Organic nitrogen The secret killer in Chinese megacities
Scientists use the principles of absorption and emission to detect the deadly chemicals in Beijing’s atmosphere
This article links to the following topics in the AQA, Edexcel, OCR, WJEC, CCEA, SQA and IB Diploma exam specifications:
• particulate matter (PM) and organic nitrogen (ON) in the atmosphere
• toxicological behaviour of chemical species
• nitration reactions
• how science works
• radical reaction mechanisms
China’s modern cities are often associated with the grey smoggy skies and hazes attributed to air pollution (see CHEMISTRY REVIEW, Vol. 28, No. 2, pp. 2–6). It has been reported that breathing this sort of air for just an hour is comparable to smoking 1.5 packets of cigarettes per day. It is therefore no surprise that some of the inhabitants in Chinese megacities try to avoid leaving their homes during heavily polluted periods.
According to the World Health Organization, 4.2 million deaths per year can be attributed to outdoor air pollution, with 3.8 million deaths per year associated with human exposure to smoke from dirty household appliances such as cooking stoves. This environmental cause of mortality has become one of the leading preventable causes of death worldwide.
With over 1.6 million people per year in China dying from the adverse health effects caused by air pollution (around 4,400 people per day), and with reports of children as young as 8 being diagnosed with lung cancer, the investigation into the quantity and quality of gases and particles in the boundary layer of China’s megacities is vital.
In 2016 it was estimated that China consumed around 11.5 million barrels of oil per day. It is the largest consumer and producer of coal worldwide. With such a large consumption of fossil fuels needed to sustain the modern lifestyles desired by its vast population, air pollution is inevitable. The people of China are fully aware and angered by the adverse health effects that are caused by the smoggy episodes clouding the country’s blue skies. But what exactly is it within this smog that is so dangerous? How are these chemicals introduced into the air that China breathes, and which chemicals are among the most threatening to life?
Organic nitrogen species
Photochemical smog is a toxic cocktail of particles and gases. Particulate matter (PM), ozone and nitrous oxides are the dominant substances found in these hazes. All are detrimental to human health. Studies have revealed that a class of chemicals called organic nitrogen (ON) compounds contribute significantly to particulate matter. ON compounds are organic molecules that contain a nitrogen atom. These include organonitro compounds, amines and amides.
The study of these compounds in urban air has been relatively neglected, despite the abundant library of medical papers that describe (in great detail) their toxicology. Many ON species have been reported as class A carcinogens, which have no safe level of exposure and definitely cause cancer.
For example, 6-nitro-chrysene has been reported to be 1000 times more carcinogenic than chrysene. The nature of this immense toxicity arises from the integration of a nitrogen atom into the structure of some typical organic molecules, for example those shown in Figure 1.
These substances may be formed in high-temperature environments, including combustion sources such as smoking or cooking, or they also may be formed secondarily in the atmosphere. Studies conducted to date have suggested mechanisms such as the electrophilic substitution of NO2 during combustion, producing species such as nitro-polyaromatic hydrocarbons (nitro-PAHs, see Box 1). Other ON species include nitroalkanes, nitroalkenes, nitrophenols, nitrosamines, amines and amides, as well as many more substances that demonstrate acute toxicity.
Box 1 Nitroaromatic compounds
When certain organic molecules are nitrated, they become more mutagenic. Nitroaromatic compounds (NACs) are of great atmospheric interest, as these are emitted directly from a variety of sources, such as from diesel emissions, biomass burning, gas burners and kerosene heaters. For example, 1-nitropyrene is thought to be one of the most prevalent nitro-PAHs emitted. It is found in diesel fumes and is produced as a result of incomplete combustion. Another example of an NAC is 2-nitrophenol, a compound used in chemical industries and agriculture.
NACs may also be formed secondarily in the atmosphere, on interaction of a PAH with oxides of nitrogen and other species. In contrast to this, other studies have suggested that nitro-PAHs where the nitro group is positioned perpendicular, or almost perpendicular, to the aromatic rings may actually demonstrate lower tumorigenic characteristics than the original parent PAH.
