17

Environment and Climate Change Committee 

Corrected oral evidence: Nitrogen

Wednesday 26 February 2025

10.50 am

Watch the meeting 

Members present: Baroness Sheehan (The Chair); Lord Ashcombe; Lord Duncan of Springbank; Lord Jay of Ewelme; Lord Krebs; Lord Mancroft; Lord Rooker; Earl Russell; Lord Trees; Baroness Whitaker.

Evidence Session No. 4              Heard in Public              Questions 31 - 36

Witnesses

I: Professor Anna Hansell, Professor in Environmental Epidemiology, University of Leicester; Professor Alastair Lewis, Professor of Atmospheric Chemistry,  University of York

Examination of witnesses

Professor Anna Hansell and Professor Alastair Lewis.

Q31            The Chair:  We are now going to move on to the second panel of the day, looking into the public health impacts of nitrogen pollution, and we are going to focus now on air pollution. I would like to welcome our two witnesses, Professor Alastair Lewis and Professor Anna Hansell. Before we move to the first question from Lord Ashcombe, may I ask each of you to introduce yourself briefly?

Professor Anna Hansell: Thank you very much for inviting me to be a witness on this panel. I am a professor of environmental epidemiology at the University of Leicester and by training originally a hospital doctor and a public health doctor. I have been working in environmental epidemiology for around 25 years now, with particular interests in air pollution, noise and chemical exposure.

Professor Alastair Lewis: Morning, everybody. I am Professor Alastair Lewis from the University of York. I am a professor of atmospheric chemistry and currently chair of Defra’s air quality expert group, which provides advice on the physical science aspects of air pollution.

Lord Ashcombe: Thank you for coming. We heard about nitrates in food and water earlier, and now, as the Chair mentioned, we are moving to air. I wonder whether we should ever be alive when you look at some of what is going on, but what are the main health effects of ambient air pollution, particularly from nitrogen oxides, ammonia and fine particulate matter? As a result of this, are some groups of people more vulnerable than others, be it age, ethnicity or whatever?

Professor Anna Hansell: This could be a fairly lengthy answer. Additional background would be helpful. I chair the Committee on the Medical Effects of Air Pollutants, and have done since 2021. I am not speaking as chair of the panel, but I will draw on a number of the documents that COMEAP has produced and also from the Chief Medical Officer’s 2022 report on air pollution.

My preamble is to say it is very welcome to have a holistic view of nitrogen. I work very much in a planetary health perspective, and I think that considering the effects of nitrogen on human health cannot be divorced from thinking about the effects on the environment. Human health and environmental health are very intricately linked. I will focus my evidence on the human health impacts here.

I have structured the evidence into different parts. There are nitrogen oxides, ammonia and particulates, the differential toxicity of particulates and then the actual health effect of particulates. Then there is the second part of the question about vulnerability. The evidence is very strong that air pollution has impacts on many aspects of health and affects many parts of the body, but the strength of the evidence varies in terms of the outcomes and the pollutant that you look at. There are some things where we would say we have some certainty that there is a causal relationship, and there are some where we can say it is likely or suggestive. Our evidence base comes from epidemiological studies, which we review in systematic reviews to try to identify good quality studies in a structured format. That is put together with evidence on mechanisms in lab studies and experimental studies. That is where the evidence is broadly taken from.

The health studies consider NO2 in the nitrogen oxides, because that is the predominant exposure. There has been some debate over some decades about whether the associations with NO2 are causal or whether NO2 is a marker for other pollutants. With the evidence that is now available, I think the majority view is that both views are correct in that there is some direct causal effect of NO2 for some outcomes, and there are some areas where NO2 may be a marker for other pollutants. For example, cars are producing particulates and NO2, so the NO2 may be in part a marker of the emissions from an exhaust.

The health effects are divided into the evidence looking at short-term exposures—that is what you are exposed to today or in the past few days—and long-term exposure, which is usually taken as at least one year but might be 10 years or more. In terms of short-term exposure and NO2, the evidence is strong for a causal relationship with all-cause mortality and respiratory effects—that is short-term exacerbation of asthma inflammation of the airways. That evidence is from both epidemiological studies and experimental chamber studies where we expose people with mild asthma to NO2. The evidence is suggestive, but not sufficient for causal relationships with cardiovascular disease. There are some other impacts that have been looked at which are metabolic disease, primarily diabetes, fertility, birth outcomes and the nervous system. There is some evidence, but it is not at the level that we would say it is causal, and we can quantify it.

