The original caption of the photograph reads: 'Like a walking laboratory, the smog detective in Civil Defence uniform goes out to seek information on the silent killer. Mr Harry Fleet, of Southwark Civil Defence, was out in last night's fog measuring the acidity, sulphur dioxide, hydrogen sulphide and carbon monoxide in the air. In his right hand is an aspirator which takes in the 'atmosphere'. Round his neck hangs a rack of test tubes and reagents to help identify the gases in the air.' London's many coal fires contributed to thick smog in the late 19th and early 20th centuries.
© NMeM / Daily Herald Archive / Science & Society Picture Library