The burning question: How can we meet the WHO’s guidelines on particulate matter?

The government is resisting peers’ attempts to amend the Environment Bill to set air quality targets in line with World Health Organization (WHO) guidelines. But why, and what policy options would ministers have to consider in order to meet the body’s recommendations? Gareth Simkins investigates

Domestic wood burning is considered to be a major source of particulate pollution. Photograph: Luke MacGregor/Bloomberg

Reducing air pollution – and with it the damage to health that it entails – is a laudable goal in anyone’s books.

Hence the recent clamour in the House of Lords to slash legal limits on fine particulates (PM2.5) during its third reading of the Environment Bill. On 6 September, it passed an amendment to set a limit of 10 micrograms per cubic metre as an annual average for PM2.5, to be attained by 2030. The figure reflects guidelines set by the World Health Organization (WHO) in 2005.

PM2.5 is the fraction of fine particulates that can penetrate deepest into the lungs and even through the bloodstream and into the brain. Scientific studies have long highlighted its baleful effects on human health, from diabetes and cardiovascular disease to kidney failure, lung cancer and glaucoma. The UK’s current legal limit for it, again as an annual average, is a comparatively generous 25µg/m3.

The pollutant is “recognised by the government to be the single largest environmental risk to health in the UK”, said Labour peer Sue Hayman when first proposing her amendment in June. She added that puting the target on the face of the bill, rather than setting a weaker one later via regulations, would answer the demands of the coroner in the case of young Ella Adoo-Kissi-Debrah, who was killed by dirty air.

The government has long resisted simply adopting the WHO’s recommendation, though it has pledged to take it “into consideration”. The amendment may well be dumped when the Commons finalises the bill.

Environment minister Lord Goldsmith told the Lords that the policies needed to reach 10µg/m3 are still being considered, “but we know that a range of restrictions on activities are likely to be needed in urban areas to meet any ambitious target.” These would include halving car traffic in cities and “a total ban on solid fuel burning”.

“I do not think it is right for us to set a target … that would impact millions of people and thousands of businesses… without first levelling with people about what would be needed and ensuring that we bring them with us in understanding the health benefits of achieving that target,” he added.

Only weeks after Goldsmith spoke, the WHO updated its guidelines, halving its recommendation for PM2.5 to only 5µg/m3, putting extra pressure on the Commons to accept the bill’s revision. Targets for other pollutants, particularly nitrogen dioxide, were also slashed.

“The WHO has been very clever here and hasn’t picked guidelines that are unattainable,” said Gary Fuller, an air quality specialist at Imperial College London. The new standards are met “quite comfortably” even in some of North America’s big cities, he added – though it’s “really clear for the UK that we have some challenges” due to Europe’s far denser population.

But consultant Tim Chatterton, formerly of the University of the West of England, doubted that the stricter PM2.5 limit would have any influence on the government.

“It’s academic, it makes little difference other than illustrating what little we have done about it,” he told ENDS, reflecting on how the UK has been in breach of its own standards for nitrogen dioxide since 2004. “Basically nothing was done seriously… until 2010, when the EU standard was broken. We had all the time in the world to demonstrate that we took our own legislation seriously,” he added.

He warned that air quality could be seriously impacted by the crisis in gas prices. The country is, “going into the worst winter yet for wood burning”, though that may be putting it charitably. “A lot of people will be burning anything,” including waste, just to keep warm, Chatterton warned.

There has been too little time for experts to properly analyse how 5µg/m3 could be reached, though a 2019 DEFRA report concluded that, “whilst challenging, it would be technically feasible to meet the [2005] WHO guideline level for PM2.5 across the UK in the future”.

Clean Air Strategy

The report suggested that existing policies, particularly those set out in the Clean Air Strategy, would ensure that only 5% of the population live in non-compliant areas by 2030, delivering annual monetised health benefits of £6.8bn. The strategy outlined plans to extend environmental permitting to intensive beef and dairy farming, a new approach to setting best available techniques for industry and an overhaul of the smoke control area framework, among numerous other policies.

But Andrea Lee, campaigns and policy manager for ClientEarth’s Healthy Air Campaign, told ENDS that there has been little progress in delivering these measures. Nothing has been heard since about the plan to regulate emissions from tyre wear or to bring non-road mobile machinery (NRMM), such as backhoe loaders and tower cranes, into the environmental permitting regime, for instance, though Brexit and the pandemic may have intervened.

Lee also criticised the government’s unwillingness to crack down on urban burning, branding it a “mostly luxury activity. Where it is an issue of fuel poverty, the government needs to address the problem. It cannot leave people to breathe dirty air because they can’t afford cleaner alternatives to keeping warm”.

A glance at the National Atmospheric Emissions Inventory and pollution attribution data for London – the UK’s most polluted city – emphasises that an outright ban on domestic solid fuel burning would be a huge boon for air quality.

The UK’s territorial PM2.5 emissions came to about 109,000 tonnes in 2019 – roughly the same mass as an American nuclear-powered aircraft carrier. Domestic wood burning alone produced 41,000t of it, or 38% of the total, with fuels other than natural gas contributing an additional 4,800t. In contrast, household gas boilers produced only 1,200t of the pollutant.

However, the Stove Industry Alliance has questioned how much wood contributes to air pollution, telling ENDS that the data is “fundamentally flawed” and that it is working with the government to address the issue.