In terms of long-term exposure, there has been quite a lot of debate over the role in mortality, but the current evidence is supportive of a causal role for both all-cause and respiratory mortality for NO2. In fact, COMEAP produces mortality figures for NO2 and for combined NO2 particulates, which I will come on to later on. In terms of long-term exposure and respiratory outcomes, there is good evidence for a causal relationship with asthma development. For the other respiratory outcomes, the evidence is less clear as causal. Some associations have been seen with cancers, particularly lung cancer and breast cancer, and similarly with dementia and cognitive decline, but it is not taken as definitely causal as yet. So that is a brief, quick run through NO2.

Ammonia is not considered as a pollutant that is looked at in terms of health impacts because it reacts in the environment to produce secondary particles, which I am sure you have had other evidence on, primarily ammonium sulphate and ammonium nitrate.

The Chair: We can speed up this part, as we have another few questions, although that is very useful.

Professor Anna Hansell: These particulates[1] are potentially maybe half of the particulate load. COMEAP has looked at different types of particles, because there are many different types, but has concluded that while different constituents are likely to have different toxicological actions, it does not consistently indicate that some PM components are more toxic than others, so we just take it as mass at the moment. I am happy to take questions on some of the evidence underpinning that, because it is quite interesting.

The strongest evidence is for particles that are less than 2.5 micrometres in diameter, because they reach very deeply into the lungs. There is evidence for short-term associations with all-cause mortality, cardiovascular and respiratory mortality, respiratory hospital admissions, CVD admissions, cardiovascular disease admissions and various physiological changes that might underpin what the mechanisms are. In terms of long-term PM2.5, there is very strong evidence for all-cause mortality, CVD, respiratory and lung cancer mortality and also for dementia, which COMEAP reported on in 2022, as this was likely to be a causal association.

Are some groups more vulnerable? Yes. There are two aspects of vulnerability: do people get exposed more, so are they more vulnerable because of higher exposure—and then are they more vulnerable because they are more susceptible? Generally speaking, children and the elderly are held to be more vulnerable to the effects of air pollution, and those with pre-existing disease, such as cardiovascular disease and respiratory disease. There is quite a number of studies suggesting that deprivation results in increased susceptibility.

It is not clear which of many potential factors might be involved in that, but one of my own studies looking at lung function and particles found a two-fold increase for impacts on lung function with lower income households.

The Chair: I am very mindful of time and that we have quite a lot to get through, but I am also very appreciative of the information that you are giving us. I understand that there is very useful information to the inquiry, which we may not have time to cover in the 45 minutes available to us, but we would be nevertheless very grateful to receive that information in written evidence, so we thank you for that. Lord Ashcombe, do you have any supplementaries or shall we move on to?

Lord Ashcombe: I think in the interests of time, we should probably keep moving.

Q32            Earl Russell: Can I ask you about ground level ozone? How significant a concern is that for you and your colleagues? What are the health risks? Are they different to other kind of pollutants that we see?

Professor Alastair Lewis: Ozone is a secondary pollutant. That means that it is formed through chemical reactions in the atmosphere, of which one key ingredient that you need is nitrogen dioxide. It is well-established science; it is essentially the processes that were discovered in California in the 1970s. That formation takes place over days, so it is a regional pollutant. The ozone climate that we get in the UK at ground level is formed by what we do and what happens in mainland Europe, and a little bit by what happens in North America as well, so it is really more appropriate to think of it as a hemispheric scale pollutant, to which we make a small contribution. The pollutant itself at ground level goes on and has some health impacts—Anna might want to talk about those—and its deposition to the surface has ecosystem impacts; it particularly reduces crop yields. It is a significant pollutant in its own right, and quite difficult to control because the ingredients that you need to make it are brought in from all sorts of different places.

Professor Anna Hansell: The health impacts are strongest for short-term exposures. There is a causal association accepted with all-cause mortality. There is strong evidence for respiratory effects on symptoms such as asthma symptoms and respiratory admissions. In terms of long-term exposure, the association with mortality is suggestive but not conclusive, and COMEAP would not produce mortality estimates for ozone. There are likely causal relationships for long-term exposure with respiratory effects such as asthma but, again, the evidence is not as strong as for the short-term exposures.

