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What are unit emissions?

This page explains what unit emissions are and what their purpose is.

In addition, the limitations of unit emissions as a tool are discussed.



Information on unit emissions

Unit emissions - definition

On LIPASTO traffic emission pages the term unit emissions refers to emissions resulting from vehicles while being used. Emissions are allocated to the transport of a transport unit over one kilometre, for example grams per passenger kilometre (g/pkm) or grams per tonne kilometre (g/tkm).

While a vehicle is being used, energy consumption equals fuel consumption that is measured in mass (g), volume (l) and energy (MJ) units. Unit emissions cover the emissions resulting from fuel combustion, measured in mass units. Electric trains are an exeption, which is further explained in Methodology for railway traffic. The compounds under examination are carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), particle matter (PM), methane (CH4), nitrous oxide (N2O), sulphur dioxide (SO2) and carbon dioxide (CO2). In addition, a value for carbon dioxide equivalent is calculated, combining the warming effects of carbon dioxide, methane and nitrous oxide.

The term transport unit refers to a unit of passengers or freight transported. On these pages the transport unit in passenger traffic is one passenger and in freight traffic it can be the entire vehicle, one container or trailer but most usually one tonne of freight. Unit emissions are calculated by dividing emissions by the product of transport units and distance (km).

Unit emissions - purpose

LIPASTO traffic emissions pages include tabled unit emissions for road, railway, waterborne and air traffic in Finland in 2007 or 2008. The results are presented separately for passenger and freight traffic. The results represent the situation in Finland, acknowledging the transport equipment used and the actual amounts of passengers and freight transported.

Unit emissions allow comparisons within or between road, railway, waterborne and air traffic. Energy consumption and emissions released can be compared between any two vehicles, but it is important to notice that full comparability can not be reached. A list of the most central reasons why not is given here: 1) There may be fundamental differences in the calculation methods and allocation between different vehicles, even though within LIPASTO unit emission pages these differences have been minimized. 2) The purpose for the transport may not allow the use of an apparently better alternative. (An example: A business traveller values time and will not choose waterborne traffic instead of air travel.) 3) The capacity of the apparently better alternative may not meet the demand. (An example: Inadequate railway capacity.) 4) Comparing unit emissions does not rank the alternatives according to the total resource consumption or total environmental impacts. Profound comparisons would require full Life cycle assessments.


When passengers and freight are transported in the same vehicle, energy consumption and emissions need to be divided between the two user groups. Allocation can be done, for example, based on the masses of the passengers and freight or based on the volume or surface area required by them. For road, railway, waterborne and air traffic there are a number of different allocation methods, but no one single method has yet been judged superios or established internationally (neither for one transport mode alone nor for all of them).

Improving unit emissions

Energy consumption and emissions can be improved, that is decreased, by many levels and by many means. Improvement can be achieved in two ways: qucikly using the existing vehicles or by long-term means. Examples on quick ways: higher load factors, optimisation of routes, fuels and speeds. Examples of long-term ways: more effective and new technologies, implementation of new policies.

Life cycle assessment

The knowledge of unit emissions alone does not tell all about the total resource consumption or envirnomental impacts resulting from a chosen vehicle. When the choise is made between existing vehicles, unit emissions are a good guide. But when the choice concerns investments on new vehicles or when the aim is to reduce environmental impacts through policy measures, more thorough indicators are needed. A suitable tool for this is life cycle assessment (LCA), which analyses the resource consumption and environmental impacts resulting from the entire life cycle, emissions from use of a vehicle being one part. The other parts inculde for example the entire production chain of fuels, production of vehicles, construction on the infrastructure, maintenance and repair and disposal of the vehicle and infrastructure.

Last updated 7.4.2009