This December scientists, journalists and policymakers will gather in Madrid for the United Nations climate change conference. Much of the media coverage will focus on carbon dioxide emissions. Reserves of fossil fuels beneath our feet are no longer being viewed as valuable national resources but as dangerous substances that must be left safely in the ground if we are to prevent uncontrollable global warming.


But carbon dioxide is not the only pollutant in our air. The exhausts from traffic, the smoke from our factories and even our home heating cause far more harm than just warming our planet. Globally, it is estimated that over 5 million people each year are dying early from breathing polluted air. In Europe it is around 400,000 people, and, in the UK it is estimated that around 32,000 people die early each year from breathing nitrogen dioxide and particle pollution.

Some of the pollutants that harm our health also harm our climate. It is therefore surprising that our actions to tackle air pollution and the climate crisis are not more joined up. In fact, some actions to reduce climate emissions had little positive impact and ended up worsening local air pollution.

Do petroleum products like petrol and diesel have a shelf-life? © Getty Images
The emissions from diesel-fuelled cars were put sharply into focus during the 2016 Dieselgate scandal © Getty Images

A perfect example is diesel cars. In Europe, for years, tax inducements and manufacturer marketing told us that diesels were better for climate change when compared with petrol vehicles. However, this is now being questioned.

Many diesel car owners found a large gap between the fuel consumption claimed in showrooms and fuel use on our roads. This not only meant spending more money on fuel at the pumps than expected, but that the real-world benefits of reduced carbon dioxide from diesel was not as great as we thought.

Additionally, the exhaust from older diesels contains sooty black carbon particles. Black carbon particles are bad for our health and they are bad for climate change too since they are strong absorbers of the sun’s heat. Due to Europe’s geographical position and mainly south-westerly airflow, it is the black carbon from Europe that dominates the soot deposited on Arctic snow, where it has a climate warming effect and encourages snowmelt, also bad for climate.

More like this

Read more about the Dieselgate emissions scandal:

But the problems with diesel do not stop there. The dash for diesel has also been a victim of its own success and demand for diesel exceeded Europe’s refineries production capacity. Extra diesel, therefore, had to be imported, much of this from refineries in Russia. These were old and inefficient, further eroding the apparent climate benefit from diesel.

Therefore, it is not clear that increasing diesels on our streets was good for climate and it certainly was not good for air quality and therefore our health. In addition to the particle emissions, the popularity of diesels is the main reason that most urban areas across Europe are now almost a decade late in meeting legal limits for nitrogen dioxide.

Wood burning is an example of a renewable energy source often seen as being climate neutral. Increasing amounts of wood are being used to generate electricity and for direct heating too. No-one would disagree that it is wonderful to sit around a wood fire and many people believe that wood heating reduces their carbon dioxide emissions compared with heating with fossil fuel, but this is less clear when you look more deeply.

Obviously, the burning of wood releases carbon dioxide in the same way as burning fossil fuels does. But for the same amount of heat, wood-burning produces more carbon dioxide than burning coal and around twice as much as natural gas. This is because of the chemical composition of wood and its moisture content. Wood is like a sponge, and even dry wood can consist of around 20 per cent water by weight. Unseasoned wood can be 40 per cent or more. This water must be driven off in the fire as the wood burns, and that takes energy.

Do London plane trees actually absorb pollution into their bark?

Asked by: Lucas Moore, Swansea

They don’t absorb it, but pollution particles do get stuck to the bark and trapped in the hairs on the leaves. A 2011 study estimated that every year, trees in Greater London remove 850 to 2,000 tonnes of PM10 pollution particles, which are the type considered harmful to humans.

Plane trees do emit isoprene though, which combines with nitrous oxide in car exhaust emissions to produce harmful ozone. Luckily, this only reaches dangerous levels in temperatures above 30°C, which are rare in the UK.

Read more about trees and carbon:

Burning wood releases in just a few minutes the carbon dioxide that was stored in the trees for decades or centuries. The idea that wood burning is climate-neutral arises from the reabsorption of this carbon dioxide as trees grow. But that process takes time. So, for a period, there is more carbon dioxide in the air from burning wood than if we burnt a fossil fuel and left the tree growing in the forest.

