As well as being a leading source of greenhouse gas (GHG) emissions, the transport sector is responsible for a large proportion of urban air pollution. An estimated 3.7 million premature deaths are attributed to ambient (outdoor) air pollution, based on WHO data from 2012. Overall, higher urban air pollution concentrations increase the risk for cardiovascular and respiratory disease, cancer and adverse birth outcomes, and also are associated with higher death rates.
Air pollution-related deaths and illness are linked most closely to exposures to small particulate matter (PM) of less than 10 or 2.5 microns in diameter (PM10 and PM2.5). Small particulates bypass the body’s defences against dust, penetrating deep into the respiratory system. They also comprise a mixture of health-harming substances, such as heavy metals, sulphurs, carbon compounds, and carcinogens including benzene derivatives.
Transport is a significant and growing contributor to particulate air pollution exposures. Road transport is estimated to be responsible for up to 30% of particulate emissions (PM) in European cities and up to 50% of PM emissions in OECD countries – mostly due to diesel traffic. However, the total contribution of transport to particulate air pollution can vary widely, from 12%-70% of the total pollution mix. Low- and middle- income countries suffer disproportionately from transport-generated pollution, particularly in Asia, Africa and the Middle East. In part, this is due to the use of old and inefficient diesel vehicles and a lack of public and active transport networks.
Another transport-related air pollutant that harms health includes ground-level ozone (O3) - a mix of urban air pollutants and key factor in chronic respiratory diseases such as asthma. Some of the precursors of O3 include nitrogen oxides (NOx); carbon monoxide (CO), which poses a particular risk for vehicle operators and passengers in closed spaces (e.g. garages); and methane, which is a strong greenhouse gas. Ground-level ozone is also known to damage ecosystem structures and functions, in turn affecting agricultural productivity and threatening food security.