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Public authorities in charge of preventing occupational diseases related to pesticide exposure mostly rely on laboratory experiments to identify and assess the toxic effects of these substances. However, in some cases, officials are able to build alternative methods to measure this kind of danger. In particular, in situ observations of farmers' daily activities offer the possibility to discover unexpected sources of occupational exposure to pesticides. Such a method has been used by the Social security agency in charge of the medical surveillance of farmers, so as to assess workers' exposure to a known carcinogenic pesticide, sodium arsenite. By describing how the investigation has been launched and conducted, we highlight the conditions of possibility of this kind of innovation in measuring the chemical dangers to which farmworkers are exposed.