In the worst-case scenario, if we cut down and burn mature trees that are at the peak of their carbon-absorbing capacity, the payback time can be more than a century compared with burning fossil fuels and leaving the tree in the forest. The payback time is fastest if we burn wood offcuts produced by forest management.

The climate change benefits from wood-burning are therefore less clear than they first seem, and we need to be cautious in describing wood-burning as beneficial to the climate. Avoiding the production of additional carbon dioxide from wood-burning over the next few decades could be critical if we are to avoid irreversible climate tipping-points and limit the maximum global temperature rise.

The renewed popularity of wood heating is adding to our air pollution too. Particles from wood burning are now commonplace on winter nights in UK cities and those across western Europe. Home heating with wood is now the largest single source of particle pollution in the UK.

We also need to talk about combined heat and power systems. Few of us will have heard of them but they are in many buildings. Converting fuel to electricity in power stations is inherently inefficient, at best half of the energy is lost as waste heat. As part of the response to the coal-smoke induced smogs that once plagued UK cities, power generation was moved into the countryside.

The chimneys of London’s Battersea and Bankside (now Tate Modern) power stations no longer belch smoke, but in the countryside, there are few opportunities to utilise the waste heat from generating electricity. By moving power stations back into cities, the waste heat could be used in district heating, but this requires a large investment to create hot-water grids.

On a smaller scale, electricity can be easily generated in the boiler rooms of individual buildings and factories. The waste heat can then be used on-site in manufacturing or in places where there is year-round demand for heating or hot water such as hotels, your local hospital or swimming pool. These small-scale power stations are not subject to the same pollution control standards that apply to large industry.

Read more about how to tackle air pollution:

Although utilising waste heat is beneficial from a climate change perspective there is a risk that a return to power generation in urban areas might increase air pollution. A recent survey of planning applications suggests that around 2,500 small-scale combined heat and power plants could be operating in London in the next few years. Separate air pollution projections by King’s College London found that widespread adoption of combined heat and power in UK urban areas over the next decade could offset the air pollution improvements being brought about by other polices.

In common with diesel and wood burning, questions are also being asked about the climate benefits of these technologies. We could do better by using heat pumps powered by renewable electricity.

There are many win-wins to be achieved for climate and air pollution and some come with massive bonuses too

My new book, The Invisible Killer, explores our current air pollution crisis and how our past solutions have been hampered by tackling one pollutant or one source at a time rather than looking at the way that we use our air as a waste disposal mechanism.

There are many win-wins to be achieved for climate and air pollution and some come with massive bonuses too. Investment in renewable energy sources such as wind, solar and wave power are clear examples of win-wins. Improving the energy efficiency of hospitals, schools and offices would reduce both air pollution and climate change emissions. Doing the same for our homes could also reduce the numbers living in fuel poverty.

There is huge scope for tackling inefficiencies in our road transport too. Around a quarter of car journeys in England are less than 2 miles, a distance that many of us could walk or cycle or take public transport, and, in many cases, it would be faster. Rather than swapping diesel and petrol vehicles for electric or hydrogen, reducing road traffic and increasing walking and cycling would help to tackle climate change emissions, health harmful air pollution, the urban noise that plagues so many as well as the chronic health conditions caused by inactivity in our everyday lives.

It does not stop there. Almost 40 per cent of vans in London are less than one-quarter full, wasting much of their carrying capacity. This should prompt us to re-think and reduce the deliveries that we have at work and at home. Incidentally, these loads of 100 to 200 kilogrammes are ideally suited to electric cargo bikes.


Looking forward there are huge opportunities from ensuring that our climate and air pollution policies take us in the same direction. This is not just an opportunity for developed countries such as the UK. The fast-growing countries of Africa are in a unique position to leapfrog dependence on fossil fuels and adopt abundant renewable sources like wind, hydro and solar power. A new study has suggested a health burden of 48,000 premature deaths each year by 2030 as well as substantial climate change emissions can be avoided if clean technologies are pursued.

The Invisible Killer: The Rising Global Threat Of Air Pollution — And How We Can Fight Back by Gary Fuller is available now (£12.99 on Hive, Melville House)
The Invisible Killer: The Rising Global Threat Of Air Pollution — And How We Can Fight Back by Gary Fuller is available now (£12.99 on Hive, Melville House)