Epidemiological studies have shown a positive trend between diesel fume exposure in urban air pollution and an increased risk of lung cancer. Many ON compounds have tumorigenic, genotoxic, mutagenic and carcinogenic properties. Animal testing and tests conducted on single-celled organisms have been used in the study of the toxicological mechanisms surrounding ON and DNA. Tumour development was found in dogs and carcinogenic activity was observed in rats (Box 2).
In 2011, 5 million cars were registered in Beijing, making vehicle emissions one of the dominant sources of air pollution in the city. Nitro-PAHs are emitted from diesel vehicles through PM emissions, and this is the route through which humans are most likely to be exposed to nitro-PAHs.
The dangerous nature of particulate matter also stems from its ability to penetrate deep into the lung due to its small size. Some particles, such as PM2.5 (smaller than 2.5μm in diameter), are so small that they are capable of reaching the bronchi and alveoli. This has the potential to induce damage to these sections of the respiratory tract. For some particles (such as PM0.1), their size is so small that they can be transferred through the lining of the alveoli and into the bloodstream.
Box 2 Ethics surrounding animal testing
A vast amount of toxicological experiments surrounding ON compounds have used animal testing as a means of assessing a compound’s toxicological potential and the mechanism of ON interaction with DNA. The question of whether animal testing is morally the right thing to do is an age-old controversy. Animal rights activists argue that animal testing is cruel and unnecessary, and that animals used in testing provide a poor template for the human body. Others argue that the knowledge acquired from animal testing is essential and is not possible to obtain in any other way (apart from using human volunteers).
Most medical treatments available today (as well as a substantial amount of toxicological studies) are possible due to extensive research involving animal testing. Scientists are encouraged to follow the 3Rs principle, in which they:
• reduce the quantity of animals being tested
• refine experiments so as to cause as little distress as possible
• replace animals with other media such as cell cultures or computer models
Once in the body, nitro-PAHs and other ON compounds are susceptible to being broken down by enzymes, forming metabolites. These ON compounds undergo enzymatic metabolism within cells through either an oxidative or reductive pathway (depending on the enzyme the ON interacts with). For example, a nitro-PAH may undergo a reductive pathway followed by other enzyme transformations to produce an electrophilic substituent, or may follow the oxidative pathway through a ring oxidation mechanism. The genotoxic behaviour of these compounds is then induced when the electrophilic species, produced as a result of this process, form covalent bonds with DNA, producing DNA adducts.
The formation of DNA adducts results in the toxic activity of the ON compounds in the body, producing illnesses such as cancers and tumours, and is what defines the carcinogenic and mutagenic behaviour of these substances.
The compound 3-nitro-7H-benz(de)anthracen-7-one (3-NBA) is an example of an ON compound in the nitro-PAH family, which has been found to exist on the surface of particulate matter emitted from diesel-powered vehicles. This compound is thought to be a leading mutagen present in urban PM and humans are predominantly exposed to it through inhalation of particles emitted from diesel engines.
In the body, 3-NBA is thought to be activated by the SULT, NAT and CYP enzymes present in the lungs. These enzymes metabolise 3-NBA to form a significant amount of specific DNA adducts residing in the organs and blood. An example of an adduct of 3-NBA with DNA is shown in Figure 2.
Interestingly, an individual’s susceptibility to the toxic behaviour of ON compounds is not only dependent on the toxicity of the compound itself, the form of the pollution in which the compound reaches the body, the exposure time or the concentration of a particular compound. It also depends on how well a specific body is able to metabolise these materials.
Genetic variants of enzymes are known to exist across a population, resulting in potentially differing responses to ON compounds from person to person. In the case of enzymes that are capable of metabolising these toxic substances, there may be different breakdown processes of these compounds, resulting in a range of metabolites across the population. A person’s vulnerability to falling ill as a result of exposure to these materials may therefore be influenced by their genetic make-up.
