Diesel Exhaust

Diesel exhaust was classified as a class 2a, probable human carcinogen by the International Agency for Research on Cancer (IARC) in 1989. This classification changed to class 1, definitely carcinogenic to humans in 2012 (IARC 2018) based primarily on a series of studies conducted on 12,315 occupationally exposed hardrock miners. The risk was greatest in surface workers with a standard mortality ratio (SMR) of 1.33 (1.06–1.66, 95% C.I.) compared with underground workers 1.21 (1.01–1.45, 95% C.I.), despite the underground workers having an average respirable elemental carbon (EC) exhaust exposure level that was over 75 times higher than the surface workers. This may be attributed to background exposures unrelated to DE in the study population or the effect of DE ageing and being exposed to sunlight, ozone and other environmental factors which can cause DE components to become more toxic (Attfield et al. 2012; Silverman et al. 2012).

Diesel exhaust can be separated into two main components: the gaseous phase and the particulate matter (PM) phase. Gaseous components include carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and sulphur dioxide (SO2) as well as additional gas phase chemical species such as polycyclic aromatic hydrocarbons (PAH) and volatile organic compounds (VOC). The PM is composed of mostly solid EC particles with potentially toxic chemicals such PAH, VOC, aldehydes, ketones and heavy metals ad/absorbed to the particles (Carrara and Niessner 2011; Fontaras et al. 2009; Hu et al. 2013; Prokopowicz et al. 2015; Riley et al. 2018). Diesel exhaust can contain hundreds of different chemical species and concentrations can change significantly depending on engine type, speed, load, whether accelerating or decelerating, starting temperature and the usage of exhaust after-treatment devices (Bünger et al. 2000; Fontaras et al. 2009; Hemmingsen et al. 2011; Hesterberg et al. 2011; Karavalakis et al. 2009; Khalek et al. 2011; Kisin et al. 2013).

Of most concern are the ultrafine particles found within DE. These particles, at less than 100 nm in size, comprise the majority of DE PM with particles smaller than 30 nm comprising over 90% of the total number of particles but only accounting for 10% of the total PM mass (Kittelson et al. 2002; Ris 2007). Ultrafine particles are capable of penetrating deeper into the lungs than larger sized particles, dispersing over a greater percentage of lung volume and thus causing a greater respiratory irritant effect (Oberdörster et al. 1995; Seaton et al. 1995). Smaller particles have a greater surface area-to-volume ratio, meaning that a greater amount of potentially toxic substances can adhere to the surface for a given mass of PM (Mullins et al. 2016; Yoza et al. 2002) and thus a greater amount of toxic chemicals are deposited in the lungs. Exposure to ultrafine particles is associated with pulmonary inflammation (Oberdörster et al. 1995) and exacerbation of existing lung diseases including asthma (Evans et al. 2014; Seaton et al. 1995). Ultrafine particles are also capable of penetrating into the cardiovascular system and cause a range of adverse health effects including increased blood pressure and heart failure (Brook et al. 2010).

Alone, each individual component of the exhaust can cause its own unique health effects and combined they can interact to cause more complicated health impacts such as cancer as well as impacting the cardiovascular, respiratory and neurological systems (Benbrahim-Tallaa et al. 2012; Cosselman et al. 2012; Heidari Nejad et al. 2015; Larcombe et al. 2014; Levesque et al. 2011a; Mills et al. 2007; Zhu et al. 2012). This makes studying the effects of whole exhaust preferable to those of isolated components, such as PM alone, where the effects of the gas components and their interaction with PM is lost (Abe et al. 2000; Larcombe et al. 2015).

Changes in Engine Technology, Exhaust After-Treatment Devices and Emission Limits

Using diesel particulate filters (DPF), diesel oxidation catalysts (DOC) and other exhaust after-treatment devices, the components of DE change dramatically. A DPF is capable of removing approximately 90% of PM by mass. Elemental carbon is preferentially removed and ratios of EC to organic carbon reduce from ~3 to 0.5 (Khalek et al. 2011). In exhaust without a DPF, EC makes up approximately 75% of PM by weight (US EPA 2002), which reduces to approximately 13% after the use of a DPF. In the ultrafine particle range, larger sized particles closer to 100 nm in size are removed from the exhaust more successfully than smaller sizes (Khalek et al. 2011).

The EURO, US EPA and the US TIER classification systems have been developed as emission standards for light-heavy vehicles on road, heavy duty vehicles on road and off road engine emissions, respectively. Most engines classified as EURO IV, US EPA 2007 or US TIER 4 and above require exhaust after-treatment devices for compliance and engines classified as EURO IV and above generally require the latest high-pressure common-rail electronic fuel injection systems (Dallmann and Menon 2016). The aim of this review is to consolidate recent DE exposure and health effects research findings relevant to setting appropriate DE exposure limits, with a specific focus on newer engine and after-treatment technologies.

