Air pollution Q&A

Scare stories about air pollution regularly come across journalists’ desks and we get questions about them. Are diesel cars a danger to our health or should we worry more about coal-burning power plants? Is the air in our cities putting children’s health at risk?

As part of our Ask for Evidence campaign we're looking at the evidence behind transport policies and claims about air pollution. We've teamed up with scientists at King’s College London, the MRC-PHE Centre for Environment and Health, the NIHR Health Protection Research Unit in Health Impact of Environmental Hazards and Asthma UK to answer your questions. Should we be worried about a ‘European killer smog’? Will cycling down a busy street put your lungs in danger?

1. "HIGH #airpollution forecast into Friday . What will you do?" @CleanAirLondon

Dr Ian Mudway, Kings College London: “On a personal level I will simply go about my daily business, but I will walk to work along the back roads and I will encourage my children not to take part in strenuous outdoor activities during the day. At risk groups should follow the government guidance to avoid strenuous outdoor activities and individuals with asthma should ensure they have their medication to hand. I would also implore individuals who drive to consider whether they could leave their car at home on Friday, if viable alternatives are available. By doing so you will not prevent the pollution episode, but you will not be adding to the problem.”

2.“Needless or necessary? Paris bans every other car to ease pollution.” Anthony Walker

Environmental Research Group, Kings College London: “In March this year and March last year Paris instigated emergency measures to tackle high levels of air pollution. These included free public transport and speed restrictions. A week later they instigated an alternate number plate restriction preventing half of cars from being driven.

These pollution episodes were caused by settled conditions when polluted air circulated very slowly over north-west Europe. Particle pollution can remain in the air for a week or more. These pollution episodes were therefore not just in cities but covered whole regions. For instance, pollution in London and across large parts of the UK was almost identical to that in Paris for several days during the peak of the episode. But in contrast to the Parisians, we did nothing about it. In fact, there was practically no reporting of the London episode in the media, which focused exclusively on pollution elsewhere. Did the Paris actions work? The Parisian pollution network says yes.”

3. "Paris adopts emergency measures to reduce pollution - should London be doing the same/more?" Dan Curtis

Environmental Research Group, Kings College London: “Long-term exposure to the air pollution that we experience in our daily urban lives has a far greater effect on public health than short episodes. Reducing air pollution every day therefore would have a bigger health impact. For this reason over 40 German cities have abandoned emergency measures in favour of low emissions zones that ban the most polluting vehicles all the time – similar to the scheme introduced in London in 2008.

While UK policies have also focused on reducing air pollution every day, this March and last March’s pollution shows these measures are insufficient to protect against short episodes. Long-term air pollution exposure adds the equivalent of 29,000 deaths per year to the UK's accumulating toll so there’s a clear need to do much more. We should absolutely focus on driving down emissions that contribute to air pollution first, as that will provide the greatest health benefit long-term, but we should also have provision to mitigate against the risks of pollutant exposures to the population during episodes.

Currently during days of poor air quality the onus for reducing exposure is squarely placed on the individual. Individuals perceived as being at risk: i.e. those with asthma, bronchitis, emphysema or a history of heart problems, are advised to avoid strenuous outdoor activity. There are also numerous websites and mobile phone apps that allow individuals to plan journeys to avoid the most polluted environments. The question is whether this is a good enough response to an established health hazard, especially when episodes are often not reported widely in the media, or are reported only after the event has occurred.

Finally, and this returns us back to the question of Paris’s emergency measures, should the polluters themselves change their behaviour? Reducing the number of cars on the road will not simply prevent an episode from occurring, but within densely populated areas, restrictions on vehicle movement (which could be voluntary) would prevent the situation from becoming worse.”

4. "Oxford St broke acceptable #AirPollution limit 1,503 times in 2014. How does this compare with other cities?" Helen Shaw

5. "Limits on air pollution were breeched 1,503 times in 2014 - but on how many different days was this? Could it just have been on one day for example?" Alex Thompson

Environmental Research Group, Kings College London: “Both questions relate to the number of times the European Union nitrogen dioxide standard was exceeded in Oxford Street in 2014. There are two limits set by the European Union for nitrogen dioxide – an annual limit and an hourly limit, not to be exceeded more than 18 times in one year. The two values reflect health evidence relating to both long term (annual) and short term (hourly) exposure to this pollutant. The monitor positioned at the kerb of Oxford Street recorded 1,503 exceedences of the hourly mean limit value. This was spread over 214 days out of 266 days where data were available (there were some power supply problems). So it is fair to say that on most days of the year at least one hour exceeded the limit.

