Particulate matter: deadly but invisible
Air pollution is often thought of as a gas hanging above our cities, but the most dangerous parts of it are particular matter (PM): ash, dust, chemical salts, thought to cause 7 million premature deaths a year.
92% of the world’s population, according to the WHO, lives in areas where PM exceeds the healthy threshold of 10 µg/m3 of PM2.5 and 20 µg/m3 of PM10 per year. In fact, there is no safe threshold: at any concentration, PM has carcinogenic, clogging and abrasive effects on our lungs, and they have been linked to heart attacks, strokes, and lung cancer.
What is it?
PM collectively describes the mixture of solid matter and liquid droplets suspended in the air by winds and air currents. Liquid droplets are seen as particles because they often nucleate around an ultrafine particle (0.1 µm in diameter). Particles with a diameter of less than 2.5 micrometres (2.5 x 10-6 m) are PM2.5, and are the deadliest, although PM10 (1 x 10-5m in diameter) take up more air space, and are more visible in the grime coating car bonnets and hedges.
90% of PM derives from sea spray but the remaining 10% is largely anthropogenic from three main sources: incomplete combustion, stirred-up dust, and the chemical reactions which occur naturally between other gases we emit, such as sulfur dioxide.
the smog due to coal-power stations in Beijing (left); the smog in Delhi due to firecrackers, traffic, paddy-stubble burning, and industry (right)
Incomplete combustion, although hugely significant, more often leads to PM10 than PM2.5. When a fuel is burnt in restricted oxygen, the heat induces pyrolysis: the thermal decomposition of the substance, leaving carbon as ash in the air, known as fly ash, as the carbon hasn’t been fully oxidised into carbon dioxide. Known as the organic matter component (OM) or black carbon, it is one of three major components of PM. It derives from an array of human practices, which often lead to localised pollution based on the dominant industry in, or near, a city.
The combustion of fossil fuels produces significant fly ash, particularly when coal is burnt; this is the cause of the smogs shrouding Beijing and much of China. Slash-and-burn agriculture, in places such as Brazil, also contributes, with an estimated 200-500 million people who rely on this practice in the world today; although, perhaps more significantly, due to its proximity to large cities in northern India, is the practice of crop burning. Here, to remove the stubble from paddy fields, the stubble is burnt, exacerbating Delhi’s air pollution, which has been likened to smoking 50 cigarettes a day. The cumulative effect of this small-scale agricultural process (outside the influence of the city’s authorities) has made the PM in Delhi difficult to counter.
Secondly, PM can be sand and dust particles in the air. The coarsest particles are often derived from soil and rock erosion: within cities, this is often due to dust on roads stirred up by vehicular turbulence, whilst mining and agriculture also play their role. However, one of the reasons why the highest concentrations of PM air pollution occur in desert latitudes – across the Sahara, the Arabina Peninsula, northern India, and central Asia – is due to the sand and mineral dust which is swept up by desert winds into cities. Zabol, a city in Iran near the border with Afghanistan, has the dubious accolade of suffering from the highest PM2.5 concentrations at 217 µg/m3, due to what is known as the “120-day wind”, which carries in dust and sand from the desert, exacerbated by the desertification of a nearby wetland.
Zabol: the most polluted city on the planet; the yellow colour is due to desert dust; the world map shows the concentrations of PM2.5 worldwide – desert areas are much worse
Lastly, PM can form in the air from other air pollutants, such as sulfur and nitrous oxides, spewed out from vehicular exhaust. Sulfur dioxide oxidises in the air to form aqueous sulphuric acid droplets, known as aerosols, which are known as secondary particles. These can then react with ammonia (from agriculture) to form sulfate particles, and because they have a yellowish colour, they are responsible for reducing visibility in smogs, and lending them a mustardy tinge.