The health risks are quite complicated to interpret in the UK because of the time/space correlations with other pollutants. In some studies, because of the chemistry you find an inverse relationship between NO2 and ozone. We think that is because of chemistry: NO2 scavenging ozone, and NO2 helping ozone form. You can look at it slightly differently by looking at the oxidative capacity of both those pollutants. That is a new approach being used to try to understand what is going on in terms of the health impacts, but that is not being widely used at the moment.

Lord Ashcombe: You mentioned that it is difficult in the UK to measure. Does that suggest it is difficult everywhere in the world, or are there are countries where it is easier to measure, such as maybe Australia or New Zealand, which are somewhat more remote?

Professor Anna Hansell: It is easier to measure where you have hotter climates and greater levels—so the US is where quite a lot of the evidence is coming from for ozone.

Professor Alastair Lewis: We experience peaks in ozone now that are much lower than they were 30 years ago. Ozone is measured in concentrations in parts per billion. Hot days in summer in the UK could exceed 200 parts per billion in the 1990s. It is very rare now for the concentrations to exceed perhaps even 90. There have been really significant reductions, so those peak events that might be kind of an epidemiologists dream to see effects are beginning to lessen. We do not see the extremes in ozone in the UK now that we have seen in the past. International policy has made that impact.

Professor Anna Hansell: It is harder to model, because we do not have very comprehensive information on volatile organic compounds concentrations, because the monitoring is very limited on that. You would need that to get a very detailed model for the epidemiological study.

The Chair: Staying with ground level ozone for the time being, to what extent does it damage crops, and where in the country can we find examples of this damage?

Professor Alastair Lewis: Crop yield reductions depend on the crop and the concentration. In terms of UK crop reductions, it is in the single figure percent to perhaps 10% depending on the crop. There are other parts of the world where ozone is much higher, and the sensitivity is much higher. Measuring that is quite complicated. It is done mainly on experiments, essentially where you perturb the ozone then you look for the difference. The places where ozone is highest and the impacts are largest tend to be towards the east of England, because you need the ingredient of nitrogen oxides, which have come from traffic and combustion, those volatile organic compounds and sunlight. There is a strong east/west split, because we obviously get drier, sunnier and warmer weather in the east of England. If you were going to point at one location, you might look at East Anglia as the most exposed to ozone.

The Chair: Before we move on to Lord Jay’s question, you have talked quite a lot about nitrogen dioxide and its role in forming ozone. Can we talk a little bit about nitrous oxide and ozone depletion?

Professor Alastair Lewis: There are a lot of nitrogen oxides out there. Some of them are quite short lived. Nitric oxide—NO—lives in the atmosphere for only perhaps a few tens of seconds; once it meets ozone, it reacts. That is the very short-lived one. The other end of the spectrum is nitrous oxide, N2O, which is a greenhouse gas, which has a lifetime of many tens of years. So it is a big spectrum, and you have to get your pronunciation right, because it is easy to mix them up. Nitrous oxide is typically emitted in the UK as a consequence of fertiliser use and from soils. Because it is long lived, it has the potential to move through the layers of the atmosphere and find itself in the stratosphere, in a way that some of the other oxides of nitrogen do not. To reach the stratosphere, you have to have an atmospheric lifetime measured in years, because it takes so long to percolate upwards and across the barrier into the stratosphere. Nitrous oxide will do that, and it is one of the components that contributes to stratospheric ozone depletion, so taking ozone in the opposite direction.

The Chair:  Would it be correct to say that nitrous oxide is now the main cause of ozone depletion in the upper atmosphere?

Professor Alastair Lewis: It is not the main cause. There are a lot of different components of which halogenated compounds are obviously the key thing that we have tried to control for, but as some of the most significant halogenated compounds have been regulated, the role of N2O has grown. It is a greenhouse gas as well, so there are many good reasons why nitrous oxide is a molecule that is the in target of regulators. There are a lot of areas where you get benefit from its emission reduction.

Q33            Lord Jay of Ewelme: I would just like to take a little further what Professor Lewis was saying about the geographic variations in emissions of ammonia, nitrogen oxides—I must be very careful about my pronunciation here—and fine particulate matter in England. I wonder whether some areas, whether we are talking about cities, rural areas or roads, are particularly affected by some pollutants more than others. If so, are there associated health impacts for certain populations?