How does this compare with other cities? Following the reporting of the high concentrations of nitrogen dioxide on Oxford Street last year, we attempted to place this into a global context. Our concentrations are very high, though the global comparison is difficult because it very much depends on where the measurements are made. London has a relatively large number of roadside monitoring sites compared with other European cities and as nitrogen dioxide concentrations are highest near the road side, our levels will appear higher than other countries where measurements are made away from  major roads. This near road monitoring in London does however make sense, due to the large number of individuals living, or working in close proximity to busy roads.”

6. “In 2014 @WHO said 54% of globe r now in cities-hows it impacting the air?” @EloiseJohnston

Environmental Research Group, Kings College London: “Yes, for the first time in history, over half the world's population now live in cities. Put simply, more and larger cities result in increased vehicle use, congestion and hence exposure of larger numbers of people to undesirable levels of air pollution. Consequently, managing the air quality in the face of growing urbanisation is a major global health issue. Only 12% of urban dwellers enjoy airborne particle pollution that meets World Health Organisation (WHO) guidelines. Globally, lowest city air pollution is in Canada and Iceland and the highest in India (Delhi and Patna) and Pakistan (Karachi and Peshwar).

Half of the world's mega-cities have air pollution that is more than 2.5 times WHO guidelines, and in most places it is getting worse. Even in the wealthiest parts of the world it is not clear that urban air pollution is getting any better. The current emphasis on technical strategies to clean up our air is not working well, and in some cases, such progress as has been made is being undone by other trends – Europe's increased use of diesel cars and more wood burning, for example. More than ever we need to transform existing cites through design; reducing growing road transport dependency and providing clean home energy.”

7. “How does #London's air pollution rank globally compared to other #megacities & how are these set to change?” @EloiseJohnston

Environmental Research Group, Kings College London: “This is a difficult question and in a sense distorts the debate about actual air quality issues at specific locations. It is complicated because air pollution is not a single entity. For legislative purposes it includes particulate matter, ozone and nitrogen dioxide, and city rankings will vary for each pollutant; Beijing will be high for particulate matter, London for nitrogen dioxide, Mexico City for ozone etc. It’s also the case that what we term particulate matter is a simple descriptor for a range of things, such that particles in one city will be significantly different from those in another, especially on a global scale, so in a sense one is often not comparing like-with-like.

People have attempted to get around these issues by deriving air quality indices that attempt to combine the relative health risks attributable to different pollutants, with their measured concentrations. This is a very inexact science, as it’s still not clear how pollutants in mixtures interact, or from a health perspective whether their effects are simply additive, or overlap.  What we can say for the UK is that our particular problem areas, like many urban areas within Europe, relate to emissions of nitrogen dioxide and particles directly emitted from vehicle (particularly diesel) exhausts.”

8. “Do countries with higher/worse air pollution have higher rates of asthma sufferers?” James Clarke  

Asthma UK: “Sadly, it’s not quite as simple as that – we know that pollution can cause asthma attacks and research indicates that it can play a role in causing asthma to develop in the first place. But asthma is a complex condition and we are learning more about it every day; sadly asthma research has been chronically underfunded for many years so there is much more to learn. What we do know is that around 30 million people in Europe are currently living with asthma and 14 EU countries feature in the top 20 worst countries globally for asthma prevalence in adults. We hope that a reduction in pollution would have an impact and we’re currently funding research at the MRC Asthma UK Centre for Allergic Mechanisms of Asthma into the impact air pollution has on people with asthma. Asthma UK is also leading on the EARIP (European Asthma Research and Innovation Partnership) project, working at channelling the collective expertise of world-leading asthma experts across the continent in order to reduce the burden the condition has across Europe.”

9. “Cycling down Oxford Street on a smog-filled day. Better to avoid completely? What if the alternative is sitting in a bus on Oxford Street?!” James Boyle

Environmental Research Group, Kings College London: “On average, petrol and diesel vehicles are the main source of air pollution in London. Therefore, it is better to avoid cycling along busy, congested roads like Oxford Street whenever possible. This is true whatever the weather. Perhaps surprisingly, pollution levels inside a bus, or other vehicles, are often higher than those outside on a bike. This is because the bus travels right amongst the exhaust emissions of surrounding vehicles, and often sits idling in the area of peak pollutant concentrations. Conversely, while cycling you breathe a greater volume of air, therefore potentially a higher dose of pollutants. Never-the-less it has been shown that the health benefits of cycling outweighs the risk of harm from air pollution.