In addition, for DNA-adduct formation to be successful within cells, the intercalation ability, hydrophobicity and the orientation prior to binding of metabolites needs to be correct. Furthermore, the sequence of DNA presented to the metabolite products available for binding also determines whether covalent bonding, and therefore adduct formation, are favourable.
There are some ON compounds that we consume without too much concern. Some, such as amino acids, are essential for life. Caffeine is much less toxic than most other ON compounds (see CHEMISTRY REVIEW, Vol. 24, No. 3, p. 34), although the material safety data sheet (MSDS) for caffeine does state that its pure form (99% purity) is harmful when swallowed. DEET (N,N-diethyl-meta-toluamide) is another ON compound and is used in some mosquito repellents. Like caffeine, DEET can be released into the atmospheric boundary layer.
In the atmosphere, compounds undergo a complicated web of gas-phase reactions through numerous secondary chemical processes. So far, our understanding of ON formation in the atmosphere covers the formation of compounds such as substituted alkenes and alkanes, nitrophenols, nitro-PAHs and nitrosamines.
For example, a possible mechanism of formation for nitro-PAHs may occur through a gas-phase nitration reaction, which is initiated by an atmospheric OH radical. Initially, an OH radical attacks the PAH (emitted from a diesel engine, for example), which forms a carbon-centred organic radical. In the presence of an NO2 radical, the VOC radical is attacked by the NO2, producing the nitro-PAH, terminating this reaction and releasing water (Figure 3).
Research has resulted in a thorough description of the biochemical mechanisms that surround the toxicology of ON compounds. Though several atmospheric studies have already been conducted, more work is needed to quantify these deadly species in urban air. This highlights the importance of communication between the sciences.
China relies heavily on rapid industrialisation to lift its citizens out of poverty. Creating and implementing initiatives to combat pollutants will be one of the most challenging tasks in the near future. The transition to new, cleaner technologies is vital, and progress is happening. From making vehicles and buildings more energy-efficient, to building nuclear reactors and high-speed rail links, there are numerous initiatives that have been set up to try and reduce emissions and reach set government targets to improve the air quality of those living and working in Chinese megacities.
Acute toxicity Causes toxic effects after a single dose or multiple doses in a short space of time.
Boundary layer The lowest part of the atmosphere (i.e. closest to the ground). It is the part of the atmosphere that directly feels the effects of the Earth’s surface. Its depth varies from just a few metres to several kilometres, depending on the local weather conditions.
Carcinogenic Causes cancer.
Epidemiological studies Studies linking diseases and poor health within a population to potential causal factors.
Genotoxic agents Chemicals that cause damage to the genetic material found within a cell.
Intercalation Inserted between layers. In the case of a molecule intercalating with DNA, it will slot in between the planes of the base pairs within the double helix.
Metabolites Chemical products produced from enzyme-catalysed processes in the body.
Mutagenic Induces mutations in the genes of an organism (i.e. changes in the sequence of nucleotide bases in DNA).
Nitration A chemical reaction that incorporates a nitro group (–NO2) into the structure of a molecule.
Radical An atom or group of atoms with an unpaired valence electron (such as the •OH radical). This unpaired electron tends to make radicals highly reactive.
SULT, NAT and CYP enzymes Specific types of enzymes (proteins that act as catalysts in living organisms). SULTs are sulfotransferases, NATs are N-acetyltransferases and cytochrome P450 (CYP) enzymes are a superfamily of mono-oxygenases.
Tumorigenic Causes growth of tumours.
• An estimated 4.2 million deaths annually have been attributed to outdoor air pollution.
• Organic nitrogen compounds contribute significantly to particulate matter in the air. They are class A carcinogens, which means that there is no safe level of exposure to them.
• Organic nitrogen compounds form toxic metabolites in the body, producing cancers and tumours.