Professor Alastair Lewis: The distribution of nitrogen oxides is the easiest one. There are quite short lived, so the highest concentrations typically occur where the emissions occur. If you looked at a map of the UK you would see the motorways and Drax power station. You can see the sources. That pollutant reacts away relatively quickly, so it is very focused at the source, and that does have impacts on different populations. Typically, the more deprived you are, the closer you are going to live to a source and the higher the concentration is going to be. The nitrogen oxides are quite straightforward. We know where they are, and we know who they affect. On the ammonia emissions, where we release these compounds is very heavily skewed to where agriculture occurs in the UK, so where dairy farming is active, and poultry farming, pig farming and so on. The reactions that convert ammonia into particulate matter take place over the course of a few hours; they mix with other pollutants, and then that kind of smears out the pollution over the whole of the UK.

So if you look at where the ammonia ultimately ends up in particles and where that ends up, it is more pushed to the south-east of England, for a couple of reasons. One is that the south-east of England is more exposed to European pollution, so we mix everything together in the south-east corner, and particulate pollution and ammonia are also quite effectively removed by rainfall. So the big difference in rainfall that we have between west and east also amplifies that effect.

For nitrogen oxides it is quite straightforward: where you emit tends to be where you experience it. For ammonia emissions, the transformations mean that it tends to occur towards the south-east of England at its maximum.

Lord Jay of Ewelme: Thank you. Professor Hansell, did you want to add to that?

Professor Anna Hansell: I think that Professor Lewis has covered the key issues there. Basically, if the exposures are higher, there are more health effects, so it is quite a straightforward relationship.

Lord Jay of Ewelme: And where would you both choose to live in order to be safe?

Professor Alastair Lewis: Well, I live in North Yorkshire, in the Yorkshire Dales, so I benefit substantially from very low nitrogen oxide emissions—but living in a rural environment I shall probably experience higher ozone levels than an urban resident would. To come back to the ozone question, just because you do not live in a city does not mean that you escape the impacts of air pollution. Rural communities are exposed to particulate matter and to ozone as well, so it is not entirely an urban phenomenon, even though we tend to talk about it mostly as an urban effect.

Lord Jay of Ewelme: And how is Leicester, Professor Hansell?

Professor Anna Hansell: It is about the suburban areas and taking care with transport routes—so not walking along the side of the road if I can walk through the park instead.

The Chair: I wonder if you can help to understand PM2.5s better. Is it that soot and dust is a prerequisite for PM2.5s, or can ammonia reacting with nitrate and sulphates to some extent in the atmosphere account for PM2.5s that we see from nitrogen pollution?

Professor Alastair Lewis: The first thing to say is that PM2.5 is a terrible metric, and I do not think that anybody in atmospheric science would ever invent it today, if we were given the chance. It is the broad classification of everything smaller than 2.5 microns, so it is a real mixture. Some of those components are liquid and some are solid, which is probably what you are referring to—the solid particles. In this room now it will be a mix of the two, probably solid particles coated with liquids. It is not a prerequisite. If you go to a very clean atmosphere, it is possible for particles to form through a process called nucleation, which is where the atmosphere oxidises a relatively volatile gas and turns it into a relatively involatile component, which wants to condense—and when condensed they stick together and a new particle forms. So you do not necessarily need the kind of things that we might think of as gunk, such as little black soot particles. We can form particles from, essentially, thin air, through oxidising, particularly sulphur dioxide.

On the actual components of PM, in an urban environment there are probably 100,000 different chemicals inside PM. That is why it is so difficult for someone like Anna to do the epidemiology on them, because it is not one component—it is hundreds of thousands of different components, of which plausibly there is a case that many of them are going to be harmful to health. If you list out all those chemicals and ask which ones people would fancy ingesting, there are relatively few on the list where you would say, “Yes, I’d be happy with that”. So it is a metric that has got a lot of use and appears in a lot of regulations, but it is not a very helpful one from the point of view of trying to control emissions, because it is this enormous basket of different chemicals. Of relevance to this inquiry are the subset that are nitrogen-containing.

Q34            Lord Mancroft: How far does ammonia from agricultural sources carry, distance-wise? What impact do you think that that has on urban air quality?

Professor Alastair Lewis: It is a very varied answer. If you were in Cornwall and it was pouring with rain and you had a farm, it would be not very far. It would be washed out very close to where the emissions occurred. On a dry day, with the wind blowing, some of that ammonia will stick to surfaces as it leaves the farm and gets into the atmosphere, and it could be transported for hours or potentially days, when it goes through those reactions and converts. It is a very variable process—but averaged over the whole year, we see that ammonia can very comfortably move between countries and across continents. What occurs in the UK absolutely affects what happens in mainland Europe averaged over a year, and what happens in our near neighbours in continental Europe absolutely affects the UK as well. I hope that that gives a feel for the scale—but if it pours with rain it does not get very far. If it is dry and the wind is blowing, it can escape the farm, the trees and the nearby environment, and it can move around hundreds or thousands of kilometres.

