Air pollution harm was seen at concentrations that were at about one half of the latest World Health Organization guideline and at just quarter of the 2040 target proposed for England.

The Canadian study was one of three funded by the US Health Effects Institute. The other two looked at more than 60 million people in the US and 27 million people in Europe. They reached similar conclusions: there is no lower limit that can be used to define safe air quality. This means governments should not constrain their ambition around setting targets for the worst air that people should breathe.

Even low levels of air pollution can damage health, study finds | Air pollution | The Guardian