Poor indoor air quality is associated with various ill-health conditions; and appropriate natural vent ilation is recognized as an effective means to keep both buildings and occupants healthy. Natural ventilation is crucial where people spend most of their time in indoors with pollutants. The purpose of the work reported in this paper was to examine the potential impact of retrofitting options of vents on improving indoor air quality by removing particulates produced during cooking. It focussed on single-room dwellings in informal settlements in Nairobi, Kenya. The dwellings are occupied by extremely low-income householders who typically live and sleep in the same room that they cook and dine. The majority of the dwellings are windowless. Where present, windows are rarely opened, either because they are occluded by crowded items, or due to insecurity, or to keep out mosquitoes. We used Computational Fluid Dynamics (CFD) to simulate air flow and particle dispersion, focusing on PM2.5. The results show that retrofits for natural ventilation can effectively reduce indoor particulates. The effectiveness is highly related to the type, size and location of retrofit vents, and to the fresh air inlet velocity.