READ MORE: PM2.5 pollution: The difficult decisions the government must make to tackle domestic wood burning

READ MORE: Smoke on the water: how London is tackling emissions on the Thames

Road transport, which released an estimated 13,000t of PM2.5 nationally, is the largest individual source of the pollutant in London, according to the London Atmospheric Emissions Inventory, accounting for 30% of local emissions.

There is confusion about where Goldsmith got the idea that urban road traffic would need to be halved to reach the WHO’s 10µg/m3 target. “I have never really heard those stats from the government,” said Lee.

She added that the country needs to “go further on road transport” in order to meet the WHO’s guidelines. Having fewer – and cleaner – vehicles on the road would deliver “huge social, health, economic benefits” from lower congestion, less noise pollution and better road safety, Lee argued.

Exhaust is now a “tiny fraction” of the sector’s emissions, said Ian Mudway, a toxicologist at Imperial College London, a greater proportion being from brake and tyre wear. Although addressing these would make sense from the perspective of reducing the mass of pollution, he questioned if they were as toxic as the fraction from poorly-combusted petrol and diesel.

“If you pick a compliance value based on mass… you could theoretically pick the wrong thing to target”, he reflected, noting that Brexit means the UK now has the flexibility to choose a more sophisticated approach, targeted at the most toxic forms of pollution. But there is “a big gap about what is the bad bit”, a topic he is currently investigating.

Wood burning’s growth in popularity over recent years has made it the second largest source, at 16%. Construction, including emissions from machinery, produces 15% and cooking contributes 13%, according to the data.

But the government clearly views actions to reduce driving in cities and wood burning as a politically sensitive intrusion into daily life – as demonstrated by its reluctant creation of clean air zones and the limited measure of banning the sale of wet wood and coal for domestic use.

The government’s failure to fulfil its promises regarding NRMM stands in contrast to the Californian authorities, which have historically pointed the way for other jurisdictions on air pollution.

Legislation signed by governor Gavin Newsom on Saturday will outlaw the sale of new petrol-driven lawn mowers, hedge trimmers, golf carts, leaf blowers and chainsaws as early as 2024. Generators must be zero-emission by 2028, although both deadlines could be pushed back if they are not considered feasible.

The legislation is backed by a $30m fund to help professional gardeners and landscapers to transition to electricity. The measure excludes the likes of all-terrain vehicles, boats, snowmobiles and off-road motorbikes, though those are expected to be phased out by 2035.

“It’s amazing how people react when they learn how much this equipment pollutes, and how much smog-forming and climate-changing emissions that small off-road engine equipment creates,” said assemblyman Marc Berman, who authored the law. “This is a pretty modest approach to trying to limit the massive amounts of pollution that this equipment emits, not to mention the health impact on the workers who are using it constantly,” he added.


Not all particulates are simply emitted. There are other factors too: passing traffic can throw it back up after settling on the roads – and it can also form chemically from ammonia, released almost entirely from manure and fertiliser. Not only is that a waste of valuable nutrients, the gas is also ecologically harmful.

The UK’s gross emissions of PM2.5, ammonia and other pollutants are supposed to be capped by the EU National Emission Ceilings Directive, and the broader Gothenburg Protocol that underpins it. Tighter caps apply from 2020, tightening further in 2030. But the compliance forecast for the two pollutants “is not looking good” said Lee, as can be seen from the figures themselves.

“With the right levels of support for farmers to help them better manage manure piles, slurry pits and fertiliser use,” agriculture’s contribution to the anticipated failures to meet the 2020 emissions ceilings could be resolved, she added. But the government’s reliance on voluntary commitments from the sector, rather than hard regulation, could limit progress, she added.

Veteran air quality campaigner Simon Birkett, founder of Clean Air in London, told ENDS that the UK could probably meet the older WHO guidelines “by banning most domestic wood burning and requiring better farming practises”. The fundamental problem is that DEFRA is a “small, weak department that seems incapable of taking the required action”, he added.

The difficulty of modelling

The activities that need to be targeted to achieve the WHO guidelines are clear, though the extent required is less so.

In March, government’s advisors the Air Quality Expert Group reported that predicting future PM2.5 concentrations is an “exceptionally difficult” task and “a major fundamental challenge in atmospheric science”.

There is consensus that most of the country would be below 6µg/m3 of PM2.5 by 2030, most from secondary particulates. But local emissions would remain critical to keep below the 10µg/m3 threshold - which central London is most likely to exceed.

But the country could be on track for better performance. The report did not consider the policies that have emerged under the net zero baner, such as the ban on new petrol and diesel cars from 2030 and an anticipated ban on domestic gas boilers within a few years. Nor did it address local factors such as London’s ultra low emission zone, which enters force on 25 October, or the plan to decarbonise the capital’s bus fleet.

As these policies continue to bite, the importance of other sources of pollution rises – hence the mayor’s unanswered 2017 plea for more powers to enforce emissions rules for construction machinery, set air pollution rules for river traffic and clamp down on wood burning.

But even that may not deliver compliance, as so much of capital’s pollution is blown in from elsewhere. “This is why, in addition to local action, London needs national, European and international action” to hit 10µg/m3 by 2030, found a 2019 report for the mayor of London.