Ultrafine particles are impossible to see and are often missed by conventional monitoring techniques, and therefore they are not covered by air pollution laws. In 2021, the Dutch Health Council and the World Health Organization (WHO) highlighted the growing evidence that ultrafine particles are damaging our health. This includes 75 studies, mostly relating to lung inflammation, blood pressure and heart problems, along with risks to foetal growth. Technical differences between the studies, however, meant that the WHO was unable to set a standard.