Yes, it is. There are thousands of extra deaths per year because of it, children's lungs are affected, and it is thought to be contributing to the rise in dementia cases, amongst other things. Increased hospital admissions for breathing difficulties on days where pollution is high.
We breach the EU air pollution limits for the whole year by around 8th January every single year. One EU rule our governments have been very happy to ignore, though you won't see the Daily Mail including that in their schtick about how we're the only mugs to follow all the rules.