Air pollution is one of the most important shared challenges among metropolises across the world, especially in developing countries such as Iran.The purpose of this study is to evaluate the effect of air pollutants and built environment on the number of unhealthy days in Mashhad metropolis in 2018. To achieve this goal, seven variables were used as independent variables (including population density, land use diversity, CO, NO₂, SO₂, PM₁₀, PM_2.5) and the number of unhealthy days was used as a dependent variable. The relationship between variables was analyzed using the Ordinary Least Squares (OLS) regression model in GIS in 40 municipal districts. The results show that the average of unhealthy days in Mashhad is 76 days. But its amount varies from 12 to 155 days in the districts. Furthermore, the correlation coefficient between the dependent and independent variables is ADJ.R² = 0.80 which represents the effect of 80% of the independent variables on the dependent variable. Among the variables, “PM_2.5” and “NO₂” have a positive relationship and “CO” had a negative relationship with the number of unhealthy days. The relationship between other independent variables and the dependent variable was not confirmed. In addition, the residual standard deviation is not normally distributed. In general, it can be argued that the establishment of industrial and workshop activities and traffic of motor vehicles on highways have a significant role in CO, NO₂ and PM_2.5 emissions in the northern, northwestern, eastern and southeastern districts of the city. Therefore, it is suggested that these districts especially those that have more than 100 unhealthy days are given priority in the planning of urban manager actions.