Lord Mancroft: The Irish must be quite safe.

Professor Alastair Lewis: Well, they are the first upwind emitter. You could say that we are second best-off, because we are second down the line. It gets harder and harder to control particle pollution the further into Europe you go. If you are in eastern Europe—in Poland, for example—what is arriving is already very polluted. So we sit in quite a privileged position in the UK and Ireland, because we have the whole of the Atlantic Ocean upwind of us. That does not mean that air pollution is not an effect here, but we have to be conscious that we are a contributor to air pollution effects in mainland Europe as well.

The Chair: As an addendum to that, can you highlight whether it adds, and to what quantity it adds, to urban air pollution?

Professor Alastair Lewis: Yes, sorry. One thing that is quite hard is to disaggregate where the nitrogen has come from. I think that Anna mentioned at the start that potentially half of the mass of PM2.5 could be nitrogen related, as ammonium nitrate or ammonium sulphate. So it is a big chunk of the PM2.5 that floats around. It varies depending on the conditions, but we are talking about a very significant fraction of that—one-third, or one-half, or sometimes even more. It makes up a big component of the PM in central London.

The nitrogen has come from two different places. Some 85% or 90% of the ammonium part has probably come from agriculture, and that could be from some way away—a farm in the south-west or in the Netherlands. The nitrate part of it has almost certainly come from combustion, a completely different place—a road or a power station. It is when the two mix together that we get a particle formed.

We have to think about this as being the mixture of pollution from disparate sources, which is why you cannot just pull one lever on one sector—and we may talk a bit more about policy. To reduce PM2.5, you have to make sure that all the contributing parts are being reduced together, because we are sort of mixing pollution to form a new pollutant.

The Chair: My supplementary to that is whether the Government have the data to be able to disaggregate where the different pollutants come from and therefore start to manage those pollutants.

Professor Alastair Lewis: Yes, there are two parts to that. Do we have good data on where the emissions are coming from, and do we have the measurements of the air to see what people are breathing and what they are exposed to? One of the big challenges with ammonia and agriculture emissions is that it is incredibly diffuse—it is coming from every farm and every agricultural installation. Huge numbers of estimates have to be made; very few places are actually monitored.

The Chair: So the two sources from agriculture are slurry, which can be managed, and presumably pinpointed—the source of ammonia can be pinpointed at a farm level—and then you have the fertilisers.

Professor Alastair Lewis: The challenge is actually what happens in the real world. This is happening in such a widespread and diffuse way, and we have relatively little data that actually tracks in real time. We know exactly what comes out of the chimney at Drax, but it is very hard to evaluate how much is coming out from individual farms. That is a real challenge with ammonia emissions, to know what is really occurring on the ground with agriculture.

The Chair: Are there metrics that can be applied—for example, in terms of fertiliser application with tonnes per square metre?

Professor Alastair Lewis: Yes, all of that data feeds into the National Atmospheric Emissions Inventory, so there is a very complicated methodology that takes all those effects into account. Even so, the people who assemble the inventories and collect all the statistics together end up with a very large uncertainty at the end of it, much larger than we have for other sources.

The Chair: I suppose what I am trying to get at is whether the Government have this data, whether in an aggregated or disaggregated form, which they could use to give scientists the tools that they need to be able to say that this is what good policy or best practice ought to look like.

Professor Alastair Lewis: There is quite extensive guidance already published at the European level and by the UK Government on best practice. The challenge is not about not having evidence of what is good practice; the difficulty has been translating that to action on the ground and then being able to evidence that the intervention has had an effect on a large scale. Emissions of ammonia have hardly budged over the last 30 years.

The Chair: I am encroaching on the last question, so I will leave it until then for you to answer on how the Government could do things better. In the meantime, shall we go to Baroness Whitaker?

Q35            Baroness Whitaker: It seems that indoor air quality is less studied and regulated, but to what extent does NOx from gas boilers or cookers contribute to indoor air pollution? How does it compare to other air pollutants, such as particulate matter from wood burning, from a health perspective?