Methods

PubMed was searched using “Diesel Exhaust” combined with the individual search term “Exposure Health Effect”, limiting the search to results published after 2005 and finding over 600 studies that matched the search criteria. In addition, the databases Embase and Cinahl Plus were searched using the term “Diesel Exhaust Exposure Health Effect”, limiting the search to results not included as part of the PubMed/Medline database and studies published after 2005. Over 200 studies matched the search criteria. Only articles from the search which matched the review criteria, as well as relevant cited references therein, were reviewed. Studies were excluded if they were not in English, if the results were based on data obtained before 2005, if the diesel fuel used was not classified as ultra-low-sulphur diesel (<15 ppm sulphur), or if it exceeded 10% biodiesel concentration, if whole exhaust was not used and finally if the concentration of the exhaust used or the health outcomes measured were not relevant to occupational exposure settings.

The cut-off date of 2005 was selected based on diesel fuel legislation to limit sulphur levels in commercial diesel fuel. The legislation was introduced in multiple countries in the mid-2000s with several years taken to complete the change over (Kavanagh 2014). If studies did not specify the amount of sulphur within the diesel fuel used, assumptions were made based on the country the study was performed in and the date that the legislation for ultra-low-sulphur diesel was introduced. If the date of publication fell outside of that range, the study was excluded (Fig. 1).

Fig. 1
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An overview of the methodology used to select appropriate articles for review

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Relevant studies were separated into occupational exposure studies, acute human exposure studies, in vivo exposure studies and in vitro exposure studies. Acute human exposure, in vivo and in vitro studies were further separated into the use of new or older technology engines. Studies that used exhaust from an engine either classified as EURO IV, US EPA 2007 or TIER 4 and above, or as being paired with a DPF and DOC, were classified as using new technology engines. Studies that did not specify engine type, used exhaust from an engine without both after-treatment devices or used an engine at a lower EURO or TIER classification were defined as using older technology.

Occupational Exposure Studies

A common method of completing occupational exposure studies is to focus on population data in order to collect potential health consequences of DE exposure. As a consequence, the majority of DE exposure data are obtained before 2000, for example (Attfield et al. 2012; Kachuri et al. 2016; Olsson et al. 2011; Silverman et al. 2012; Vermeulen et al. 2014), when sulphur levels in fuel were high (>500 ppm and in some cases >5000 ppm) (Kavanagh 2014) and diesel engines were not equipped with exhaust after-treatment devices. In order to measure the consequences of a lifetime of occupational exposure to a substance, a lifetime has to have passed, making such studies difficult when the substance being measured (i.e. new technology diesel engine exhaust) is still newly introduced into the workplace. Thus, few studies have looked at new technology exhaust and fewer still have looked at the health consequences of exposure. Since the occupational studies reviewed did not specify the type of engine technology used, all are classified as old technology studies.

That said, studies focussing on populations that have been more recently exposed to DE at occupational concentrations have been published (Table 1), including a series of studies on a cohort of diesel engine testers in China (Bassig et al. 2017; Dai et al. 2018; Niu et al. 2018; Wang et al. 2017). These studies found that the greater the concentrations of DE exposure and the longer the time period the workers had been exposed for, the more the immune response was dysregulated (Dai et al. 2018; Wang et al. 2017), with the highest exposure levels (greater than 397 μg/m3) resulting in significantly lower inflammatory cytokine levels in blood serum (Dai et al. 2018). This was theorized to be a mechanism for increased lung cancer risk as the immune system has an important role in eliminating cancerous cells. Exposure to ~282 μg/m3 resulted in decreased lung function, compared to control subjects with an exposure level of ~92 μg/m3 (Wang et al. 2017). At DE exposure concentrations of ~268 μg/m3, increased DNA damage was found in peripheral blood lymphocytes, as well as DNA hypomethylation changes associated with DNA damage in blood samples of exposed subjects, compared to control subjects exposed to DE concentrations of ~92 μg/m3 (Zhang et al. 2015, 2016b). Occupational DE exposures that resulted in above 1.08 μg/g urinary creatinine, a marker of PAH exposure and corresponding to an exposure of ~170 μg/m3 of fine PM (~110 μg/m3 total carbon), were associated with higher levels of cancer biomarkers (Niu et al. 2018). An exposure level of approximately 100 μg/m3 found immune alterations similar to published lung cancer risk studies, suggesting a significantly higher risk of lung cancer (Bassig et al. 2017).

