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Not convinced that is particularly accurate. A street in Colchester that is considerd the most polluted in the town and exceeds WHO safe levels of nitrogen dioxide is rated 1 out of 6?
I think this is just an average. I suspect any area that is not 1 out of 6 is actually quite badly polluted some of the time. It's just the quiet times pull the average down.
I just put in a couple of postcodes along the A4 in Chiswick to check, and got a message saying sorry this postcode is not covered by the data.
From what I can see this just looks to be the worst kind of clickbait.