Your faeces, my friend, are blowin' in the wind

GO FOR a bracing winter stroll in a major US city and you will be inhaling more than vehicle fumes. 

A new study has demonstrated for the first time that during winter most of the airborne bacteria in three large Midwestern cities come from dog faeces.

Noah Fierer at the University of Colorado, Boulder, found the high proportions of airborne dog faecal bacteria after analysing samples of winter air from Cleveland, Detroit and Chicago. 

His team checked the DNA in their samples against reference banks which "barcode" organisms according to their genes.

They discovered that most of the bacteria they found came from dog faeces by checking the bacterial profiles against reference samples of bugs typically present in soils, leaves and faeces from humans, cows and dogs.

In summer, the proportions of bacteria in the air come almost equally from soils, dog faeces and the leaves of trees. But come winter, the trees have shed all their leaves and aerosols from soils are limited by overlying snow or ice, reducing absolute counts of airborne bacteria by about 50 per cent. 

This means that dog faeces becomes the dominant remaining source.

Fierer says that at the relatively low concentrations found - 10,000 bacteria per cubic metre of air sampled - the bugs are unlikely to cause disease (Applied and Environmental Microbiology, DOI: 10.1128/aem.05498-11).