BACKGROUND: The impact of past air quality improvements on health and equity at low pollution levels near the revised WHO air quality guidelines remains largely unknown. Less is known about the influence of simultaneous reductions in multiple major pollutants. Leveraging real-world improvements in air quality across Canada, we sought to directly evaluate their health benefits by quantifying the impact of a joint shift in three criteria pollutants on mortality in a national cohort. METHODS: In this population-based cohort study, we assembled a cohort of 2·7 million adults living in Canada in 2007 who were followed up through 2016. Annual mean concentrations of fine particulate matter (PM FINDINGS: Between 2007 and 2016, annual mean exposures to PM INTERPRETATION: In Canada, substantial health gains were associated with air quality improvements at levels near the revised WHO guidelines between 2007 and 2016, with notable heterogeneity observed across socio-demographic subgroups and regions. These findings indicate that modest declines in air pollution can considerably improve health and equity, even in low-exposure environments. FUNDING: Health Canada.