There is almost always an alternative ‘lower pollution’ cycle route in any city, so minimize that risk by cycling along quieter routes. From a personal point of view I never walk or cycle along Oxford Street as the roads that run parallel above and below are largely free of traffic and all of the shops have entrances at the rear.”

10. “Can we solve the air pollution problems in our cities by growing plants on our roofs?” Katie Roberts

Environmental Research Group, Kings College London: “Simply put, no. There are some studies using modelling approaches that have suggested that greening of highly polluted streets can significantly reduce pollution, but there is little validation of this research and numerous other studies have not supported this idea. Overall, these solutions are too small compared to the magnitude of the problem and in fact can even, in certain circumstances, make things worse.  Trees planted along roads can limit the dispersion of pollutants away from the roadside and certain plants are high emitters of volatile organic compounds during the summer months that contribute toward ozone generation. However, it is a politically appealing solution, because it provides an apparent alternative to targeting and legislating against the polluter/vehicle owner. There are many other benefits of greening the environment, but the potential impacts on urban air quality have been overstated.”

11.“Will a £20 idling charge actually make any difference [to air pollution levels] in the city?” David Norton

Environmental Research Group, Kings College London: “The £20 idling charge enforced by some London boroughs will not have a pronounced effect on overall urban air quality, but should be viewed as part of a range of measures that local government can use to reduce air pollution at specific hot spots, such as outside railway stations, amenities, hospitals and schools. Whether the scheme is effective will ultimately depend on how well and how fairly it is policed.  If the approach is too heavy handed, then ultimately, the public health message could simply be drowned out by the view that it is simply a revenue generating scheme. If the scheme simply encourages individuals to think about their own vehicle emissions it will be positive. Also, somewhere within the debate the fuel cost savings of not leaving a vehicle idling for long periods should also be emphasized.”

12. "As someone who suffers from asthma should I think twice before working in the city? Or commute at off peak times?" Jennie Field

Asthma UK: “You’re not alone, 1 in every 11 people has asthma and two thirds of them find that pollution makes their asthma worse. Decisions about where you work or when you commute are big ones, and ones that shouldn't be taken lightly. The best way to manage your asthma so that you don’t react when you come into contact with asthma triggers, like pollution, is to take your asthma medicines as prescribed. If pollution triggers your asthma, and you are taking your preventer inhaler daily then you shouldn't experience symptoms as the medicine builds up your resistance over time. In addition we recommend that you carry your reliever inhaler with you at all times in case you do have an attack. Understanding your condition and knowing what to do if you do feel unwell is vital.

The best way to do this is to use a written asthma action plan – if you have one you are 4 times less likely to need to go to hospital because of your asthma - yet only 30% of people have one. If you’re managing your asthma you should be able to live relatively symptom free – in which case it shouldn't impact on where you work or how you travel in to your office. You can understand your risk of an asthma attack by taking our risk test.”

13. "What are the EU regulations for air pollution? Are they fit for purpose?” Chris Peters

Environmental Research Group, Kings College London: “EU air pollution regulations cover 12 pollutants: fine particles, larger particles, Nitrogen dioxide (NO2), Ozone, Sulphur dioxide (SO2), Carbon monoxide (CO), Polycyclic Aromatic Hydrocarbons, Benzene and a range of metals (lead, arsenic, cadmium and Nickel). Full details can be found on the EU’s air quality webpage.

European Union air quality standards are based on the best available scientific knowledge at the time, where there is an overall consensus, with the aim of minimizing the harmful effects of air pollutants on human health. However, unlike the World Health Organization’s guidelines, which are based solely on health impacts, the EU limits are more pragmatic and account for the feasibility of compliance and the likely economic impacts. The EU limits are therefore often set higher than the equivalent WHO or other national guidelines and this is the critical issue in relation to the question.

For example, the EU limit for fine particles is set at 25μg/m3. For the same pollutant the WHO set a limit of 10μg/m3 and the US National Ambient Air Quality Standard is 12μg/m3(averaged over 3 years). Given that health effects of fine particles have been reported below the EU limit value, they cannot be said to protect the public from adverse health effects, they simply minimize risk. Therefore, not meeting the EU limits should be viewed in a very dim light. So are the current EU limit values fit for purpose? In terms of absolute protection of public health, probably not. As a legal mechanism to drive down emissions to reduce risk, a conditional yes. Though in the latter case this requires commitment of national governments to drive down emissions, even once the limit values have been attained. A requirement for Member States to do this is incorporated into the Directive via the ‘Exposure Reduction’ obligations, which require targets to be met for fine particles even if the above limits are achieved."

