PROJECT SUMMARY/ABSTRACT Per- and polyfluoroalkyl substances (PFAS) are a group of persistent and bioaccumulative chemicals used in a large number of industrial and consumer products that have been measured in blood of more than 97% of the population above 6 years in nationally representative studies. PFAS are commonly assessed as individual chemicals, but exist as diverse and complex chemical mixtures of known and unknown compounds in the environment and human body. Due to rapid lung development and growth that occurs during the prenatal period and childhood, these developmental periods may be particularly susceptible to the adverse effects of PFAS on lung function. Impaired lung function in childhood is associated with impaired lung function in adulthood, which in turn is associated with early onset of chronic diseases and premature mortality from all causes. Animal studies suggest PFAS adversely affect lung maturation, surfactant function, and alveolar integrity, but there are limited epidemiological studies exploring the association between PFAS and lung function in children. The findings from the previous epidemiological studies are limited, inconclusive, and lack longitudinal assessment. Our study proposes to investigate the effects of PFAS and PFAS mixtures on children’s lung function in Project Viva, an ongoing pre-birth cohort study of 1116 mother-child pairs recruited in Massachusetts between 1999 and 2002. Specifically, we will 1) evaluate the association between prenatal exposure to PFAS and PFAS mixtures and lung function in mid-childhood, and 2) assess the association between mid-childhood PFAS and PFAS mixtures and lung function in mid-childhood and adolescence. To our knowledge, this will be the first study to assess the effects of PFAS mixtures on lung function in children, and the longitudinal effects of these chemicals on lung health. As we seek to identify potential causal pathways for the impact of PFAS on lung function during developmental periods, our findings may support PFAS mitigation and regulation, and the development of targeted interventions to reduce the risk of lung illness later in life.