Air pollution is known to have detrimental effects on human health, contributing to an estimated 3.4 million premature mortalities globally in 2010. By looking at both different types of air pollution (PM2.5, ozone, etc.) and pollutant sources (road transportation, agriculture, household energy, shipping, etc.) it is possible to produce a global picture of air pollution distribution and how to most effectively reduce the impact on human health. This paper will look specifically at the road transportation source sector, discussing the current predictions of health effects and how these can be reduced in the future.
On-road transportation, which includes diesel and gasoline vehicles, accounted for approximately 240,000 of the global premature deaths associated with PM2.5 and ozone in 2015. The health effects associated with road transportation, in particular diesel vehicles, was brought to public attention after it was discovered that approximately 11 million Volkswagen light-duty vehicles (LDVs) had been fitted with a defeat device between the years 2009 to 2015; this device was used to detect when a vehicle was undergoing emissions testing and control the emission, making it appear as though the vehicles met the emission certification limits.
These excess emissions pose a significant impact on human health since diesel vehicles are estimated to generate 20% of global nitrogen oxide (NOx) emissions, a direct cause of increasing PM2.5 and ozone levels. To help reduce the adverse effects to human health associated with road transportation (studies have found vehicle emissions to be associated with an increased risk of health conditions such as asthma, lung cancer and cardiovascular disease) many major vehicle markets have implemented emission standards required for all new heavy-duty vehicles (HDVs) and LDVs. Whilst these policies have been crucial to significantly reduce exhaust emissions, as discussed above, there are still differences in the real world and certified emission limits largely due to excess diesel NOx. When evaluating road transportation associated health effects all main fuel types must be considered, these are gasoline, diesel, liquefied petroleum gas and compressed natural gas. The distribution of fuel types varies by region and vehicle type, HDVs tend to run on diesel whilst for LDVs approximately 40% of vehicles in Europe and nearly all of the LDVs in the USA use gasoline. In order to consider many types of fuel used for on-road transportation, a transportation attributable fraction (TAF) is often calculated. By definition, the TAF is the proportion of ambient particle matter attributable to surface transportation modelled on a global scale. The TAF can be used in association with different types of air pollution, this paper looks specifically at methods discussing PM2.5 and ozone related TAFs. To look specifically at the health effects related to road transportation the TAF can be separated into four subsectors. In particular, this paper will focus on the results from the on-road diesel vehicles and on-road non-diesel vehicles subsectors.
Considering the baseline scenario, it is estimated from the emission factors that 4.6 million tonnes of excess NOx were produced by diesel on-road vehicles in 2015. The total excess NOx emissions account for 31% (±3%) and 56% (±9%) of all HDV and LDV emissions respectively despite certification limits being in place to reduce on-road transportation air pollutants. It was found that HDV emissions across all regions are estimated to be 45% higher than emission limits, with real-world emissions from buses being the furthest from current certification limits (the worst-performing buses are in China measuring between 4-4.5 times the Euro IV and Euro V limit values).
Of the LDV emission factors, it is estimated that for the Euro 4 and Euro 6 certification standards real-world NOx emissions are 3.2 times and 5.7 times the respective emission limits; in the US LDVs, including vehicles fitted with a Volkswagen defeat device, produce approximately 5 times the emission limits for Tier 2. Increased concentrations of PM2.5 and ozone, correlated to excess NOx from on-road diesel vehicles, in 2015 lead to approximately 38,000 premature mortalities and 625,000 years of life lost across the globe.
Since PM2.5 has a higher concentration-response relationship than ozone, more than 80% of the total excess NOx related health effects occurred due to increased PM2.5 concentrations. The Euro 6VI scenario effectively reduces diesel NOx emissions by up to 90% compared to the baseline emission rates for 2040 (see figure 1), achieved by putting the Euro VI standards in place in regions where they had not already been implemented. Whilst the emissions for LDVs still increase between 2015 and 2040 after the implementation of the Euro 6 standards, shown in figure 1, a significantly higher increase in NOx emissions has been avoided.
The benefits of these reductions of NOx emissions include an approximate decrease of 104,000 premature PM2.5 and ozone related deaths in 2040, with more than 80% of this decrease in human health effects being in China. Overall, the scenario reduces PM2.5 levels across the globe and Australia, Brazil, China, Mexico and Russia (the regions where more stringent standards are enforced) see a reduction in ozone levels. To reduce the emission factors from LDVs the strong RDE scenario expands the RDE programme implemented by the EU-28; the strengthened RDE standard reduces emission factors from 4 times the Euro 6 emission limits to just 1.2 times these same limits.