If you want to talk through your asthma with a specialist asthma nurse, you can call the Asthma UK Helpline on 0800 121 62 44, or speak to your GP or asthma nurse if you are struggling with your asthma symptoms. To get alerts about pollution levels, follow Asthma UK on Twitter at @asthmauk.


Smog headlines



Professor Ross Anderson

Ross is a respiratory epidemiologist with a long-standing interest in the epidemiology of asthma and the health effects of air pollution. His early air pollution research was into the effects of indoor biomass burning in Papua New Guinea but over recent decades he has been mainly concerned with outdoor air pollution. Most of his research in this area has been concerned with time-series studies in the UK, Europe and Hong Kong and has included extensive systematic reviews and meta-analyses of time-series studies. More recently, he has been investigating the effects of long term exposure to air pollution using health datasets from the UK and ISAAC. His main current interest is in investigating the health effects of traffic pollution in London.

Dr Ben Barratt

Ben has worked at KCL’s Environmental Research Group since 1994 and helped to formulate and develop the London Air Quality Network. He now specialises in assessing the impact of local air quality management initiatives and assessments of personal exposure to air pollution. Recently, he has helped assess the impact on traffic and air pollution of the London Low Emission Zone and the London congestion charging scheme on air quality.

To facilitate this research Ben has developed an expertise in assimilating and handling large datasets from diverse sources. He is currently developing a research resource combining pollution monitoring data from across Europe and North America, representing over 1.5 billion data points from 15,000 monitoring locations.

Dr Sean Beevers

Sean is currently involved in developing KCL's CMAQ-urban modelling capability, which can model air quality from a European scale down to local street scale, as well as a new generation of road traffic emissions inventories in the UK.

His group's work is widely used for measuring exposure to air pollution around London. KCL's emissions inventory capability has also provided inputs to EU projects and for use by the Greater London Authority as part of the London Atmospheric Emissions Inventory. Sean has also looked at the impact on air pollution of measures to tackle climate change, which was used as evidence at the Copenhagen climate change conference. As part of the ongoing 'Traffic' research project, Sean is also developing a hybrid exposure model for London, combining CMAQ-urban with people’s movements through the city.

Dr Gary Fuller

Gary is a senior lecturer in air quality measurement at KCL. He led the development of the London Air Quality Network to become the largest such network in Europe. His research looks at trends and changes in urban air pollution. Working closely with toxicologists, clinicians and epidemiologists, his work promotes the best use of air pollution measurements in health studies, working towards a better characterisation of pollutant exposure.

Professor Frank Kelly

Frank is director of KCL's Analytical and Environmental Sciences division. He is also director of the Environmental Research Group at the Centre for Environment & Health. From these dual positions he is able to combine his two main research interests: free radical/antioxidant biochemistry and the impact of atmospheric pollution on human health.

Frank is a past president of the European Society for Free Radical Research and former chair of the British Association for Lung Research. He is also involved in providing policy support to the World Health Organisation on air pollution issues and is a member of the Committee on the Medical Effects of Air Pollution.

Dr Ian Mudway

Ian is a lecturer in the Analytical and Environmental Sciences division at KCL. His research on respiratory toxicology is mainly focused on how the human lung responds to air pollution. He has been involved in several projects assessing air pollution and its effects. He initiated a study into the impact of London’s Low Emission Zone on the respiratory health of children. His work has also involved considerable outreach activity in schools in East London.

Dr Martin Williams

Martin's research interests lie in applying atmospheric science to policy on air quality, the relationship between air quality and health, and links between air quality and climate change. He is particularly interested in the effectiveness of air quality policies on urban and regional air quality as evidenced by measurement. The impact of aviation on air quality is a big part of this work, and Martin was involved in assessing the impact on air quality of developments at three major UK airports. He also holds a continuing interest in ozone's role as a pollutant, having developed a model used to forecast ozone levels.

Asthma UK

Asthma UK’s mission is to stop asthma attacks and cure asthma. We do this by funding world leading research, campaigning for improved care and supporting people to reduce their risk of a potentially life threatening asthma attack. We are entirely funded by voluntary donations.

Featured image: Ruben de Rijcke/Wikimedia Commons

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