When a US sinus and nose doctor had lab mice breathe US city air for four months, the lining of their nose became inflamed. The cause was not allergens, since the air taken from Baltimore had been filtered so that only particles below 2.5 microns remained. His conclusion: air pollution causes sinusitis-type symptoms, at least in lab mice.
There has been a torrent of evidence in recent years for ill-effects from particulate matter (PM), small particles invisible to the naked eye. Early evidence found spikes in deaths in a city during higher pollution episodes, even at relatively low particulate levels. Even in Europe, it is estimated that average life expectancy is 8.6 months lower than it would otherwise be, due to PM exposures from human sources (http://www.who.int/mediacentre/factsheets/fs313/en/). Particles have now been linked to cancer, asthma, stroke and heart disease, diabetes, obesity and even dementia. Diesel engines and biomass burning are the largest contributors to particulate matter pollution. A group in Barcelona reported poorer attention scores and slower brain growth in children exposed to higher amounts of particulate matter at school (PLOS Medicine, doi.org/10.1371/journal.pmed.1001792). Proximity to a busy road determined pollution levels in classrooms.Even daily spikes in pollution lowered attention (Epidemiology, DOI: 10.1097/EDE.0000000000000603).
‘It started with the impact on lungs; then it was linked to heart attacks and strokes. In the last five years, there have been questions asked about many major conditions which affect society,’ says Frank Kelly, environmental health scientist at King’s College London. ‘For nearly every case, there’s been a positive association with particles, including diabetes and neurodegenerative diseases such as Alzheimer’s and Parkinson’s.’ The more researchers dig, the more harm they find.
Inhaling particulates causes around 29,000 deaths each year in the UK, noted a 2016 report from the Royal College of Paediatrics and Child Health. Damage occurs across a lifetime, from a baby’s first weeks in the womb to the years of older age. Maternal exposure to ambient concentrations of fine particulate matter – with a diameter of 2.5 microns or less – is a known risk factor for pre-term births. This year it was reported that an estimated 18% of births globally are associated with these pollutants, particularly a problem for Asia and Africa (Environmental International, http://dx.doi.org/10.1016/j.envint.2017.01.023).
The suspected route of travel is through the lungs and into the bloodstream, where the particles can cause systemic inflammation. ‘Once in the bloodstream these particles cause inflammation of the walls of blood vessels. They become thicker and calcified and get blocked. It's a similar mechanism to cholesterol,’ says Carlos Dora, coordinator of Interventions for Healthy Environment at the World Health Organization (WHO), which puts worldwide mortality from air pollution at 7m/year. ‘It causes as many deaths as tobacco.’
The particles may have a carbon core, but be coated in metals and hydrocarbons. Which chemistries are most associated with inflammation or ill health is not known. ‘That is still an unanswered question. There is a difference in toxicity depending on the source. What we can say is that particles from combustion seem to have these impacts,’ says Michael Brauer, public health scientist at the University of British Columbia, Canada. ‘What the lung tissue sees is not the core, but all sorts of compounds coated on the surfaces.’
Particles containing lots of transition metals, like iron and copper, seem to be particularly damaging. They oxidise proteins in the lung, for example. Such metals are associated with fumes from combustion engines, but also brake wear from vehicles. What kind of exposure matters most remains uncertain. ‘Is it cumulative exposure over a number of years, or the number of episodes where you were exposed over certain levels?’ asks John Henderson, professor of paediatric respiratory medicine at University of Bristol, UK. He is looking at the impact of pollutants on lung development and asthma.
Henderson views children as especially vulnerable. A study in southern California that measured lung function as children matured found a strong relationship between air pollution exposure and diminution of lung function growth. ‘It might not make you ill as a child. You may have no symptoms, show no effects, but it is storing up a problem that you carry throughout your life,’ says Henderson.
There is also renewed interest in ultrafine particles, typically smaller than 0.1 microns or 100nm, at the lower end of the so-called PM 2.5 size range. ‘The ultrafine particles don’t weight a lot, but on a number basis - meaning if you counted the particles in the air - they dominate what we are exposed to,’ says Brauer. ‘Some argue that our lung is not a balance. It doesn’t weigh the amount of particles. What it is seeing is the number of particles.’ For policy, it may not matter too much about relative toxicity of particles; merely controlling PM 2.5 mass concentrations results in health benefits.
