The western United States experienced a record-breaking wildfire season in 2020. This study quantifies the contribution of wildfire emissions to the exceedances of health-based National Ambient Air Quality Standard (NAAQS) for fine particles (PM_2.5) by comparing two CMAQ simulations, with and without wildfire emissions. During August to October 2020, western wildfires contributed 23% of surface PM_2.5 in the contiguous US (CONUS), with a larger contribution in Pacific Coast (43%) and Mountain Region (42%). Consequently, wildfires were the primary contributor to the 3,720 observed exceedances. The wildfire influence peaked on September 14^th, 2020, when 273 exceedances were recorded and wildfire emissions contributed 41%, 81%, and 72% to surface PM_2.5 concentrations in the CONUS, Pacific Coast, and Mountain regions, respectively. Our finding highlights the predominating influence of wildfires on air quality, and potentially human health, that is expected to grow with increasing fire activities, while anthropogenic emissions decrease.