Environmental impacts

Road transport emissionsReturn to top

Vehicle emissions contribute to the increasing concentration of gases that are leading to climate change. In order of significance, the principal greenhouse gases associated with road transport are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Road transport is the third largest source of UK greenhouse gases and accounts for over 20% of total emissions. Of the total greenhouse gas emissions from transport, over 85% are due to CO2 emissions from road vehicles. The transport sector is the fastest growing source of greenhouse gases.

Road transport also remains the main source of many local emissions including benzene, 1,3-butadiene, carbon monoxide (CO), nitrogen oxides (NOx) and particulates (PMs). Within urban areas, the percentage of contributions due to road transport is particularly high - in London road transport contributes almost 80% of particulate emissions. There is a growing body of evidence to link vehicle pollutants to human ill health including the incidence of respiratory and cardio-pulmonary disease and lung cancer. In 1998 the Committee on the Medical Effects of Air Pollutants estimated that up to 24,000 people die prematurely each year in the UK as a direct result of air pollution. Similar findings are emerging from international research. According to the World Health Organisation, up to 13,000 deaths per year among children (aged 0-4 years) across Europe are directly attributable to outdoor pollution. The organisation estimates that if pollution levels were returned to within EU limits, more than 5,000 or these lives could be saved each year.

References: Holgate, S. Quantification of the Effects of Air Pollution on Health in the United Kingdom. The Stationery Office, London, 1998; WHO (2004) One in three child deaths in Europe due to environment. World Health Organisation, June 2004.

European vehicle emissions standards Return to top

European directives have been instrumental in reducing what are known as the regulated emissions. These include carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HCs) and particulate matter less than 10 microns in size (PM10). First introduced in 1992, these form a set of rolling regulations designed to become more stringent year on year. Currently limits for new cars and light-duty vans must conform to 'Euro IV' standards.

The effect of tighter Euro standards on vehicle emissions has been to accelerate the introduction of greener vehicle technologies. For petrol cars, this has been achieved in part through the use of the three-way catalytic converter and the move to fuel injection systems. For diesels, NOx and particulate emissions have been reduced through the development of direct injection engines and diesel particulate filters (DPFs). These technological advances, together with the cleaner fuels that made these developments possible, have led to a dramatic reduction in regulated pollutants; so much so, that a car manufactured today produces twenty times fewer emissions than a car made in 1970. Car manufacturers are well aware that future cars will have to conform to yet tighter regulations – indeed, tighter standards will come into force in 2012 (Euro V).

In contrast to the legislation for regulated pollutants, there is no current EU law which limits the amount of carbon dioxide produced by cars. However, the European Commission reached a voluntary agreement with the ACEA to reduce the average CO2 emissions to 140g/km by 2008 for all new cars sold in the EU with a possible second target of 120g/km by 2012. However, although this agreement is succeeding to reduce carbon emission from new car by around 1% per year, it now seems unlikely that the targets will be met (these would require reduction of around 5% per year).

Green cars and the environment Return to top

All of the green fuel and car options considered by this Guide provide a reduction in life cycle emissions of greenhouse gases as compared to conventional petrol cars.

Battery-electric cars using renewable energy provide the largest reduction in GHGs of almost 100%. Even if 'standard-mix' electricity is used, battery electrics reduce greenhouse gases by around 40%. Commercially produced pure bioethanol and biodiesel also provide significant reductions in greenhouse gases in the range 35%-90%, the reduction depending on feedstock crop used. Due to their high fuel-efficiency, most petrol hybrids offer a reduction in greenhouse gases of around 25%. Bi-fuel LPG and natural gas cars also produce modest reductions of around 15% due to better combustion and to their lower carbon content.

Conventional petrol and diesel cars can also offer significant emission reductions. Within each vehicle class, diesel cars typically emit around 20% fewer greenhouse gases than petrol cars. However, the many new petrol engines achieve greenhouse gas emission similar to diesel through the use of Gasoline Direct Injection (GDI) or Fuel Stratified Injection (FSI) technology. That said, for all conventional fuels, increasing fuel-efficiency (by definition) reduces greenhouse gas emissions - a 10% improvement in 'mpg' will reduce GHGs by 10%.

Some (though not all) green fuel and car options also offer reductions in regulated emissions. However, in comparison to greenhouse gas emissions, the comparison is a little more complex.

Battery-electric cars using renewable energy provide the largest reduction in regulated emissions of almost 100%. However, if 'standard-mix' electricity is used, battery-EVs lead increase life cycle NOx and PMs (power station emissions). Petrol hybrids provide a significant reduction in regulated emissions (up to 90%) as compared to a petrol car. For bi-fuel LPG and natural gas cars, PMs are also virtually eliminated and NOx emissions are typically reduced by up to 15%. For commercially produced pure bioethanol/biodiesel, CO, HCs and PMs are reduced (compared to petrol/diesel), although NOx is often slightly increased (due to higher combustion temperature and/or engine compression ratio).

In contrast to the link between 'mpg' and GHG emissions, petrol and diesel cars with better fuel economy do not necessarily have lower regulated emissions. That said, diesels typically produce more NOx and PMs than petrol cars (due to higher compression ratio/efficiency of compression engines), but are lower in CO and HCs. However, the many new diesel cars reduce PMs to very low levels through the use Diesel Particulate Filters (DPFs), which are now becoming standard on many new models.