Car emissions types and impacts

transport emissions air quality climate change

Emissions from petrol and diesel engines include carbon dioxide, carbon monoxide, hydrocarbons such as methane, particulate matter, and nitrogen oxides, all of which have significant environmental impacts.

Road transport emissions

Vehicle emissions contribute to the increasing concentration of gases that are leading to climate change. 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 more than 20% of total emissions. Of the total greenhouse gas emissions from transport, over 80% are due to CO2 emissions from road vehicles.

Road transport also remains the main source of many local pollutants including 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.

Figures from the Royal College of Physicians say that around 40,000 deaths in the UK each year are attributable to outdoor air pollution, costing approximately £20 billion annually. Of particular danger are highly concentrated levels of particulates and NOx emissions, putting people living in urban zones at greater risk than those in rural areas. Although this looks at air pollution from all sources, road transport accounts for a significant proportion of the findings.

Sources: Royal College of Physicians of London, Accessed 2017

Vehicle emissions standards

European directives have been instrumental in reducing four key 'regulated emissions': carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HCs) and particulate matter less than 10 microns in size (PM10). First introduced in 1992 (Euro 1), 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 6 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 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 reduction in regulated pollutants; so much so, that a car manufactured today emits an order of magnitude fewer emissions than a car made the 1970s. However, there is now evidence that, Euro 1 to 4 saw the greatest reductions and that, more recently, the rate of reduction has slowed. In addition, there is now clear evidence that some key pollutants, including NOx and particulates, have not in fact measurably improved since Euro 4.

The European Parliament passed legislation capping manufacturer's average new car CO2 emissions at 95 g/km from 2020 onwards. This is a subsequent article, following on from the initial cap of a manufacturer average of 130 g/km CO2 for new cars by 2015. Model specific CO2 limits permit higher emission for heavier vehicles; however, average figures for each manufacturing group must comply with the overall target.

As part of the CO2 legislation, manufacturers exceeding targets have to pay a penalty for each car registered, which amounts to €5 for the first g/km of over the limit, €15 for the second g/km, €25 for the third, and €95 for each subsequent gram. From 2019, stricter penalties will be introduced; every exceeding gram costing €95. Conversely, ultra-low emission vehicles count as 'super-credits' which can be used to lower manufacturers' overall emissions. These see every car registered that emits 50 g/km CO2 or less count as 1.5 vehicles towards the manufacturer's average emissions cap.

The following table lists the maximum emissions permitted by each Euro standard. These figures are based on the NEDC vehicle test cycle which will change to the WLTP protocol by September 2019 - see below.

European tailpipe emissions standards for passenger cars (in g/km)

Euro Standard Implementation date* CO
Euro 1 July 1993 2.72 - - - 0.97 0.14
Euro 2 January 1997 1.00 - - - 0.70 0.08
Euro 3 January 2001 0.64 - - 0.50 0.56 0.05
Euro 4 January 2006 0.50 - - 0.25 0.30 0.025
Euro 5 September 2010 0.500 - - 0.180 0.230 0.005
Euro 6 September 2015 0.500 - - 0.080 0.170 0.005
Euro 1 July 1993 2.72 - - - 0.97 -
Euro 2 January 1997 2.20 - - - 0.50 -
Euro 3 January 2001 2.30 0.20 - 0.15 - -
Euro 4 January 2006 1.00 0.10 - 0.08 - -
Euro 5 September 2010 1.000 0.100 0.068 0.060 - 0.005**
Euro 6 September 2015 0.100 0.100 0.068 0.060 - 0.005**
* Market placement (or first registration) dates, after which all new engines placed on the market must meet the standard. EU emission standards also specify Type Approval dates (usually one year before the respective market placement dates) after which all newly type approved models must meet the standard.
** Applies only to vehicles with direct injection engines.

Sources: DieselNet, Accessed 2017, International Council on Clean Transportation

Vehicle testing

Emissions standards form part of the system of vehicle ‘Type Approval’, the regulatory mechanism for ensuring that cars and vans sold within the EU meet minimum environmental and safety standards. The process involves the testing of a representative production vehicle and component parts at an accredited facility.

