Glasgow, the city I mentioned, is 32% covered in trees/park, and it's surrounded by a great deal more.
I don't believe that figure includes folks gardens, so that will add a great deal more to the whole 'green' air.
Since the 1960's Glasgow has made enormous strides to improve the air quality of the city, and as a child of that time, who didn't live in the city, but just visited, I watched in astonishment as wonderful architecture slowly emerged from the grime of the industrialisation hub.
Now the air quality is among the best of the UK's cities.
The Council still spends significant sums and energy improving that too.
I think this is pretty much true for most UK towns and cities though, we're very aware of air quality and pollution these days.
Trees growing on old industrial sites and the like still grow good food. The best cherries in our village are growing on an old bing (pit waste) where they were planted as part of a scheme to green up eyesores. Now it's a small park
at a crossroads.
One of the folks in the village is a food scientist and she checked the fruits (mostly since it's near the school and the kids found the trees
and her daughter was one of them who'd come home covered in purple juice) and she said that they were safe to eat, no worries, and what one would expect to find in supermarket fruits too.
Personally I do try to forage away from roadsides, above dog piddling height, etc., but I suspect that the city centre nuts are fine