Possible Airborne Contamination of Legionella Bacteria in the Lens Region of France


Between November 6, 2003, and January 4, 2004, outbreaks of Legionella epidemics were reported in the Lens region of France. Some of the subsequent environmental investigations identified the Noroxo petrochemical factory located in Harnes as the possible origin of the outbreak. However, subsequent large-scale outbreaks raised doubts about the actual origin of the epidemics.

The consulting company INERIS was asked by the Prefecture of Pas-De-Calais to conduct an atmospheric modeling study to evaluate the atmospheric dispersion of potentially contaminated aerosols originating from the cooling towers and the lagoon at the Noroxo plant.

EnviroComp's Role

We were retained by Exxon Mobil, the owner of Noroxo, to examine and evaluate the air modeling report prepared by INERIS, visit the Noroxo site, and develop an opinion on the case based upon our knowledge of atmospheric dispersion and the evaluation of the existing claims.

Our investigation of the modeling work performed by INERIS indicated that the case for contaminated water emissions from Noroxo plant operations causing the Legionella outbreak was very weak; the ADMS model used by INERIS produced concentrations that explained only about 30% of the variance of the observed infection rates. Moreover, this correlation analysis did not include the many points where infection rates were zero but the modeled concentrations were high.

To illustrate the events and the dynamics of the epidemic, and to provide an analysis tool to investigators, we prepared an animation (Video 1) showing a map of the region with the time evolution of the Legionella cases and the prevailing winds.


Video 1 (low resolution)