Lichen

Further to the recent post about Dartmoor having its own weather system, I think because our air usually comes straight from the Atlantic, when it reaches us it is still very pure, i.e. it contains very few pollutants. This is not all good. Roses, apparently, like some pollution in the air and black spot - an unpleasant fungal infection (of roses) - although controllable in an urban garden, thrives in our clean air.
The other beneficiary (apart from our lungs, of course) is lichen, which grows everywhere. This picture:

shows the lichen that has grown all over one of our garden seats - the pic just shows part of a single strut. On close inspection it is very like coral - it certainly has coral's variety of form.
Lichen will only grow like this where the air is clean enough. So, if you've got it, you know your air is in pretty good shape.