From September 2017, all new models launched must be tested using the Worldwide Harmonised Light Vehicle Test Procedure (WLTP), which replaces the New European Drive Cycle (NEDC) as the mandatory test protocol. WLTP aims to counter many of the deficiencies in the NEDC tests, which failed to reflect what the vehicle produces in real-world driving.

Vehicle testing - NEDC vs WLTP

Table courtesy of ACEA via

Although still conducted in a laboratory, WLTP has a more realistic test cycle to be completed than NEDC. Coming into force in September 2017, there is a period of transition where only NEDC values should be used on customer information, to allow customers to compare different cars - especially those older cars tested only on the NEDC cycle.

In the UK, the Government confirmed in the Autumn Budget 2017 that it will use WLTP figures for official CO2 emissions from April 2020 onward. This will have knock-on effects on systems such as car tax (VED) and company car tax rates.

However, like the NEDC, the WLTP test is still far from perfect, undertaken as it is in laboratory conditions and not able to recreate all the demands put on vehicles when driven in the real-world. The discrepancies between official test figures and those discovered via real-world testing are expected to be reduced (by around half), but will still be present.

Europe has also introduced the Real Driving Emissions (RDE) test for some emissions which effectively supercedes WLTP in effectiveness, since tests are carried out on real roads using a Portable Emissions Measurement System (PEMS).

RDE testing involves driving vehicles fitted with a PEMS on a route containing three sections - urban, rural, and motorway. The sections must be completed in that order, with speeds of < 60 km/h, 60-90 km/h, and > 90 km/h aimed for respectively, where conditions allow. Each section covers at least 16 km and accounts for around a third of the complete test.

A phased implementation of RDE (as well as WLTP) will see official emissions figures effectively increase as tests begin to return more accurate results. RDE initially came into force in September 2017, with all new vehicles needing to be tested using the new protocol from September 2019.

Emissions Analytics logo Reflecting the improved accuracy of RDE, NGC has partnered with Emissions Analytics who conduct RDE monitoring and publish an EQUA Index for each measured pollutant.

Dynamometer (rolling road) and real driving vehicle emissions testing

Images show 'rolling-road' (dynamometer) and Real Driving Emissions (RDE) testing. Images courtesy of European Commission Joint Research Centre (left) and WhatCar?

Environmental impacts of climate change emissions

According to a report by the Royal College of Physicians, in the UK around 40,000 premature deaths each year are attributable to outdoor air pollution. Effects of air pollution have been linked to a number of conditions, with road traffic emissions having a significant impact on air quality.

Carbon Dioxide (CO2)
Each year an estimated 36 billion tonnes of carbon dioxide are emitted due to human activity, 1% of which originates from the United Kingdom.

While carbon dioxide is non-toxic, its main environmental effect is as a greenhouse gas which, by enhancing the greenhouse effect, contributes to increases of the Earth's atmospheric, land and sea temperatures. NASA reports that 16 of the 17 warmest years since records began 136 years ago have occurred since 2001 - with 2016 the warmest on record. With a reduction in sea ice, rising sea levels, and longer periods of extreme temperatures and weather systems, the results of climate change will be of great (and as yet unknown) significance to all patterns of life on Earth.

Nitrogen Oxides (NOx)
As a result of the high temperatures occurring during combustion, nitrogen combines with oxygen from the air forming oxides of nitrogen (NO, NO2, N2O etc.). These gases are known to have a significant detrimental effect on air quality and thus public health - impacting upon respiratory conditions. Particularly prevalent in large urban areas, around 40% of European NOx emissions come from road transport.

Particulates (PMs)
Particulates are fine particles produced by incomplete combustion, the burning of lubrication oil and by the presence of impurities within the fuel. Typically with a dimension of the order of 10 microns or less (known as 'PM10'), they are known to cause and aggravate human respiratory diseases and are thought to be carcinogenic. The World Health Organisation has issued a report stating that there are no concentrations of airborne micro-sized particulate matter that are not hazardous to human health.

Carbon Monoxide (CO)
Produced during the incomplete combustion of carbon compounds such as fossil fuels, this gas is known to be deleterious to human health. During respiration it readily combines with haemoglobin in the blood thus hindering the body's ability to take up oxygen. It is thought therefore to aggravate respiratory and heart disease.

Chris Lilly

Author:Chris Lilly
Date Updated:31st Oct 2017

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