How the big smoke was cleaned up | Letters

The most effective proxy that the western coalition has against Isis is the Kurdish YPG militia operating in northern Syria (Report, 28 July). Given that the territory the YPG controls is now under threat from Turkey, its fighters will engage this new enemy and turn away from fighting Isis. Isis will be free to consolidate its gains in Syria and to continue its regime of terror. How did the US and its allies allow this situation to come to pass?
Dr Rod Thornton
Defence studies department, King's College London

" London fogs, which were tackled by the Clean Air Act 1956, did not mean we were "groping about in airborne industrial waste" (Notebook, 28 July). The cause was overwhelmingly the discharge from domestic fireplaces burning bituminous coal, dealt with by a rolling programme of smoke control areas, replacing bituminous coal with coke, low-volatile steam coal, gas, electricity and other "smokeless" fuel.
Bill Gabbett
London