The sanitary survey is not without its dangers. On this occasion Hallé's companion Boncerf, while facing the breeze and wading along the edge of the black mud of the notorious Gobelin tributary, finds himself overcome by noxious fumes:
Monsieur Boncerf, who at this point had turned more directly into the south-easterly breeze and had descended to the riverbank, was overcome by a biting, alkaline, stinging, and stinking odour. It affected his respiratory system so badly that his throat began to hurt within half an hour and his tongue became noticeably swollen. Affected by these poisonous vapours, he warned me to return to the road straightaway; because I had remained at the easternmost point of the bank that had been infested by these sediments, and hence with my back to the wind, I myself did not experience anything unpleasant.
(Hallé, "Procès-verbal", p.lxxxvi)
In this extract from the TV documentary Filthy Cities, introduced by Dan Snow, historian Andrew Hussey retraces Hallé and Boncerf's route. Andrew Hussey concludes that Hallé's findings were truly revolutionary; it was Hallé who for the first time clearly differentiated smells which were merely unpleasant from those which were truly noxious, in this case the poisonous fumes produced by the tanneries and other industries of eastern Paris.
Alain Corbin, The Foul and the fragrant: odour and the French social imagination
Dan Snow Filty cities: Revolutionary Paris (2011 TV series)
Hallé's original account:
J.-N. Hallé, "Procès-verbal de la visite faite le long des deux rives de la rivière Seine, depuis le pont-Neuf jusqu'à la Rappée et la Garre, le 14 fevrier 1790,"
Histoire et Mémoires de la Société Royale de Médecine, 10 (1789). BIU Santé website