in diesel smoke (Lancet Oncology, 2002; 3: 581). There is also an association between excess lung cancer and diesel, but not petrol (gasoline), exhaust (AmJ Epidemiol, 2007; 165: 53-62).
The main concern with diesel is that it contains a large percentage of particles-including ultrafine 'nanoparticles'-that are easily inhaled. It's been estimated that the particulate emissions from diesel engines per travelled distance is over 10 times higher than emissions from the equivalent-sized petrol engines running on unleaded petrol, and over 100 times higher than that from petrol engines fitted with catalytic converters (Eur Respir J, 2001; 17: 733-46).
Even worse, the small size of diesel exhaust particles allows them to penetrate deeply into the lungs, allowing them to deposit their toxic chemicals throughout the respira-tory tract.
And researchers are increasingly finding that diesel particulate matter (DPM) doesn't just affect the lungs. More and more studies show that DPM can trigger a range of effects in the body-especially in the cardiovascular and reproductive systems (Environ Health Perspect, 2002; 110: A458-64).
In one of the latest studies, diesel fumes were found to interact with the fatty acids in LDL ('bad') cholesterol, raising the risk of heart disease (Genome Biol, 2007; 8: R149; doi:10.1186/gb-2007-8-7-r149). Although this was based on mice, and so may not apply to humans, human studies also show diesel to have deleterious effects on the heart.
One demonstrated that even brief exposures to dilute diesel exhaust, at concentrations found in urban road traffic, can induce adverse cardiovascular effects in people with preexisting stable coronary heart disease as well as in healthy volunteers (N Engl J Med, 2007; 357: 1075-82).
In addition, albeit from a study in animals, it's been found that diesel exhaust particles can affect sperm production (J Reprod Dev, 2007 Sep 4; Epub ahead of print).
It's also been hypothesized that the toxic components found in diesel exhaust can cause oxidative and DNA damage, which can lead to significant reproductive and cardio-vascular effects (Mutat Res, 2007; 636: 95-133).
Other health problems linked to diesel-exhaust exposure include diabetes, neurological problems and a range of cancers in addition to lung cancer (Environ Health Perspect, 2002, 110: A458-64; Int J Cancer, 2004; 111: 286-92).
To stem the adverse health effects of diesel emissions, regulators in both the US and UK have asked for cleaner diesel engines for several decades-and the standards are becoming tighter. As a result, the total mass of particulate emissions from new or retrofitted engines has been sharply reduced.
Recent studies of low-emitting diesel engines with catalyzed par-ticulate filters have shown that emission rates for several worrying chemicals are lower than that with comparable compressed natural gas (CNG)-fuelled engines (Inhal Toxicol, 2004; 16: 889-900).
However, the fact remains that millions of the older diesel engines are still in use, and there are still questions as to precisely what other tiny pollutants the new fuels and emissions-control methods may be creating (Environ Health Perspect, 2002; 110: A458-64).
A 1998 report by Sweden's EPA warned than new diesel cars cause more harm to the environment and our health than new petrol-fuelled cars. Said Reino Abrahamsson, a spokesperson for the agency, "For the sake of the environment, it is better if car buyers choose a fuel-efficient environmentally classified petrol-driven car rather than a diesel."
Other alternative fuels
- Biodiesel. Derived from sources such as soybeans, switchgrass, corn and many other crops, biodiesel can reduce particulate emissions by more than 55 per cent compared with regular diesel (Environ Health Perspect, 2002; 110: A458-64). Moreover, it can significantly reduce greenhouse gas emissions. Even when just 20 per cent of a blended fuel is biodiesel, as it often is, particulate emissions-along with carbon monoxide, hydrocarbons and other toxic compounds-drop by about 15 per cent (Inhal Toxicol, 2007; 19: 1033-9). The downside, however, is that smog-causing nitrous-oxide emissions may slightly increase, and there is also the potential for higher biodiesel blends to damage engine components.
- Ethanol. This was one of the world's first vehicular fuels, and it's now the most widely used alternative to gasoline. The primary source is corn. How-ever, while it's typically estimated that today's average corn ethanol cuts global-warming pollution by about 20 per cent compared with petrol, some researchers estimate that it may actually increase global warming (Natural Resources Res, 2006; 15: 255-70).
- Liquefied petroleum gas (LPG). Most cars powered by LPG in the UK
are hybrid vehicles, and are more environmentally friendly and fuel-efficient than petrol engines. They produce fewer emissions of carbon dioxide, hydrocarbons, carbon monoxide and nitrous oxides than either petrol- and diesel-powered vehicles. However, being a fossil-derived fuel, it still releases greenhouse gases into the atmosphere.
- Compressed natural gas (CNG). CNG-powered vehicles are quieter, more fuel-efficient and less toxic than conventional cars. But they are produced from a non-renewable fossil-fuel energy source, and its driving range is generally less than with comparable petrol- or diesel-fueled vehicles.
- Hydrogen. Fuel-cell vehicles powered with hydrogen are currently under development by automakers around the world. The advantage is that hydrogen is widely available, and the only tailpipe emission is water vapour.
Driving towards fuel efficiency
How you drive and take care of your vehicle affects its fuel economy and emission levels, says the Union of Concerned Scientists, a not-for-profit research group (www.ucsusa.org). They offer the following tips for reducing the environmental impact of your car today:
- Drive less. Driving as little as possible is the best way to reduce the environmental impact of your transportation needs. Link errands, carpool, use mass transit, or bike or walk whenever possible. Choose a place to live that reduces your need to drive. If your family already has one car, consider trying to meet your travel needs without buying a second one.
- Drive moderately. High-speed driving and jack-rabbit starts increase both fuel use and emissions.
- Keep your vehicle well tuned. Simple maintenance such as regular oil changes, air-filter changes and spark-plug replacements will lengthen the life of your car as well as improve fuel economy and minimize emissions.
- Check your tires regularly. Keeping your tires properly inflated saves fuel by reducing the amount of drag your engine must work to overcome.
- Keep track of your fuel economy. A drop in your car's fuel usage can be a sign of engine trouble. Keeping track of your fuel economy on a regular basis lets you know when something is not working properly. Check your gas mileage by recording the odometer reading and number of gallons you purchased when you fill up. Divide the number of miles travelled between fill-ups by the number of gallons purchased.
- Don't let your car idle for more than a minute. During start-up, your engine burns extra gasoline. However, letting your engine idle for more than a minute burns more fuel than turning off the engine and restarting it.
- Park in the shade. Minimize evaporation of fuel and keep your car cooler in the summer by parking in the shade.