Critical Windows of Exposure: why do they exist and what do they tell us?

Abstract

BACKGROUND AND AIM: This is part of symposium 22 entitled “Innovative Methods for Identifying Perinatal Critical Windows with Mixtures” The concept of critical exposure windows is often equated with developmental origins of disease, although the concept is found in multiple disciplines such as cognitive sciences i.e. language development. In reality, critical exposure windows are the intersection of developmental biology and exposure science, in which time varying environments coincide with age varying biological development. METHODS: When time dependent environment and age dependent biological processes intersect, they create dose response curves that differ in slope uniquely from the slopes derived at other life stages. Critical exposure windows inform mechanisms as discovering them provides insights to both the biological effects of an environmental exposure and the complex age dependent processes that are being modified by that exposure. RESULTS:With regards to mixtures, consideration should also be given to the biological matrix in which exposures are measured, as the half lives and rates of excretions for different exposures vary in blood and urine and other tissues, creating potential biases in results. Variations of even a few months in exposure timing may also introduce bias if the life stage reflects accelerated growth and development (i.e. fetal life). CONCLUSIONS:Finally, from a public health perspective, knowing critical exposure windows means knowing when to intervene and has benefits to creating targeted interventions that are both more effective and more cost efficient. In this talk, we will discuss these issues and the developmental biology principles that underlie these phenomena, while also considering how time varying phenotype expression affects results. KEYWORDS: critical windows, developmental biology, exposure timing, bias