Professor Alastair Lewis: I will take the question on the source if Anna takes the one on health effects. More or less anything that you burn with a high-temperature flame is going to generate some particles and some NOx. So gas cookers and gas fires are unusual; they are a thing that we have in our homes that burns unconstrained with a hot flame, and they are a source of NOx.

Historically, we were worried about much bigger sources outdoors. One of the reasons that we are thinking much more about indoors is partly that we have dealt with a lot of the outdoor sources of NOx, but have moved down a trajectory of increasing the energy efficiency of our homes. In the past, you might have said that all the emissions from your gas cooker or gas fire would just blow under the doors and through the rattling windows, but we have very energy-efficient homes now. If you stick appliances that we probably used quite safely 40 years ago—the same cooker—in a very energy-efficient house, you will potentially trap in the pollution. That is why there is such a focus on some of these indoor sources now, not because the sources have changed but because the way that we use them has changed. Anna, do you want to comment on the health impacts?

Professor Anna Hansell: There is evidence that gas cooking is linked to asthma. There has been some discussion about how robust the evidence is, because there was a systematic review in 2013 that has been used quite a lot because it was able to quantify—but a more recent one in 2023 could not quantify—the associations between gas cooking and respiratory outcomes in children. There have been some publications on the burden of disease for Europe, which I will put in the written evidence, which have produced some quite large numbers of people affected, but there is a lot of uncertainty that was not acknowledged in that report.

So NO2 inside is definitely linked to asthma and respiratory effects, but there is no conceptual reason why NO2 exposure indoors would differ from that outdoors. There are some issues about using the same coefficients[2], which I can go into in the written evidence, but it is complicated.

Particulate matter indoors from wood-burning stoves is not a question that we would ask in health: is it better to have a wood-burning stove or to cook on a gas cooker? It is also complicated because you need to take into account the ventilation in the home, which gas cooker you are talking about and which wood-burning stove. However, there has been action in a number of countries around the world, particularly the US and Australia, to try to remove or reduce pollution from wood-burning stoves indoors. We know that indoor stoves are also a substantial component of outdoor particulate exposure, so there are some real concerns about the use of wood-burning stoves indoors.

There have been some studies that have looked at both indoor and outdoor exposures, suggesting that both are important for health. One might predict that, but it is about being able to quantify it in people with COPD—chronic obstructive pulmonary disease.

Baroness Whitaker: I believe that asthma is on the increase in the UK, particularly among children. Is there another reason besides decarbonisation—health grounds—why the Government ought to recommend the move from gas to electricity?

Professor Anna Hansell: There is. Asthma stabilised in children, but it still affects a significant number of children, depending on which figures you look at. It could be one in 10 children.

Baroness Whitaker: When did it stabilise then?

Professor Anna Hansell: It has stabilised over the last 10 years or so, but it is very much a concern, and reducing exposure to NO2 and particulates should help reduce the incidence and exacerbations of asthma.

The Chair: I will take two supplementaries, but time is very tight and we still have to get through a very important section on policy and regulation. Please ask your questions succinctly, Lord Ashcombe and Earl Russell, then I will move to Lord Krebs.

Lord Ashcombe: My question is very quick. What are the differences between cooking on gas and on electricity? Do you have the same issue? Also, what are the differences between an open fire and a stove?

Professor Alastair Lewis: There are two components to this. Obviously if you do not use a gas hob, you do not get nitrogen dioxide. That is a function of the flame. In a way, that is a no-regrets option: if you remove the flame, you have removed the nitrogen dioxide.

A substantial component of the particles that you get from cooking is generated from the cooking process itself, so you would get them whether you were cooking on an electric or a gas hob. It is not a silver bullet, but you would particularly reduce NO2 emissions—well, eliminate them—by moving to electric. Importantly, you remove any risk of carbon monoxide as well, which is a different pollutant, but still a significant source of harm in poorly optimised systems. Although the evidence is a bit mixed, it is a no-regret option to me.

Earl Russell: Very briefly on human health and indoor air pollution, as we have moved to far more energy-efficient homes, should building regulations look more at air purification within the indoor environment?

Professor Alastair Lewis: It is not a given that you have poor indoor air quality in a high energy-efficiency building, but it can be a consequence, particularly of retrofits. A modern building built today will have considered appropriate ventilation, but when retrofitting buildings—which we will have to do an awful lot in the UK for net zero—it can be more complicated to put in ventilation.