Table 1 Key data from selected occupational human exposure studies using old technology DE
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Studies outside of the diesel engine tester cohort found that mechanics occupationally exposed to DE, at an estimated level of 250 μg/m3, exhibited cytotoxic and genotoxic damage to buccal epithelial cells, and peripheral blood lymphocytes. In addition, damage to DNA in both cell types was correlated with years of service, suggesting that longer periods of exposure to DE resulted in a greater amount of DNA damage (León-Mejía et al. 2019), which has implications for lung cancer risk (Bonassi et al. 2011; León-Mejía et al. 2019). A lifetime exposure (approximately 45 years) to 14 μg/m3 of EC in Western Australian miners was estimated to result in an increase of 5.5 (2.7–9.2, 95% confidence interval) lung cancer deaths per 1000 male workers. An exposure of 44 μg/m3 of EC was estimated to result in an increase of 38 (19–97, 95% confidence interval) lung cancer deaths per 1000 male workers (Peters et al. 2017). Exposure was measured between 2003 and 2011 when new technology engines were still being introduced, such that the total PM exposure levels are estimated to be 19 μg/m3 and 59 μg/m3, respectively. Norwegian tunnel finishing workers occupationally exposed to DE at approximately 37.8 μg/m3 of EC, (~50 μg/m3 PM assuming that the majority of exposure was from old technology engines) found that in comparison to non-exposed control subjects, the tunnel workers had more DNA damage in their peripheral blood mononuclear cells, altered blood plasma profiles and dysregulated expression of several microRNAs, including some related to carcinogenesis, cell death and oxidative stress (Rynning et al. 2019).

Occupational DE exposure studies reported effects on lung function and biomarkers that correlated with increased cancer risk (Bassig et al. 2017; Dai et al. 2018; León-Mejía et al. 2019; Niu et al. 2018; Rynning et al. 2019; Wang et al. 2017). All studies reported increased risks of lung cancer in workers occupationally exposed to DE. Occupational exposures below 100 μg/m3 of PM increased DNA damage, immune alterations in a pattern related to increased lung cancer incidence and an estimated risk of 38 lung cancers per 1000 workers exposed to approximately 44 μg/m3 EC (approximately 59 μg/m3 PM).

Acute Human Exposure Studies

We found no studies that examined the effects of new engine technology exhaust exposure on humans and only one study that focussed on the health effects of acute exposure to DE with and without a DPF on humans. Thus, all studies that involve acute exposure of humans to high levels of DE have used old technology diesel engines. All but two studies used exposure chambers and participants were exposed to either diluted whole DE at a variety of concentrations and/or air as a control. The measured end points focussed primarily on the cardiovascular system with fewer studies focusing on the respiratory system. No study exposed participants to DE for more than three hours. Further information on the exposure methodology can be found in Table 2.

Table 2 Key experimental data from selected acute human exposure studies using old technology DE
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Studies using DE exposure concentrations between 350 and 300 μg/m3 found minor and major cardiovascular effects with increased arterial stiffness (Lundbäck et al. 2009), increased endothelial dysfunction in patients at risk for heart failure (Vieira et al. 2016), reduced vasodilation (Lucking et al. 2011; Mills et al. 2011), increased thrombus formation (Lucking et al. 2011) and increased blood pressure after two hours of exposure (Mills et al. 2011; Tong et al. 2014). In addition, altered blood plasma profiles were found in healthy individuals and altered blood plasma profiles and altered microRNA expression in peripheral blood were found in individuals with an allergy or asthma (Giles et al. 2018b; Yamamoto et al. 2013; Zhang et al. 2016a). DNA hypomethylation was found in genes associated with oxidative stress and inflammation in asthmatics (Jiang et al. 2014). Respiratory effects have also been reported, with altered microRNA and transcription profiles and DNA hypomethylation associated with increased oxidative stress in epithelial cell brushings (Clifford et al. 2017; Rider et al. 2016) and increased airway hyperactivity and obstruction in individuals with asthma or allergies (Hussain et al. 2012; Zhang et al. 2016a). Healthy individuals exposed for 30 min to 300 μg/m3 of DE reported significant irritation of the nose, throat and chest after exposure (Giles et al. 2018a). In comparison, heart rate was not affected (Lucking et al. 2011; Vieira et al. 2016). Some studies reported no changes in blood pressure after 30–60 min of exposure (Giles et al. 2018a, b; Lucking et al. 2011) and no changes in markers of inflammation and platelet activation (Giles et al. 2018a; Lucking et al. 2011); balance was not affected and no changes were found in central nervous system biomarkers (Cliff et al. 2016; Curran et al. 2018).

