Water fluoridation is controversial. Over the past decade, concern about the potential toxicity of fluoride has grown, but there are still questions. Most human studies of fluoride toxicity only studied children who were exposed to high levels of fluoride and only a few studies measured fluoride exposure during critical periods of development. Urine, which is commonly used to quantify a person’s exposure to fluoride, reflects recent exposure and requires serial sampling to measure fluoride during different periods of development. Our proposed study will analyze tooth dentin – the tissue that lies beneath enamel – to measure the level and timing of fluoride exposure. Tooth dentin is an optimal biomarker because it provides a historical record of both serial and cumulative exposure to ingested fluoride. We will test the following specific aims: Aim 1. To quantify prenatal, early childhood, and cumulative fluoride exposure levels using shed baby teeth collected from 800 children; Aim 2. To test whether dentin fluoride concentrations in baby teeth is associated with lower IQ scores (n=610) and attention problems (n=898); Aim 3. To test whether the urinary and dentin fluoride concentrations impact thyroid hormones, and ultimately IQ scores and attention problems. This study is innovative because it will employ state-of the-art analytical methods to measure the level and precise timing of exposures to fluoride and toxic metals in tooth dentin. The study capitalizes on an existing Canadian pregnancy and birth cohort, the Maternal-Infant Research on Environmental Chemicals. We will leverage existing data from our ongoing NIH-funded study to find out if fluoride exposure during early brain growth alters children’s IQ scores and behavioral problems. In addition, we will test the endocrine disrupting potential of fluoride during pregnancy. Our measurement of fluoride exposure in tooth dentin and serial urine samples offers an unprecedented opportunity to test for neurotoxic effects of early-life exposure to fluoride. The large sample size will let us find out if to child sex, timing of exposure, or exposure to powdered infant formula impact fluoride’s toxicity. Major contributions include: 1) quantification of exposure to fluoride during fetal development and early childhood using dentin; and 2) results that will directly impact decision making concerning the safety of fluoride exposure during fetal development and early childhood at levels relevant to the U.S. and Canada. Given our archived resources, we will be able to accomplish this study efficiently and cost-effectively.