Filtering air is a possibility. It is very widespread in some countries; it is very common in homes in China. There is an issue about whether it is something that certain people can afford and others cannot, and it generally is not very effective for gaseous pollutants. You could filter out particles, but probably not NO2.

The Chair: And you can open windows indoors when you are cooking.

Q36            Lord Krebs: I am moving on to a topic on which we have touched already, about government policies and regulations on air pollution. I want to ask your view on whether the current policies are focusing on the right things. What more could be done to reduce the public health impacts? Although the Government reported meeting their emissions-reduction commitments for air pollution, it nevertheless remains a leading health concern. Does that mean that the commitments are not stringent enough?

Professor Anna Hansell: I have a very short answer to the second question and it is yes, in terms of the commitments not being stringent enough. From a health perspective, we recommend that the WHO air quality guidelines are followed, because below those levels there are very unlikely to be health effects. So they are ambitious, but certainly very important to health, and we are not legislating to WHO air quality guidelines outside.

Indoor air is more complicated, because of the evidence of what is indoors in homes and direct evidence looking at health impacts. One of the problems that we have come across with COMEAP is that there are many sections of government that are involved in looking at indoor air—DLUHC, Defra, DESNZ and the building safety regulator involved in DHSC. There was a cross-Whitehall working group on indoor air quality at one point, but it needs to be reactivated. That would be very helpful to discuss policy and the evidence base.

The other area that I would be very happy to see is more information on outdoor VOCs and enhanced monitoring.

Professor Alastair Lewis: Emission reduction commitments are the national accounting for overall emissions. You add up all the sources in the UK and that is what our emissions are. That does not always translate one-to-one to improvements in air quality on the ground.

So if you close down a gas-fired power station, you subtract quite a large chunk from your national emissions and you get a big reduction. The impact that has on people is quite small, because that one reduction could have been a chimney out on the east coast. You have to separate apart emissions reductions from actual changes in concentrations. Lots of our big reductions in emissions over the last few years have actually come from decarbonising the energy supply. The size of reduction commitments that we have got for nitrogen oxides is pretty ambitious. That is reducing at the moment at the rate of about 5% a year, which is pretty quick. Reducing that to a very low, manageable level is a question of when, not if. It is just the timescale for the transition.

Ammonia is rather different. It has hardly changed in 40 years. Our commitments to reduce that are very modest. I suspect they are largely informed by limited progress over previous decades, and multiple Governments have struggled to deliver those reductions. For ammonia in particular, the Air Quality Expert Group is doing some more work on ammonia. There are actually some future risks on ammonia. It is one of those pollutants which you can plausibly see increasing in the future around pressure on farming and productivity, but also the use of ammonia as a fuel and its potential release from things like anaerobic digestion. For most pollutants, we have a kind of trajectory downwards and we see an end point. We are not quite sure how low it is going to be. Ammonia is quite rare as a pollutant, where actually we are looking out into the future and seeing quite a lot of risk in terms of increase. The other thing layered on top of that is that a warmer climate drives more ammonia into the atmosphere, as well. So there are a lot of things about ammonia that make it hard to see a positive downward trajectory that we have seen for lots of other things. The level of ambition probably has to increase relative to where it has been for the ​​last 40 years but it has clearly been hard to deliver on the ground.

Lord Krebs:​ Could I ask a couple of follow-ups? Anna, you referred to guidelines but should there be legislated targets rather than just guidelines? My second question relates perhaps to something Alastair said, which is that, to the extent there are targets on what is emitted rather than the impacts on health, should the emphasis be on the end purpose of reducing air pollution or improving air quality to reduce the impacts on human health, and should that be in the metrics by which any policy is judged? Would that be possible? 

Professor Anna Hansell: The word “guidelines” applies specifically to the World Health Organization guidelines, because they are not targets. We have targets in the UK that were set in the Environment Act. They are not the WHO guidelines, they are higher. We have already met those in most areas of the country in terms of PM2.5, so we could revisit those, be more ambitious and set them as legislative targets. The issue of whether health outcomes should be included in the target setting is complicated and I suggest that the focus is on the concentration level rather than the outcomes, which can be affected by a large number of things. If you are looking at trying to reduce the component of air pollution that is producing health impacts, you reduce the air pollution. That is the best way to legislate.