Exposures to DE at concentrations between 300 and 200 μg/m3 resulted in similar health effects. Eye irritation was reported by 11 out of 18 healthy subjects exposed for up to three hours, and nose and throat irritation was diagnosed by a medical professional (Wierzbicka et al. 2014). Healthy subjects had decreased induced vasodilation (Barath et al. 2010), increased vasoconstriction (Peretz et al. 2008b) and increased blood pressure and inflammation after two hours of exposure (Cosselman et al. 2012; Tong et al. 2014) as well as altered gene expression in peripheral blood mononuclear cells (Peretz et al. 2007). In contrast, 60 min of exposure did not affect heart rate variability, systemic inflammation or blood pressure in 18 healthy male volunteers (Barath et al. 2010). No indications of oxidative stress was found in individuals with metabolic syndrome (Allen et al. 2009) and no changes in heart rate variability were found after two hours of exposure (Peretz et al. 2008a).

Studies examining concentrations of DE below 100 μg/m3 report half the amount of vasoconstriction in comparison 200 μg/m3 exposures completed in the same study, suggesting a linear dose–response although these levels were not significantly different to air exposures (Peretz et al. 2008b), increased airway inflammation in healthy subjects (Behndig et al. 2011), allergic inflammation and viral-induced immune responses in allergic rhinitics (Pawlak et al. 2016) and decreased lung function, increased airway acidification and increased respiratory inflammation in asthmatics exposed for two hours at exhaust concentrations up to 75 μg/m3 (Zhang et al. 2009).. No thrombotic effect was found in subjects with metabolic syndrome (Carlsten et al. 2008), no impact on heart rate was observed (Lucking et al. 2011; Peretz et al. 2008a; Tong et al. 2014), no impact on vasoconstriction was found (Lucking et al. 2011) and there was no evidence of respiratory epithelial cell damage in healthy, allergic or asthmatic individuals following exhaust exposure at 100 μg/m3 for two hours (Behndig et al. 2015).

One study compared the health impact of exposure to DE with and without a DPF on 19 healthy volunteers. The use of a DPF decreased PM concentration from 320 to 7.2 μg/m3. Study participants who were exposed to whole, unfiltered exhaust for one hour had increased thrombotic formation and reduced vasodilation. The use of a DPF negated the effects of exposure on vasodilation and decreased the thrombotic effect as well (Lucking et al. 2011).

Approximately 60% of acute human exposure studies used exhaust exposure concentrations between 250 and 350 μg/m3, suggesting that this level of exposure is the concentration where an observable response is likely to occur using short exposure periods (Tong et al. 2014). At this level of exposure, health effects are noticeable by the participants themselves with reports of irritation to mucosal surfaces such as the nose, throat and eyes. Most studies examined the cardiovascular system (Steiner et al. 2016). As exposure levels decreased to 100 μg/m3, reported health effects lowered in severity and more studies began reporting negative outcomes. Those that reported positive results mostly involved individuals with asthma or allergy, suggesting that they may be an at risk population that requires closer monitoring.

In addition, combining the results of both the occupational and acute human exposure studies shows potential to explore genetic alterations and DNA damage as potential biomarkers for disease and lung cancer risk after DE exposure. Rynning et al. found DNA damage in peripheral blood mononuclear cells associated with DE exposure (Rynning et al. 2019) and León-Mejía et al. found DNA damage of the buccal cheek cells and lymphocytes after continued DE exposure, with the amount of DNA damage correlating with years of service and thus years of exposure (León-Mejía et al. 2019). Thus, with more testing, there is potential to use either a cheek swap or a blood test to quantify DNA damage as a marker of increased cancer risk after DE exposure. Hypomethylation is another potential marker of DE exposure (Clifford et al. 2017), particularly hypomethylation and other changes to genes related to DNA damage responses such as p16, RASSF1A, and MGMT which are frequently found to be dysregulated in cancer (Zhang et al. 2016a, b). Alternatively, the plasma miRNA profile could be another marker as studies have found it to be significantly altered after DE exposure (Rynning et al. 2019; Rider et al. 2016), with several of the altered miRNA expressions such as miR-31–5 p, miR-20b-5p, miR-196b-5p, miR-4500 and miR-340 being associated with cancer (Rynning et al. 2019).

In Vivo (Animal Model) Exposure Studies

Approximately 74% of studies involving animal models focus on the respiratory and central nervous systems. A strength of animal studies is that long-term exposure can be compressed into the relatively short life span of experimental animals, meaning that lifetime exposures can be completed in a much shorter period of time than in comparative human occupational studies. In addition, animal models can also be exposed to much higher concentrations than used in either occupational or acute studies involving human subjects. Thus, the majority of in vivo studies reviewed expose animals for a large range of exhaust concentrations over longer periods of time, including months or even years, which also represent greater proportions of their life expectancy, than human studies. Six in vivo studies examined the effects of acute exposure. Information on animal type and exposure methodology can be found in Tables 3 and 4.

