Why would a health insurance company study air pollution? Because, as Christian Horemans, an environment and health expert with the Belgian mutual insurer Mutualités Libres, explains: “It has an enormous impact on public health and an enormous cost for the Belgian compulsory health and disability insurance. We consider it our duty to analyse risks for the financial sustainability of health and disability insurance, part of social security.” The insurer’s recent study , carried out by a team that included Belgian and Dutch universities and research centres, compared health insurance claims from 1.2 million people in Belgium in 2019 with particle pollution in their neighbourhoods. The financial results were startling. The team focused on the costs of nearly 4m GP visits and more than half a million visits to hospital emergency departments made by young people and adults, aged 10 to 64. After removing the social and economic factors that affect people’s health, the study found associations between GP and hospital emergency visits and particle pollution (PM2.5), and when it applied the results to Belgium as a whole, researchers estimated that reducing air pollution to match the least polluted 25% of places in Belgium would have saved €43m (£37m) in GP and emergency hospital visits in 2019. It is likely that even greater savings would be achieved by meeting World Health Organization guidelines for air pollution, as proposed by the European parliament for 2035. Horemans said: “The lower the concentrations of PM2.5, the fewer people need to see the general practitioner. So improving air quality not only benefits public health, but also ensures the financial sustainability of the social security system.” The study also found that more tree and grass cover in a neighbourhood was associated with less demand for GP and hospital visits. This was especially true of cities and accords with wider evidence on green space in urban areas. The World Health Organization already recommends that we have access to 0.5 hectares (1.2 acres) of public green space, about two-thirds the size of a football pitch, within 300 metres of our homes. The urban forester Cecil Konijnendijk recently suggested a 3-30-300 rule of thumb for urban forestry and urban greening – every citizen should be able to see at least three trees (of a decent size) from their home, have 30% tree-canopy cover in their neighbourhood and not live more than 300 metres from a park or green space. Prof Mark Nieuwenhuijsen from the Barcelona-based research institute ISGlobal, who was not involved in the Belgian study, said: “GPs are often overburdened with patient visits. Lowering air pollution and increasing tree cover may be an effective way to reduce their workload. “Based on these results, we need more, not fewer initiatives, like Barcelona’s Superblocks. Political pressure to reverse urban planning initiatives that reduce air pollution and increase tree cover seem ridiculous.” The Belgian study only looked at part of the health costs from air pollution. It did not include the impacts on young children, including childhood asthma, and in old age. There is strong and growing evidence that air pollution affects our health from pre-birth, as children develop and grow, and also at the end of our lives. One of the most important developments in air pollution research over the last 10 years has been associations between air pollution and brain health. This includes cognition and attention in children, mental illness in adults and dementia at the end of our lives. Published in 2022, a previous study by the insurer analysed incapacity and long-term sickness from work due to mental health. In Belgium, about one in four cases of incapacity are related to mental health conditions and these have become the biggest driver of the increase in work incapacity benefit payments. The study found links between short periods of high air pollution and the onset of long periods of time off work for mental health problems.