Professor Alastair Lewis: Yes, we have targets for annual average for PM2.5, and that is a legally binding target. But as we have discussed, one of the challenges is that it is a winding path from an individual source, farm or installation through to that PM2.5 in the end, because so many things are feeding in and contributing to it. I suggest that perhaps what we have is almost too big a gap between a national total for a particular pollutant and the end point, which is PM2.5, and not a lot of staging posts along the way—for example, “This particular sector of society or business will emit a certain amount”. We tend to deal with it as totals at one end of emissions and then we wait for all the atmospheric chemistry to do its work and end up with a single metric at the end. It makes it rather hard for people designing policy to have some fine controls over the system.

The Chair:​ Just on that point before we go back to Lord Krebs and then Baroness Whitaker to finish off the session, outdoor air pollution and particularly particulate matter has been classified by the International Agency for Research on Cancer as a group 1 carcinogen. That should really concentrate minds, and yet, as you were just saying, the information that we have on particulate matter is not commensurate with the action that needs to be taken. So, again, there is the question that I asked previously: does that data exist and is it in the public domain?

Professor Alastair Lewis: There are probably about 150 monitoring stations around the UK for PM2.5. There is a reasonable map of where the pollution is. Is there a map of where your individual street is? No, there is not. There is probably a reasonable ability to predict that but that is not communicated particularly well. Defra has recently done a review on air quality information systems and there is clearly a long way to go in communicating air pollution to the public, both from the health perspective—why are we interested in reducing it?—and then: what is occurring in your neighbourhood and what can you do about it? So, we have quite an expensive system of monitoring but it is very much framed around regulation—what we are legally obliged to collect in terms of data. It is rather less focused on actually informing people about what they could do about it. There is a stretch. If you are a farmer somewhere, it is difficult to understand that you really are a contributor to air pollution in central London. That is a difficult stretch to make.

We have a lot of data out there. A lot of money is spent collecting monitoring information and emissions information. We have not probably made the most of that information in saying what is going on and why some of these actions in individual sectors are important. If you are asking people to take action it has to be clear to them why they are doing it. On air pollution, we have not always made that very clear.

​​The Chair:​ Professor Hansell, how hard should we push for this data to be made available?

Professor Anna Hansell: I would be very supportive of having more data, particularly on the composition of the particles. We suspect there are differences in terms of toxicity with different types of particles but we have not been able to quantify that in any substantive way to date because of this issue about the data that are available.

Lord Krebs:​ The Office for Environmental Protection noted in its report that our standards are weaker than the revised equivalent EU standards. We seem to have diluted our environmental and health protections since 2000 and since we left the European Union. Should we be aiming to be at least at the level of the rest of Europe?

Professor Anna Hansell: Yes is the answer to that. They are not completely like for like. For example, we have a population exposure reduction target which is not in the European outdoor air quality guidelines and there are some differences in how that[3] is assessed. But we certainly should be looking at what they are doing in Europe, where the legislation is covering not just PM2.5 but all the other pollutants as well, and is on a much more ambitious timescale.

Baroness Whitaker:​ Returning to ammonia, is the implication of what particularly Professor Lewis was saying that farmers should be encouraged or perhaps obliged to monitor their individual output of ammonia more precisely?

Professor Alastair Lewis: It is not easy to measure ammonia. If it were easy to measure, we would be at that place already. It is one of the most difficult atmospheric gases to detect. This is one of the reasons that we do not do this routinely in lots of places. If we could get some breakthroughs there—in the way in which everybody’s car has a NOx sensor on it, though you might not realise it; those sensors are cheap and reliable—it may be possible to get more to the farm level but there are reasons why and, unfortunately, at the moment they are limited by measurement. The other thing that is missing is that we do not have lots of good evidence of case studies of how effective interventions are. So, even if you are not monitoring on every farm, it would be nice to have that strong evidence that when there was a farm-level intervention, you were actually seeing the benefits and it was really clearly articulated that that this was having an effect.

Baroness Whitaker:​ It sounds as if we should have a trial or something like that.

Professor Alastair Lewis: There have been trials and people have done that but there is not a substantial evidence base. It is really nothing like as good as we have, for example, on the evidence around controlling vehicles.

​​The Chair:​ With that, it just remains for me to say thank you very much on behalf of all the committee for the time that you have taken to impart your wonderful knowledge to us. Please write to us with any gaps that you think may need filling. I know, Professor Hansell, you have quite a lot of information there at hand which you were unable to impart, so thank you very much for the written evidence that we will be receiving on that. With that, I formally end the public session of this meeting.