Table 3 Key experimental data from selected in vivo animal exposure studies using old technology DE
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Table 4 Key experimental data from selected in vivo animal exposure studies using new technology exhaust
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Older Engine Technology

The majority of in vivo exposure studies use old technology diesel engines and exposure concentrations vary greatly (between 50 and 3000 μg/m3).

Studies that exposed animals to DE concentrations between 3000 and 2000 μg/m3 for 1–12 weeks resulted in negative respiratory and neurological effects. A 3.2-fold greater DNA mutation frequency was found in the lungs of mice exposed for 12 weeks compared with air exposed controls suggesting greater cancer risk (Hashimoto et al. 2007), large increases in neuroinflammation were found in the brains of mice exposed for 4 weeks (Levesque et al. 2011b) and increased lung inflammation was found in mice exposed for less than a week, with allergic mice exhibiting greater symptoms (Stevens et al. 2008). In contrast, more recent studies exposing rats for one or 4 weeks to ~2000 μg/m3 found only minor histopathological changes and inflammatory effects in the lungs (Magnusson et al. 2019) and minor oxidative stress in the brain (Valand et al. 2018).

Exposing mice to DE between the concentrations of 1000 and 2000 μg/m3 has effects on the transcription of stress-related genes in the brain (Lung et al. 2014). Similar to an exposure concentration of 3000 μg/m3, a 3.1-fold increase of DNA mutations was found in the lungs of mice exposed to 1000 μg/m3 for 12 weeks, suggesting greater cancer risk (Hashimoto et al. 2007).

Studies exposing mice and rats to DE concentrations between 500 and 1000 μg/m3 found oxidative stress and increased inflammation in the lungs of rats exposed to 950 μg/m3 for < 1 week (Tsukue et al. 2010), increased neuroinflammation in mice exposed to 650 μg/m3 for 4 weeks (similar to that found in 2000 μg/m3 exposure concentrations) (Levesque et al. 2011b), increased flu severity in mice exposed to 500 μg/m3 for < 2 weeks (Gowdy et al. 2010) and an increased effect of chemically induced arrhythmia in hypertensive rats exposed to 500 μg/m3 for < 1 week (Hazari et al. 2015). Increased respiratory inflammation was found in allergic mice, but not healthy mice, exposed for < 1 week and the effects were lower than that found in mice exposed to 2000 μg/m3, suggesting dose–response relationships in these particular outcomes (Stevens et al. 2008).

In vivo exposure to DE concentrations between 300 μg/m3 and 100 μg/m3 results in neurological effects including impaired neurogenesis in male mice exposed to 250 μg/m3 for < one day (Coburn et al. 2018), increased neuroinflammation in mice exposed for 4 weeks to 173 and 149 μg/m3 (Gerlofs-Nijland et al. 2010; Win-Shwe et al. 2008) and impact on object recognition ability in mice exposed to 129 μg/m3 for 12 weeks (Win-Shwe et al. 2012). No impact was found on spatial learning abilities in mice exposed to 149 μg/m3 for 4 weeks (Win-Shwe et al. 2008). Health impacts on other systems included increased respiratory inflammation in normal mice and increased respiratory inflammation and oxidative stress in asthmatic mice exposed to 200 μg/m3 for 7 weeks or 169 μg/m3 for 8 weeks, respectively (Bai et al. 2011; Tanaka et al. 2013a; Tanaka et al. 2013b), unfavourable changes in atherosclerotic plaques in mice exposed to 200 μg/m3 for 7 weeks (Bai et al. 2011), changes in steroidogenesis in male rats exposed for 4 weeks to 149 μg/m3 (Yamagishi et al. 2012), an increased effect of chemically induced arrhythmia in hypertensive rats exposed to 150 μg/m3 for less than a week (Hazari et al. 2015), increased allergic symptoms in asthmatic mice exposed to 100 μg/m3 for 12 weeks (Matsumoto et al. 2006) and increased oxidative stress in the lungs of rats exposed for 3 days to 100 μg/m3, although no impact on respiratory inflammation was found (Tsukue et al. 2010).

