Ambient Particulate Matter; PM2.5, with an aerodynamic diameter smaller than or equal to 2.5 µm, has emerged as the most critical health hazard concerning air pollution. The small size enables ambient particulate matter to go through the respiratory system, easily entering the lung or blood stream. Chiang Mai is one of the cities with the highest level of PM2.5 that exceeds the standard level of PM2.5 concentration (10µg/m3, recommended by the World Health Organization). High concentration levels have severe consequences for the health of the population in Chiang Mai. The objective of this study is to estimate the risk area of health impact due to exposure to PM2.5 in Chiang Mai. This study illustrates the data of PM2.5 concentration gathered from ground-based monitoring sites named DustBoy and data of hospital admissions from the Chiang Mai Provincial Public Health to reveal the population exposure related to human health effects such as heart diseases, chronic obstructive pulmonary disease, lung cancer, cardiovascular disease. In addition, correlation coefficient is employed to estimate the relationship between population exposure to the high ambient PM2.5 and the health effect due to PM2.5 pollution. The results are presented in the Chiang Mai Risk Map as a spatial pattern of population exposure using the spatial distribution method. These results support the high correlation between population exposure to PM2.5 and health impact and strongly suggest priority areas to prevent and control air pollution and social equality in health.