Collectively, smog is the complex soup of all these particles, though the ingredients vary each time: in Riyadh, the incineration of oil is to blame alongside desert winds; in Delhi, fire crackers at Diwali, crop-burning, industrial and vehicular exhaust, as well as open wood fires on stoves known as challahs, are the toxic mix. In Bamenda in Cameroon, soil erosion and urbanisation are a thick combo, whilst in Onitsha – the highest for PM10 particles – it is diesel fumes, from traffic and ageing ships, waste incineration, and oil refineries which are culpable. 87% of the deaths from PM pollution, overall, will be borne by low- and middle income-countries because rapid informal population growth catalyses and exacerbates all these causes of PM.
Effects of PM on the body
Although scientists are only beginning to uncover the pathways through which PM damages the body, the damage that PM does has been well catalogued. But only now is it being recognised by world leaders, even though, according to the WHO, no safe threshold exists at which PM2.5 is not dangerous to human health
Particles with smaller diameters penetrate the lungs more deeply: PM10 are filtered from the nose and throat by cilia and mucus, whereas smaller particles can travel into the bronchi and then bronchioles of the lungs. What makes PM2.5 (also known as respirable particles) so dangerous is that these particles can reach an alveolar level.
The cell membranes which separate each alveolus from the capillaries are only 0.5 µm thick, 5 times thinner than a PM2.5 particle; when these particles settle on the alveolar walls, they can prevent gaseous exchange by clogging them up, especially if the air pollution is chronic, in which case a whole layer of sediment can build up. This exacerbates existing pulmonary conditions, such as asthma and bronchitis, with children and the elderly most affected. Acute exposure, in a single event or day, (as often happens in anticyclonic weather in European cities) can make people short of breath, irritate their lungs, and it can cause their heart to beat irregularly.
the lungs on the left of a rural Indian; the lungs on the right of somebody living in Delhi │ plaque building up in an artery
Scientists have shown, validated through meta-analysis, that PM exacerbates cardiovascular conditions. As little as a 5 µg/m3 increase in PM2.5 concentrations has been shown to lead to a 13% increase in the risk of having a heart attack. A 2011 study concluded that 7.4% of all heart attacks are caused by breathing in traffic exhaust, which is the largest of any preventable cause. Cardiac-related strokes and atherosclerosis – the build-up of plaque inside arteries – are also caused by the particulate matter in air pollution.
The mechanism for this exacerbation is still unclear, but some scientists believe it is associated with inflammation: the body recognises the presence of potentially-carcinogenic particles in the lungs and initiates an inflammatory response, stimulating bone marrow and response-proteins (cytokines). These inflammatory responses have been shown in both mice and humans to increase the stress on the lining of blood vessels, known as the epithelium, which makes atherosclerotic events more likely, especially as the cytokines can change the nature of the plaque itself.
What’s even more worrying arguably is that PM2.5 are classified by the WHO as Group 1 carcinogens. The surfaces of these particles released in incomplete combustion are covered in compounds known as polycyclic aromatic hydrocarbons, such as benzopyrene, which are carcinogens. Benzopyrene is well-known for causing mutations in DNA, and it has been linked extensively to lung cancer. A rise of just 10 µg/m3 of PM2.5 provokes a 36% increase in the risk of lung cancer, according to a 2013 study published in Lancet Oncology which took place across 9 EU countries and involved 312,944 people.
The particles from diesel exhaust are potentially even more deadly: they are smaller, in the nanoparticle range, therefore can diffuse into the blood stream, and they have been shown to break the blood-brain barrier, carrying their carcinogens directly to organs. This is exacerbated by traffic jams, and breathing air from the road through air conditioning: idling traffic releases 4-8 times as many pollutants and PMs as free moving traffic.
Particulate matter is a complex, but increasingly understood picture. At its heart is the inescapable fact that our urban culture is based on carbon. Leaders are waking up to the dire situation which hangs thick above our cities, blackening, yellowing, and hazing the air, but until our culture of carbon is eradicated, air pollution is here to stay.