Annual Research Review: Exposure to environmental chemicals and psychosocial stress and the development of children's learning difficulties.

Although awareness of the role of environmental exposures in children's cognitive development is increasing, learning difficulties have not yet been a major focus of environmental health science. Learning difficulties disproportionately affect children living in economic disadvantage, yielding an 'achievement gap.' Studies examining the neurobiology of reading and math have mostly included economically advantaged youth, leaving a great deal unknown about the neural underpinnings of reading and math difficulties in youth living in disadvantaged contexts. Critically, due to environmental injustice, these youth are disproportionately exposed to environmental neurotoxicants. Herein, we review literature supporting a theoretical framework of environmentally associated phenotypes of learning difficulties. We propose that prenatal exposure to neurotoxicants and early-life exposure to psychosocial stressors increases risk for learning difficulties via effects on neural circuits that support cognitive processes which, in addition to literacy and numeracy, are integral to acquiring and performing academic skills. We describe models in which (1) prenatal exposure to air pollution has a main effect on learning via brain structure and function or associated domain-general cognitive processes and (2) a joint 'two-hit' pathway in which prenatal air pollution exposure followed by early life stress-when combined and sequential-increases risk for learning difficulties also via effects on brain structure, function, and/or associated cognitive processes. We review a select literature documenting effects of exposure to pollutants and early life stress on relevant neural circuits and associated cognitive processes in animal models and parallel findings in human epidemiologic studies. We advocate for team science in which researchers, practitioners, and policymakers collaborate to increase health literacy about environmentally associated phenotypes of learning difficulties and support the development of precision-oriented instructional and environmental intervention methods for youth living in economic disadvantage.