This report considers the moral, legal and economic case for doing more to tackle air pollution in London. Air pollution is arguably the most significant environmental issue facing London, as well as one of the most significant public health issues. Although London's air pollution problem is less visible than in the past, the coal smoke has being replaced by nitrogen oxides from diesel fumes, gas boilers, and other sources. Levels of nitrogen dioxide are well above European legal limits, and London levels of particulate matter also exceed World Health Organisation guidelines. Research estimates that air pollution was responsible for up to 141,000 life years lost or the equivalent of up to 9,400 deaths in London in 2010, as well as over 3,400 hospital admissions. Analysis shows that 328,000 children attend schools in London where nitrogen dioxide concentrations exceed the legal limit and healthy limit, representing nearly 25% of all pupils in London. The most polluted schools in London experience concentrations of nearly twice the limit. Analysis also indicates that 3.8 million people work in parts of London which are above legal limits, representing 44% of London’s workday population. More deprived parts of London generally experience higher levels of air pollution, although there is considerable variation. This is the first report in a series of two, with the second report considering specific policy proposals in more detail.