Blog 21 July 2026
London Climate Action Week is always full of surprising encounters, but I certainly didn’t expect to find myself speaking to King Charles about black carbon. What is black carbon? you might ask (and so did he). It’s a traditionally overlooked super pollutant that is increasingly recognised for its impact on health and climate change.
That the King is interested in the subject is testament to how far the world has come in growing awareness of super pollutants. And the irony of the setting was not lost on attendees. We were in a royal palace – St James’s – during an intense heatwave, and it was boiling. International attendees were amazed that the UK is so ill equipped for these temperatures that even the King doesn’t have aircon.
That week, London wasn’t the only place experiencing a heatwave: cities across Europe were facing similar extremes. Record temperatures broke national records, hospitals came under strain and heat-related air pollution intensified.
Heat is not just a climate issue. It is also an air quality issue — and cities are on the frontline of both.
Heatwaves and air quality: the overlooked connection
When temperatures rise, air pollution often gets worse, particularly in urban areas. Heatwaves mean stagnant air, trapping pollution closer to the ground. Higher temperatures also make pollutants more chemically active, meaning even more pollution is produced through chemical reactions in the atmosphere.
One of the clearest examples is tropospheric ozone, or ground-level ozone. It forms faster in heat and sunlight, so during heatwaves it can spike rapidly. During the June heatwave, concentrations of tropospheric ozone were continuously raised in Europe. Black carbon can also rise during heatwaves in cities, as observed during London’s 2022 heatwave.
These two super pollutants damage health during heatwaves, but they also directly contribute to global warming. This creates a dangerous feedback loop: climate change causes heatwaves that worsen super pollutants, and super pollutants contribute to further warming.
Why action on tropospheric ozone matters now
High tropospheric ozone levels are linked to increased hospital admissions, lost working days and reduced outdoor labour capacity. These effects are particularly serious during heatwaves, when health systems, public services and workers are already under strain.
The good news is that action can deliver fast benefits. Because tropospheric ozone is formed when other pollution particles (or precursor emissions) react, cutting those emissions can quickly reduce peak ozone levels. That means cleaner air within days, helping cities protect residents in the short term. Tropospheric ozone is also a greenhouse gas, so cutting levels of it also protects us from the effects of climate change in the long term.
Urban solutions to tropospheric ozone
Traffic is a major source of the precursor emissions that form tropospheric ozone. Electrifying transport, introducing clean air zones, and increasing walking and cycling can all help reduce these emissions.
London provides a clear example. The Ultra Low Emission Zone (ULEZ) and wider efforts to reduce traffic are cutting nitrogen oxides (NOx), a key ozone precursor, helping to address spikes in ozone formation. The city also published its first ever extreme heat plan, recognising that heat is now an urgent urban risk.
Buildings are another important source. Heating systems that rely on coal, oil, biomass and inefficient gas release NOx, volatile organic compounds, carbon monoxide and black carbon — all of which contribute to the formation of tropospheric ozone.
Cities can cut these emissions by moving to cleaner heating, including heat pumps, renewable electricity, clean district heating and energy-efficient retrofits. Paris is retrofitting older buildings to phase out oil-fired heating and expanding cleaner district heating, reducing nitrogen oxides and volatile organic compounds from fuel combustion.
Cleaner air, cooler cities: the opportunity ahead
Many solutions to address escalating heatwaves exist and are ready to scale. Cities are on the frontline of extreme heat and polluted air, but they can also lead the response. By cutting super pollutants such as tropospheric ozone and black carbon, they can deliver cleaner air, cooler streets and better protection for residents.
To assist their efforts, tropospheric ozone needs to be included in global policy frameworks and national investment decisions. Our strategic roadmap sets out how ozone can be embedded into existing climate and air quality frameworks to accelerate progress on this dangerous pollutant and greenhouse gas.
