On September the 19th 1973, some toxic gas was released in the open air at Tolochimie Factory in Toulouse (France). Through archival records, we can analyze this accident, challenging the way to measure and deal with open air in an industrial context. In the perimeter of the factory, the accident is framed as a loss of gas normally confined in a system controlled by sensors and parameters. In town, people speak of a toxic cloud, which measure is fuzzy and that could trigger a spill over that needs to be regulated: two ways of “containing” air are revealed in a particularly obvious case of normal pollution.