This paper investigates the flow field and particle trajectories in the GE90 pyrometer purge air design in order to illustrate the fouling mechanism that drives the deposition of particulates onto the lens. It is shown that the predominant flow feature within the purging system is a swirl pattern that develops at the mouth of the unit's still tube. This feature has the ability to draw contaminant particles into the still tube and thus significantly increase the likelihood of particle deposition resulting in optical fouling of the pyrometer's optics.