The dose response relationship had not been demonstrated in any of the case-control studies performed. Years later and after a laborious and careful selection of oils, a case-control study was developed which resulted in a step forward in the knowledge of the real etiology. An oil container with a characteristic shape and linked to a specific company demonstrated a relationship between the concentration of chemical compounds in the oleyl-anilide and the log of Odds Ratio. This study was known as “Toxi-Epi study”.

Kilbourne EM, Bernert T, Posada de la Paz M, et al. Chemical correlates of pathogenicity of oils related to the toxic oil syndrome epidemic in Spain. Am J Epidemiol 1988;127(6):1210-27.