Exposure studies that used DE concentrations below 100 μg/m3 found mild increases in the effect of chemically induced arrhythmia in hypertensive rats exposed to 50 μg/m3 for < 1 week (in comparison to exposure to 150 and 500 μg/m3) (Hazari et al. 2015), increased oxidative stress in the lungs and minor impact on respiratory inflammation of rats exposed to 60 μg/m3 for < 1 week (in comparison to exposure to 950 μg/m3) (Tsukue et al. 2010), minor increases in respiratory inflammation in asthmatic mice exposed for 8 weeks to 39 μg/m3 (in comparison to exposures to 169 μg/m3) (Tanaka et al. 2013a), some impact on steroidogenesis in male rats exposed to 38 μg/m3 for 8 weeks (Yamagishi et al. 2012) and no impact on object recognition in mice exposed to 47 μg/m3 for 12 weeks or oxidative stress in mice exposed to 36 μg/m3 for 8 weeks (Tanaka et al. 2013b; Win-Shwe et al. 2012).

New Engine Technology

Seven in vivo studies exposed animals to the exhaust generated from new technology diesel engines. Exhaust concentrations never exceeded 200 μg/m3 and all studies were published in the past 5 years. Rats exposed to 182 μg/m3 for one and 4 weeks displayed changes in gene expression of the brain which suggests minor oxidative stress, although no histopathological effects were found and the differences compared to rats exposed to old technology exhaust at a concentration of 2000 μg/m3 were minor (Valand et al. 2018). Magnusson et al. found minor respiratory inflammation and oxidative stress in the lungs of rats exposed to approximately 170 μg/m3 for one and 4 weeks. No differences were found when compared to rats exposed to old technology exhaust at a concentration of 2000 μg/m3 (Magnusson et al. 2019). Douki et al. found minor indications of accumulated lung DNA damage in rats exposed to less than 100 μg/m3 for 3 weeks; however, effects were found to be worse with new technology exhaust when compared to old, suggesting that toxicity was associated with the ultrafine particulates and the gas phase of the exhaust (Douki et al. 2018). A series of studies exposed rats to 12 μg/m3 of exhaust for 28–30 months and found only limited effects, including minor histopathological changes and mild increases in inflammatory and thrombotic markers; however, no damage to DNA was recorded and no increases in tumour development were found (Bemis et al. 2015; Conklin and Kong 2015; Hallberg et al. 2015; McDonald et al. 2015).

In in vivo exposure studies using old technology exhaust, exposure concentrations varied greatly with the highest exposures resulting in a range of health impacts to the respiratory, cardiovascular and neurological systems of mice and rats. The severity of these effects decreased as exposure decreased. Results were similar for new technology studies; however, the few studies available limit the conclusions that can be drawn. Once again, animals with conditions simulating asthma or allergy displayed worse symptoms and the study with the lowest exhaust exposure concentration that still reported exposure health impacts used asthmatic mice as subjects, highlighting potentially susceptible populations. Some studies also reported increased influenza severity in mice exposed to DE, which may highlight another susceptible population that was not found in the human exposure studies.

In Vitro (Cell Model) Exposure Studies

Most in vitro studies into the effects of DE exposure use DE particles collected on quartz filters and added directly to the media the cells are grown within (Zarcone et al. 2016). Using this approach to estimate the potential health consequences of exhaust exposure is limited as it ignores the health consequences of the exhaust gases entirely. In addition, the particles collected on the filter agglomerate, sticking together to generate an artificial particle spectrum made of larger particles, often removing the ultrafine particles from the sample and thus from the subsequent analysis of exposure health effects (Morin et al. 2008). This approach often underestimates health consequences of exposure and over 16 times higher concentrations of particles are needed to generate the same health consequences as exposure to whole exhaust (Lichtveld et al. 2012). All in vitro studies included in this review use whole exhaust instead of pre-collected particles and focus on the damage caused to the respiratory epithelium, either using primary human epithelial cells or the alveolar carcinoma cell line A549. All cells were grown at an air–liquid interface, exposing one side of the cell model directly to the diluted DE (Tables 5 and 6).

Table 5 Key experimental data from selected in vitro exposure studies using old technology exhaust
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Table 6 Key experimental data from selected in vitro exposure studies using new technology exhaust
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Older Engine Technology

Studies exposing cells to old technology diesel engine exhaust have mostly focussed on cell damage, oxidative stress and inflammatory responses. A549 cells exposed to 1600 μg/m3at air–liquid interface displayed inhibited proliferation and increased oxidative stress (Okubo et al. 2015). The same cells exposed to exhaust after the use of a DPF, at a concentration of 470 μg/m3, exhibited suppressed immune reactivity in comparison to air exposed controls. Oxidative stress was decreased in comparison to the DE exposure concentration of 1600 μg/m3; however, the decreased immune response after exposure was only found in those cells exposed to the DPF-equipped exhaust (Okubo et al. 2015). A549 cells exposed to 1300 μg/m3 at air–liquid interface displayed increased cell death and increased oxidative stress (Kooter et al. 2013), while differentiated primary human bronchial airway epithelium grown at air–liquid interface and exposed to DE at a concentration of 850 μg/m3 displayed increased oxidative stress and increased PAH adduct formation but no loss of viability (Hawley et al. 2014).

