Reduction of adverse health and environmental effects related to air quality has been the primary driving force for tightening the exhaust emission limits of transport sector for decades. While struggling with the emission regulations and challenging targets for engine efficiency, ever even cleaner cars and engines equipped with dedicated emission control technologies have been introduced.  At the same time, concerns on energy security and global warming are driving towards new liquid and gaseous fuel alternatives. All new technologies should meet the “no harm to health and environment” principle, however, verification of this is challenging for a variety of technologies with potential presence of new, unknown species in the exhaust gases.

Air pollution is world’s largest single environmental health risk. WHO (2014) estimated that approximately 7 million premature deaths are attributable to air pollution exposure. Air pollution also increases respiratory and cardiovascular diseases and cancer. In addition to health effects, air pollution deteriorates vegetation, water and soil. Air pollution causes substantial adverse economic impacts (EEA 2015).

Table 1. Classification of emissions based on their adverse effects on health, environment and global warming.

^a Light olefins, aromatics, aldehydes

Legislative limits are rare for the emissions species other than carbon monoxide (CO), total hydrocarbons (HC), nitrogen oxides (NO_x) and particulate matter (PM) for the transport applications. However, a number of unregulated exhaust species are harmful to human health and to the environment, and some of them are also strong greenhouse gases. Moreover, transformation of primary (tailpipe) emissions into secondary products is an important aspect when transport emissions are assessed.

There are several lists of “priority air toxics” that define the most harmful compounds to be taken into account when evaluating exhaust gases from the transport sector. These lists have been defined from various starting points, and consequently, outcomes are not uniform. Diesel engine exhaust itself has been classified as carcinogenic  to humans, Group 1, while gasoline engine exhaust is classified as possibly carcinogenic to humans, Group 2B (IARC, 2013).

The U.S. Environmental Protection Agency (EPA) has defined key mobile-source air toxics (MSATs). The US EPA (2001) MSAT list included 21 compounds, among them acetaldehyde, acrolein, benzene, 1,3‑butadiene, dioxin/furans, diesel exhaust, ethylbenzene, formaldehyde, n-hexane, six metals, MTBE, naphthalene, styrene, toluene and xylene. The US EPA (2007) includes eight key MSATs and gasoline particulate matter. The emission species in this MSAT group are listed below (US EPA 2007).

• Benzene, formaldehyde and 1,3-butadiene are classified as human carcinogens (IARC, 2010, 2012). The lifetime of 1,3-butadiene is short, but it is highly reactive and may also form formaldehyde, acetaldehyde, and acrolein in atmospheric reactions.
• Acetaldehyde has been classified as possible carcinogen by IARC (1999). It may also produce peroxyacetyl nitrate (PAN), a phytotoxicant and mutagen, through reactions with NO_x.
• Acrolein is highly irritating, and long-term inhalation results in chronic inflammation.
• Polycyclic organic matter is found in the gaseous exhaust, particles, or both. It contains a mixture of compounds, for example benzo(a)pyrene, which is classified as a human carcinogen. Many other polyaromatic hydrocarbons (PAHs) and nitro-PAHs have been classified as proven, probable or possible carcinogens to humans.
• Naphthalene is the most abundant PAH in ambient air. There is evidence in rodents that exposure to naphthalene leads to inflammation of the nasal tract and tumors of the nasal epithelium, for example. However, there are no data on carcinogenicity in humans. Case reports suggest that exposure may cause effects in blood cells, such as haemolysis and haemolytic anaemia. (HEI 2007).
• Diesel exhaust has been classified as carcinogenic to humans (Group 1, IARC),
• Gasoline PM: Gasoline engine exhaust is classified as possibly carcinogenic to humans (Group 2B, IARC).

Review of HEI (2007) concluded that the contribution of mobile sources is greatest for 1,3‑butadiene, followed by benzene, formaldehyde, acetaldehyde and acrolein.

In addition to the defined MSAT species, number of particles (PN), especially number of nanoparticles in the lowest size classes below 50 nm, is addressed with potential adverse health impacts. In addition, reduction of mass-based PM emissions does not necessarily reduce PN emissions. (HEI 2002, Kittelson et al. 2002, IARC 2016).