The biggest benefit on health effects from this NOx emission reduction is seen in India, which has the second highest number of passenger diesel vehicles (after the EU-28). Globally the PM2.5 and ozone, caused by NOx, associated premature deaths are reduced by an estimated 31,000 from the execution of this scenario. Diesel NOx emissions would nearly be eliminated by progression towards next-generation emission standards across all regions. For 2040, the near elimination of diesel NOx emissions leads to the avoidance of 38,000 global PM2.5 and ozone related premature mortalities.
The next-generation standards can be compared to the baseline for 2040 to gain the full impact of using more stringent vehicle emission limits. Between the baseline and the next-generation scenarios, a reduction of 2% of global PM2.5 associated mortalities is seen and this percentage is higher (7%) for ozone related premature mortalities. This is a significant reduction to global premature mortality as it is estimated diesel vehicles account for 55% of all road transportation and 2010 land traffic (a combination of on-road and off-road transportation) is responsible for approximately 5% of the total global PM2.5 concentration related premature deaths. This comparison shows the importance of reducing road transportation emissions to ease the health burden of air pollution. The effect of reducing vehicle emission on air pollution is further backed by localised studies in China during the coronavirus (COVID-19) lockdown implemented in January 2020. With the introduction of a travel ban, it was found that NO2, PM2.5 and carbon monoxide (CO) reduced in 44 Chinese cities by 24.67%, 5.93% and 4.58% respectively.
Air pollution from road transportation originates from many different fuel types, in order to compute the impact of different road vehicle types a TAF can be calculated for specific air pollution sources. The global distribution of PM2.5 concentration related TAF for the year 2005 can be seen in figure 2. From this, we know that surface transportation contributes most heavily to overall PM2.5 concentration levels in high-income North America, Europe and high-income areas of Asia, whilst surface transportation contributes the least to concentration levels in sub-Saharan Africa, with the exception of southern sub-Saharan Africa. High-income areas of the globe are likely to have larger TAF values due to the population having more disposable income and a larger share of the population being able to afford on-road vehicles. For 2005, the global average TAF is 8.5%, equivalent to a population-weighted PM2.5 concentration of 1.75μgm−3 related to surface transportation. The health effects are unable to be directly drawn from the TAF since it does not indicate the volume of emissions, for this reason, areas with lower TAF values, such as Southeast Asia (TAF of 7.2%), may still have equal or higher health impacts than for example Western Europe (TAF of 23.5%). When compared to total global PM2.5 concentration distributions, South Asia, East Asia and North Africa and the Middle East all have higher than average overall PM2.5 levels but lower than average TAFs, this suggests that emission contributions toward health impacts are effected largely by other source sectors.
The above method of using TAF encompasses all surface transportation types, these typically include but are not limited to road, rail and off-road agricultural transportation; surface transportation types are powered by engines which use diesel and gasoline, amongst other fuel sources. Whilst the TAF is useful for gaining a wider understanding of the distribution of PM2.5 associated with road transportation, the health effects of on-road vehicle related air pollution has not been evaluated. In order to look more specifically at the human health effects from on-road transportation, the TAF can be split into subsectors of surface transportation.
The global health effects from tailpipe emissions for 2010 and 2015 are calculated from the TAFs for PM2.5 and ozone for those years. In 2010 it is estimated that 312,000 and 49,000 premature mortalities (these are within the extremities of confidence intervals of estimates from other studies) were attributable to PM2.5 and ozone concentrations respectively due to surface transportation; the total excess mortalities increase for 2015 to approximately 385,000, representing 330,000 PM2.5 and 55,000 ozone premature deaths.
The global TAFs associated with the PM2.5 mortalities for 2010 and 2015 were 11.9% and 11.6% respectively, these fractions agree with the TAF for 2005 (different chemical transport models have been used and surface transportation types vary between methods accounting for slight variation). East and South Asia were the two regions with the highest number of combined PM2.5 and ozone premature deaths attributable to transportation in 2015, with East Asia and South Asia excess deaths approximating 120,000 and 80,000 respectively, despite this the regions with the highest TAFs for all road vehicles were Europe and North America.
Of the global premature deaths associated with transportation in 2015, the on-road diesel vehicle subsector accounts for 47% of the excess mortalities, this totals to be approximately 180,000 deaths. The diesel excess mortality estimate using TAF is higher than the estimate when considering NOx emissions, this is in part due to the TAF estimate including both PM2.5 and ozone concentrations and also due to the NOx estimate only considering one emission source associated with diesel vehicles. It is estimated that 17% of the global premature deaths were from the on-road non-diesel vehicle subsector, showing how important it is to consider all vehicle fuel types when calculating health burdens. Diesel vehicles are found to be the leading contributor to transportation attributable to excess deaths and in France, Germany, India and Italy the diesel vehicle subsector is estimated to be related to two thirds of transportation mortalities. The tailpipe emissions associated with PM2.5 and ozone concentrations in 2015 can be seen in figure 3 for each trade bloc. Trade blocs are groups of countries that have formal trade agreements, they are significant to road transportation emissions since each one has the opportunity to coordinate action to standardise vehicle emissions standards. As seen by the reduced NOx emissions earlier, through using cohesive emission certifications, global human health impacts due to road transportation are undoubtedly reduced. Figure 3 shows that road transportation contributes hugely to overall surface transportation emissions, with ozone emissions nearly all directly related to road transportation.
