August’s heatwave led to the UK’s second major summer smog of the year. Air pollution across most of England and Wales reached between seven and eight on the government’s 10-point scale. Summer smogs are caused by many pollutants that chemically react together in strong sunlight. There is no simple connection between a single pollution source and the air quality problem, making it hard to reduce and keep these smogs under control. Prof James Lee from the University of York has tracked 20 years of changes in summer smog, specifically changes in greatest values of ozone breathed at six locations in the UK. He explained: “The top 5% of measurements each year are rising, especially over the past few years, following a dip in the early part of last decade. There are still many days a year that breach the World Health Organization’s guidelines for ozone. This is certainly not decreasing. We need a better understanding of the way that these smogs form under current and future conditions.” Currently our controls focus on the pollutants that we targeted though the last century, and not the ones of today. A 2020 report by UK government advisers has pointed to problems from the growth in pollution from the personal care products, aerosol sprays, glues and paints that we use in our homes. These are subject to little or no legal controls. Some pollutants are also masking the impact of summer smog. When I was analysing ozone around London 20 years ago, it was clear that this was mainly a suburban and countryside issue. Places with the worst nitrogen dioxide had less ozone. But now ozone is creeping into the big towns and cities where most of us live. Prof Ally Lewis from the University of York explained: “As vehicles clean up and electrify, this masking effect is disappearing. Getting nitrogen dioxide out of our cities is crucial, but it will leave behind some ozone in its place, and that will have to become a focus of policy attention.” Lewis’s team also have an important warning for the regions of the world that have the worst particle pollution. This includes much of India, where hazy smogs are a familiar image, and large parts of China. These severe particle pollution problems are also masking an underlying problem with ozone. A new report from the State of Global Air initiative has highlighted the health burden from particle pollution and nitrogen dioxide in over 7,000 cities. Reducing this is critically important, but people will not see the full benefit unless governments also take steps on ozone-forming pollutants.