Apart from over-stimulating the body’s immune system and causing inflammation, air pollution may also impact the mucous membrane lining various cavities in the body. The hypothesis is that air pollution may damage this defensive lining, perhaps changing its permeability. ‘This may allow more allergens in,’ notes Brauer. In Baltimore, Maryland, Murugappan Ramanathan at Johns Hopkins found two different effects in his mice exposed to particulate matter (Am J Respir Cell Mol Biol. 2017, doi: 10.1165/rcmb.2016-0351OC). There were more pro-inflammatory compounds. But the particulates also disrupted the epithelial cells that formed the lining of the nose. The findings were supported by a human study. ‘The particulates almost punch holes in the epithelium, thereby allowing other things to get in. If you are exposed to particulates for a long time, you are probably more prone to adverse effects later on,’ suggests Ramanathan, who led the study. ‘It might also make allergens more allergenic.’
Meanwhile, the bad news about particulate matter keeps flowing. One surprise for researchers is that negative effects seem to continue even at relatively low levels. Ramanathan notes that 60mg of particles per square metre caused inflammation in the nose of the mice, whereas some of the world’s cities have levels in the hundreds of milligrams. The US has made progress in air quality; Europe, not so much. ‘Europe stagnated. This is partly due to the increasing use of diesel [vehicles],’ says Dora. And it seems there is no safe level. ‘Just because regulatory bodies set a threshold below which we are encouraged to reduce particulate matter levels, that doesn't mean that threshold level is harmless,’ Henderson explains.
Brauer agrees and says there is bad news here for Western cities in particular. ‘We’ve been trying to understand the relationship between particulate matter levels and, say, mortality and different health impacts. We’ve tried to find a safe level and we haven’t been able to identify one yet,’ he explains. The relationship is non-linear and this may be counterintuitive. ‘It seems the curve flattens out as you get to high levels, such as in countries such as China or India, but it is steep at lower levels,’ says Brauer. North America and Europe can still get significant benefit from relatively small reductions in PM, because the harm curve is steep at relatively low PM levels.
Brauer reaches for an analogy to explain why this means wealthier countries need to prioritise reductions just as much as more polluted countries such as China. ‘Think about smoking. If you go from two packs to one pack a day you don’t get that much benefit. That is China or India reducing its levels of particulate matter. You get some benefits if you cut down to half a pack or quarter a pack, but the real benefits come from quitting completely. That is the situation North America is in right now,’ he explains. The World Health Organization guidelines recommend an average of 10 micrograms per m3 of PM 2.5 averaged over a year, but health impacts are seen down to 6 micrograms per m3. Europe’s target is 25. That means action on air quality are cost effective, though the public is unlikely to notice improvements, given that particulate matter is invisible, odourless and tasteless.
How to cut down on particulate matter and reduce exposure is clear. First, older diesel vehicles are the worst offenders or heaviest emitters, so governments should encourage scrappage schemes or keep them away from cities. Older buses in London are being retrofitted with particle traps, as an example. And from January 2018, every single-decker bus added to the London fleet will be electric. ‘Most of the pollution problems in London come from the surface transport fleet. Buses probably contribute 12 to 15% of all particulate matter pollution. Taxis another 10%,’ says Kelly.
The harm from car exhausts fumes can be reduced by keeping tailpipes away from cyclists and pedestrians. ‘You get benefits from moving a few metres from traffic if you are a cyclist. The further you go away from a road the better; it is almost an exponential decay function. At 150m, you are near background levels,’ says Brauer. This is adds to the support for pedestrianizing city streets, which is under discussion for busy Oxford Street in London, for example.
It is also possible to reduce particulate matter getting into people’s lungs using physical barriers. Jordi Sunyer at the ISGlobal in Barcelona, who reported on cognitive performance in Spanish schoolchildren and pollution levels within schools, found that trees were quite effective. There are some European funded projects trialing various barriers. For instance, in Guildford researchers are trying out hedging between roads and pedestrians.
A study in Berkeley, California, reported in June, showed that pollution levels vary from street to street and there are hotspots associated with locations such as warehouses or intersections (Environ. Sci. Technol., 2017, 51 (12), 6999). Sensors or apps could, however, show such information and empower pedestrians to make choices, allowing us to plot healthier walking or cycling routes. King’s College London’ City Air app has 20,000 registered users. It maps out the quickest walking route from A to B, but also a second route that may take a little longer but with lower pollution levels. ‘Until we clean up these busy roads and reduce emissions, we should give people alternative ways to protect their health,’ says Kelly. Meanwhile, progress in reducing PM emissions or how much of them we breathe in will pay health dividends for years.
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