The Advertising Standards Authority's ruling against Brighton & Hove City Council concerning a digital poster campaign about the dangers of particulate pollution from domestic wood burning raises some interesting but troubling issues. (3rd June 2026)
The poster featured a wood-burning stove emitting smoke in the shape of an adult and child, alongside the claim that "harmful particle pollution near 4 city primary schools was 78% higher last winter compared with last summer." The tagline? "Wood burners and open fires. The cosy killer".
Above: The completely objective and disinterested complainant. He later went on to become an advertising lawyer.
The ASA upheld the two complaints, one of which came from a chimney sweep, finding that the Council had failed to substantiate the direct causal link between the use of wood burners and open fires with the claimed increase in particulate pollution. The Council was told to ensure that "robust evidence was held to substantiate any claims where a direct causal link between the use of wood burners and open fires and an increase in particle pollution was stated or implied."
Let's be clear about one thing from the outset. The burden of proof in the CAP Code sits squarely on the advertiser. Rule 3.7 of the CAP Code requires that claims capable of objective substantiation must be substantiated before an ad is published. This is not controversial. What is controversial, or at least problematic, is the standard to which that burden must be discharged. And this is where the ASA's reasoning becomes genuinely troubling.
Brighton & Hove City Council submitted a wealth of evidence from highly credible sources. The most recent data published by the Department for Environment, Food and Rural Affairs (Defra) showed domestic combustion was the second-highest source of national PM2.5 emissions. They provided a memorandum of scientific support from an academic who specialises in air pollution, who stated agreement with the messaging in the ad campaign and that the results were consistent with the burning of wood and solid fuel. They submitted two peer-reviewed studies and detailed monitoring data from 50 air quality sensors installed across Sussex.
Crucially, the Council also went to considerable lengths to eliminate alternative explanations for the seasonal spike in PM2.5. They observed no significant differences in PM2.5 levels at roadside or suburban detectors, ruling out traffic. PM emissions from road traffic had dropped due to cleaner exhausts and more electric vehicles. Winter PM2.5 concentrations peaked late at night, long after traffic had fallen. There were no motorways or heavy industries near the city centre. Ships at nearby Shoreham Port used electric charging, major airports were at least 40 km away, and local power generation was principally from offshore wind. Even international agricultural ammonia, which accounted for some excesses, occurred mostly in April, after the heating season.
The ASA acknowledged that domestic wood burning was "a significant contributor to urban PM2.5 emissions" but said it "had not seen evidence in support of the direct link claimed in the ad." This begs the rather obvious question: if it's not the wood burners and open fires causing the seasonal spike in particulate pollution near these schools, what is it? The Council had systematically excluded traffic, aviation, industry, and energy generation as potential causes.
The burden of proof is not the problem here. The problem is that the standard of proof remains undefined. Is it the balance of probabilities? Is it beyond reasonable doubt? Or something more akin to absolute scientific certainty? When an advertiser produces Defra data, peer-reviewed studies, expert academic support, and a systematic elimination of alternative causes, but the ASA still concludes the evidence is insufficient, one has to ask what would be enough? And having given a 'free-hit' to the complainant, regardless of his vested interest, the ASA risks finding itself on the same side of the debate as the climate-change deniers, despite its sterling efforts over many years to fight greenwashing and promote sustainability.
This matters because it has a chilling effect. The Virgin Atlantic SAF ruling from 2024 demonstrated how overly strict enforcement of green claims can lead to green hushing, where companies simply stop communicating their environmental efforts for fear of regulatory sanction. In that case, a genuine technological breakthrough was penalised because a minority of listeners might overestimate its environmental benefits. Here, a local council trying to protect the health of schoolchildren from air pollution has been told its messaging is misleading, despite overwhelming circumstantial evidence supporting its claims.
We hope this decision may be reconsidered by the Independent Reviewer. But whatever happens in this particular case, it would be immensely helpful for the ASA to adopt a clearly defined standard of proof. Advertisers need clarity on the requisite standard of proof. For a regulator whose own guidance emphasises proportionality, that seems entirely appropriate.