It is known that populations living in urban areas tend to have a higher exposure to road transportation emissions, consequently, the TAFs and associated air pollution mortalities can be calculated for 100 major urban areas across the globe for the year 2015. This analysis reveals that the areas with the highest excess mortalities related to transportation air pollution have a combination of large populations and high transportation emissions, most of these urban areas are major cities in Asia with exceptions of Mexico City, Cairo, Moscow, London and Los Angeles. The highest attributable deaths per 100,000 people are mainly found in European urban areas, the same is also true for the urban areas with the largest TAFs. Some urban areas are found to contribute a large share of a country or regions premature mortalities due to transportation, for example, Tokyo, Japan has a 39.7% share of its regions excess deaths. Although the TAF is likely to be an overestimation for road transportation, due to the inclusion of other surface transportation types, health effects related to transportation is expected to have been underestimated. The TAF has been evaluated using tailpipe emission estimates ignoring other sources of air pollution and health impacts, such as road dust, brake and tyre wear releasing fine particles, evaporative emissions and noise.
Contributions towards air pollution from gasoline vehicle emissions are mainly CO and nonmethane volatile organic compounds (NMVOCs) and account for approximately 20% of global sources of these pollutants. Gasoline emissions are highest in China, India and the USA, with global emissions of CO and NMVOCs leading to an average PM2.5 concentration increase of 6.0μgm−3. Average global increases of PM2.5 concentrations due to diesel vehicle emissions (3.0μgm−3) are half that of gasoline, with regions most effected found over China, India and the Middle East. Diesel vehicles are related to large percentages of global black carbon (BC) and NOx emissions. The gasoline and diesel transportation subsectors are responsible for contributions to the global average increase of surface ozone levels of up to 8.5 and 6.7 ppbv respectively. Global premature deaths due to PM2.5 and ozone are calculated to exceed four million and the global total years of life lost is close to 80 million years. It is estimated that diesel emissions contributed to 89,100 excess mortalities and 1.66 million YLL in 2015 related to PM2.5 and ozone. Of the global premature deaths, 46,900 (25,400, 67,700) mortalities were associated with diesel emission induced PM2.5 agreeing with the baseline scenario for diesel vehicle NOx emissions.
Gasoline emissions are responsible for approximately 86,400 premature mortalities associated with PM2.5 and ozone concentrations, as well as 1.56 million years of life lost in 2015. Combined gasoline and diesel excess deaths total 237,000, this agrees with the TAF mortality estimate for the on-road diesel vehicle and on-road non-diesel vehicle subsectors. Compared globally the excess mortalities and YLL for gasoline and diesel emission are very similar. However, the health effects from diesel and gasoline vehicles can be normalized over the global total distance travelled by each vehicle type. The estimated total distance travelled for gasoline vehicles (1.55×1013 km) is 2.6 times higher than the distance travelled by the diesel sector in 2015. The global mean excess mortality rate for diesel vehicles is 15.2×10−9 deaths km−1. The gasoline global mean premature death rate is 2.7 times lower than the diesel vehicle rate. The highest premature mortality rates for gasoline and diesel are found in India, which are caused by a combination of large population density and high numbers of short distance vehicle journeys. Although diesel and gasoline emissions are found to contribute to comparable PM2.5 and ozone excess deaths, the calculation of the premature death rate suggests that diesel vehicle emissions are more harmful to human health.
The road transportation sector is attributable to 5% of all premature deaths related to PM2.5 and ozone concentrations. The highest levels of transportation attributable to air pollution are found in regions with high-income, with the biggest health effect occurring in regions with dense populations. As the human population continues to rise and urbanisation increases the health impacts of road transportation will become more prominent, even with current standards vehicle emissions will rise by 2040. Currently analysis of road transportation attributable health effects estimated annual premature deaths to be 240,000. On-road diesel and non-diesel vehicle emissions were also linked to long term health conditions such as asthma, cardiovascular disease and lung cancer. Whilst other sectors have a larger percentage of contributions to air pollution excess mortalities, significant changes can be made to reduce the health burden from road transportation. By reducing the emissions produced by on-road transportation, air pollution lowers and subsequently decreases premature mortalities associated with transportation. In order to reduce emissions, stricter certification limits are needed by major vehicle markets and trade blocs. The implementation of the most stringent policies for diesel on-road vehicles alone avoids 174,000 premature mortalities in 2040, compared to continued use of current emissions limits, and nearly eliminates diesel NOx emissions.