Ozone is harmful to both human health and the environment. It causes irritation of the respiratory system, reduction of lung function and induced asthma, and there is also evidence of induced cardiovascular related morbidity. Ozone contributes to the damage of plants and ecosystems, which may lead to shifts and losses of species (US EPA 2007). Many volatile organic compounds (VOCs) contribute to the formation of ground-level ozone together with nitrogen oxides (NO_x) in the presence of heat and sunlight through complex atmospheric chemistry. (Gaffney and Marley 2011, Carter 2001).

NO_x, sulphur dioxide (SO₂) and ammonia (NH₃) contribute to the acidification causing the loss of animal and plant life. In addition, NH₃ and NO_x bring nutrients in land and water disrupting these ecosystems and leading to eutrophication and changes in species. NH₃ is associated with harmful effects on health and vegetation, and can form ammonium aerosols that affect climate and visibility. SO_x, NO_x and CO₂ can damage materials and buildings by corrosion, biodegradation and soiling caused by particles and by acidifying compounds. (EEA 2015). NH₃ is a catalyst induced emission, both from the three-way catalysts (TWC) and urea-based selective catalytic reduction (SCR) systems for NO_x control for diesel vehicles, though it mainly originates from agricultural sources. (Meija-Centeno 2007, EEA 2012b). 

Besides CO₂, methane (CH₄), black carbon (BC, a constituent of PM), nitrous oxide (N₂O) and ozone precursors increase global warming. (EEA 2015). In the opposite, the PM associated organic carbon, ammonium (NH₄⁺), sulphate (SO₄^2–) and nitrate (NO₃^–) have a cooling effect on climate (Myhre et al. 2013 in IPCC AR5). Methane is emitted from engines and vehicles, particularly when fuelled with natural gas or biomethane. N₂O is induced by catalyst chemistry of (TWC) of the spark-ignited gasoline cars. (Meija-Centeno 2007).

Overall cancer potency of the exhaust gases can be calculated using risk factors (OEHHA 2009, US EPA IRIS, the Nordic Ecolabelling 2008). There are also methodologies to evaluate ozone forming potential, acidification potential, photochemical oxidation creation potential, particulate matter formation potential and marine eutrophication potential of exhaust gases (Querini et al. 2011). Individual VOC species contribute differently to formation of ozone and oxidants, which is evaluated by using a maximum incremental reactivity (MIR) scale to assess the ozone-forming potential (OFP) of any emitted molecule (Carter and Atkinson 1987). These methods do not take into account possible presence of “super-toxics”, which can be harmful at concentrations below detection limits of the analysis methods (for example some nitro-PAHs), or previously unknown exhaust species that are not characterized at all. Thus comprehensive methods and tests are desired. Screening of biological activity of exhaust gases can be conducted by using different tests, for example Ames test for mutagenicity, which is a term related to genotoxicity and carcinogenesis. However, the bacterial test does not reflect in vivo mutagenic activity, nor it is capable to accurately predict the risk of carcinogenicity in mammals. Reactive oxygen and nitrogen species are thought to be related to oxidative stress associated with inflammation and tissue damage in the cells and lungs. Many test methods are available for monitoring oxidative stress or oxidative potential of exhaust gases. IEA-AMF Task 42 studied toxicity of exhaust gases (Czerwinski 2014 link).

Exhaust emissions from new cars and heavy-duty vehicles have been reduced drastically with tightening emission legislations. For example, HEI (2015) reported that the 2007 compliant diesel engines equipped with exhaust gas recirculation (EGR), diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) reduced levels of PM by 90% and those of VOCs and semivolatile organic compounds (SVOCs) by over 90% compared with emissions from old engines. Emissions for new-technology engines were not carcinogenic in in-vivo studies (rats), though a few effects in rat lungs were observed resembling changes seen in earlier studies after long-term exposures to gaseous oxidant pollutants, in particular nitrogen dioxide (NO₂). Low emission level of new cars and heavy-duty vehicles is compensated to some extent by their increasing numbers on road, and by  emission species attached with the emission control technologies, e.g. nitrous oxide (N₂O) and ammonia (NH₃), or with alternative fuels e.g. methane and aldehydes. (Meija-Centeno 2007, IEA-AMF Task 35-2: Rosenblatt et al. 2014, IEA-AMF Task 22: Aakko and Nylund 2003). Many emission species are significant even at low concentrations. In addition, engine-out emissions from internal combustion engines are still high, which is immediately reflected in the tailpipe emissions when emission control technologies are not working properly. All in all, exhaust emissions from on-road transport are still a relevant, particularly when new technologies and alternative fuels are introduced.