Three studies exposed differentiated primary human airway epithelial cells collected from healthy volunteers and volunteers with COPD to a range of exhaust concentrations and types (Zarcone et al. 2016, 2017, 2018). In a study that used old technology exhaust, Zarcone et al. (2016) found that exposing the cells to ~1200 μg/m3 induced the production of inflammatory markers, oxidative stress, cellular death and increased permeability after 150 min of exposure. At 430 μg/m3, they found increased oxidative stress after 150 min and increased permeability after 375 min. At 140 μg/m3, only decreased permeability was recorded (Zarcone et al. 2016).

New Engine Technology

Only three in vitro exposure studies were found that examined new technology DE exposure. Exposure to 1500 μg/m3 for 60 min induced oxidative stress and decreased the defence response to infection, although no cellular death occurred (Zarcone et al. 2017). Primary human airway epithelial cells exposed to three different, much lower, exhaust concentrations found that the lowest concentration (34 μg/m3) had no impact on healthy cells, the second lowest concentration (82 μg/m3) increased oxidative stress in healthy cells and the highest concentration (206 μg/m3) increased oxidative stress in healthy cells and decreased host defence in COPD-derived cells (Zarcone et al. 2018). Differentiated primary human airway epithelial cells exposed to a DE concentration of 35.3 μg/m3 found increased oxidative stress and increased PAH adduct formation. No difference in health effects was observed between new technology exhaust and old technology exhaust at a concentration of 800 μg/m3 (Hawley et al. 2014).

Occupational Exposure Limits and Their Applicability

The Australian Institute of Occupational Hygienists recommends a DE occupational exposure limit of 100 μg/m3 as a time weighted average over 8 h, measured as elemental carbon (AIOH 2017). In America, the DE exposure limit in a non-coal mining setting was set in 2008 at 160 μg/m3 total carbon (MSHA 2016). However, there is no particle mass exposure limit set for non-mining settings (OSHA 2013). As of 2019, the European Union have introduced occupational DE exposure limits of 50 μg/m3 EC, to be put into effect in 2023 in non-mining settings and 2026 in a mining setting (EU 2004). Previous DE exposure health effect reviews have recommended an occupational exposure limit of 100 μg/m3 of diesel PM in total, which is equivalent to approximately 75 μg/m3 EC (Taxell and Santonen 2017). Using the acute human studies reviewed in this report as the basis for the cross comparison, this limit is accurate for reducing the health effects of short-term exposure in healthy workers. However, this limit fails to take the safety and comfort of workers with asthma or allergy into account and is far above occupational exhaust concentrations where studies found significantly increased lung cancer risk. Previously published reviews recommended the lower occupational exposure threshold of 50 μg/m3 of respirable EC (approximately 67 μg/m3 of PM) in order to reduce lung cancer risk (Möhner and Wendt 2017). This current review, based on the acute human exposure studies and the occupational exposure studies, suggests a limit below 50 μg/m3 of PM, approximately 35 μg/m3 EC, would be more suitable. This level is below the exposure concentrations where effects were observed among asthmatics and below the concentrations that found the higher lung cancer risks (an increase of 38 (19–97, 95% confidence interval) lung cancer deaths per 1000 male workers (Peters et al. 2017) and a significant increase in DNA damage and dysregulation of microRNAs, some of which were associated with carcinogenesis (Rynning et al. 2019)). In addition, this limit is supported by in vivo exposure studies, where exposure concentrations at 50 μg/m3 only resulted in mild health effects.

However, it should be noted that exposure limits based on both the mass of EC, as well as the mass of total PM, are limited in their long-term applicability. In order to meet the suggested 50 μg/m3 PM occupational limit, most if not all diesel equipment must be fitted with exhaust after-treatment devices, including a DPF. Diesel particulate filters remove particles from the exhaust; however, they preferentially select for EC above other particle types (Hawley et al. 2014; Khalek et al. 2011) skewing the exhaust output and eliminating EC as a predictive measure for overall exhaust exposure, making any occupational limits based on EC unreliable.

Occupational limits based on particle mass have their own drawbacks. Evidence is accumulating that it is particle size and particle number that contribute more towards health impact than total particle mass (Cauda et al. 2012; Hawley et al. 2014; Ramachandran et al. 2005), making occupational limits based on mass, without accounting for particle size and number, a questionable decision. The latest European Emission Standards take this into account and have set limits for both particle mass and particle number (EU-Commission 2011).

In addition, multiple studies published in the last decade are reporting little to no change in health impacts after the use of a DPF. In in vivo and in vitro exhaust exposure studies that compare exposure health effects before and after the use of a DPF, few to no decreases in health impact are found (Douki et al. 2018; Gioda et al. 2016; Hawley et al. 2014; Karthikeyan et al. 2013; Magnusson et al. 2019; Okubo et al. 2015; Steiner et al. 2013; Valand et al. 2018) with only a few adverse cardiovascular events, including thrombosis and vasoconstriction, being decreased or prevented in an acute human exposure study (Lucking et al. 2011). Diesel particulate filters remove more than 90% by mass of particles from the exhaust (Hawley et al. 2014; Lucking et al. 2011; Magnusson et al. 2019; Valand et al. 2018). However, they cannot be 100% efficient given pressure drop constraints of the system, and therefore some particles (generally in the smaller size ranges) will pass through the DPF. Also at the operating temperatures of a DPF, many particles (such as PAHs) are liquid and can migrate through the filter and be resuspended (PubChem; Hawley et al. 2014; Khalek et al. 2011). Indeed PAH can melt as low as 80 °C and boil as low as 200 °C, both of which are well below typical exhaust temperatures (PubChem). The addition of after-treatment devices, such as a DOC, may even generate additional nitro-PAHs (Carrara and Niessner 2011; Inomata et al. 2015). This suggests that either the exhaust gases are having a greater effect on health than previously thought or that ultrafine particles, and the toxic chemicals potentially adsorbed to their surface, are responsible for the majority of health impacts caused by diesel PM (Douki et al. 2018; Hawley et al. 2014; Karthikeyan et al. 2013; Tanaka et al. 2013b). Thus, using occupational limits based on particle mass, an exhaust exposure that was over the limit where negative health consequences occur would read as under with the use of a DPF, and yet the DPF would have little to no impact on decreasing the health impacts on an exposed worker.

Limits on particle number should also be addressed. Studies have found NOx to be a reliable indicator of DE exposure, as long as the majority of sources contributing to the NOx concentrations are diesel engines (Hedmer et al. 2017; Taxell and Santonen 2017). Equipment that measure NOx concentrations are also less expensive than the equipment needed for EC measurement (Hedmer et al. 2017) and thus an additional occupational limit based on NOx should not prove to be an expensive burden on industry. However, a more thorough review on the health effects of NOx and its applicability as a DE exposure predictive measurement should be conducted before any sort of limit is put into effect. In future, more research needs to be conducted on the health effects of exposure to new technology diesel engine exhaust and further occupational studies need to be based on the possible health outcomes of the increasing application of new technology engines in industry.

Limitations

The majority of literature was sourced from PubMed using strict search criteria and thus it is possible that relevant studies were missed. Studies were only included if they were written in English and thus relevant studies in other languages were also excluded. This review focussed on studies relevant to occupational exposure settings and thus studies that used exhaust concentrations not relevant to occupational exposure conditions were not included.

The studies included in this review use a wide variety of engine types with varying emission classifications and after-treatment devices. Details of engine specifications and settings used during the exposures are limited, if they are reported at all. This, combined with the wide range of exposure outcomes measured, makes firm conclusions difficult for setting occupational DE exposure limits. Consistency in experimental designs and strict guidelines for reporting engine specifications and settings in DE exposure research would help immensely in solving this issue.

Many of the occupational exposure and acute human exposure studies also use exclusively male subjects and more research needs to be done to verify that occupational DE exposure has similar health impacts in both men and women. The few animal studies that compare both sexes often show differences in response between males and females (Coburn et al. 2018; McDonald et al. 2015; Conklin and Kong 2015). There is insufficient evidence to assess whether these differences also exist in humans. In addition, very few studies exist that exposed human, animal or tissue to “new technology” exhaust and thus further research is needed to confirm the findings of this review. Future studies in DE exposure effects should concentrate on using newer technology engines and after-treatment devices in order to consolidate the health effects of exposure to “new technology” engine exhaust before it becomes more widely used in an occupational setting.

Conclusion

In conclusion, an occupational exposure limit of 100 μg/m3 is too high as it does not take increased lung cancer risk caused by high levels of DE exposure into effect. A limit of 50 μg/m3 is more appropriate if lung cancer risk and the effects of exposure on workers with asthma, allergy and respiratory disease are accounted for. An occupational exposure limit based on EC is not appropriate as after-treatment devices preferentially remove it from the exhaust, making it an unreliable indicator of exhaust exposure. After-treatment devices also make occupational limits based on particle mass unreliable at best and additional limits, such as ones based on particle number or NOx concentrations, are needed in order for occupational exhaust exposures to be reliably monitored.