Health and human capital effects of lead exposure

2020 Fall.Includes bibliographical references.The legacy of lead in the United States is complex and intertwined with public health. As concerns over the toxicity of lead increased with time, policy makers responded with a series of national policies aimed at minimizing the risk of lead exposure across society. One such policy, the Clean Air Act (CAA), set a timeline for the removal of lead from gasoline beginning in 1975. This policy would target the anti-knock lead additive tetraethyl-lead (TEL), which was used to boost gasoline octane and improve engine performance (Needleman, 2000; Reyes, 2007). Over the following two decades, the flow of lead entering the environment from automobile emissions decreased precipitously. This dissertation exploits a natural experiment in lead exposure arising from the differential phase-out of leaded gasoline across states under the CAA. Though the policy was implemented at the national level, enforcement took place at the producer level, creating exogenous variation in lead emissions from automobile exhaust across states and over time. Since lead dust from automobile emissions was a significant source of lead exposure over the period, we leverage this spatial and temporal variation as a quasi-random vector of lead exposure. Chapter one summarizes the CAA, and the historical significance of the policy as it relates to public health. Using blood lead levels (BLLs) from The Second National Health Nutrition and Exercise Survey (NHANESII) as a bio-marker for lead exposure, this paper models the lead exposure effect of the policy. Combining annual gasoline sales and gasoline lead concentrations at the state level, the steps taken to construct the variables proxying for lead exposure following the CAA are detailed at length. The empirical strategy applied in this chapter is used to identify the causal effect of the phase out on lead exposure, and is carried over in the following two chapters. Much of the research focusing on the effects of lead exposure emphasize the risk faced by children, who are particularly susceptible to even minute quantities in the first five years of life. Chapter two tests the hypothesis that lead exposure in childhood impacts cognitive ability and the presence of abnormal latent preferences toward risk and uncertainty in adulthood. Applying the identification strategy detailed in Chapter one, to a nationally representative sample of individuals born during a period of significant reductions in leaded gasoline emissions, we find considerable evidence supporting the causal effect of childhood lead exposure and later in life outcomes. Across a series of tests, we find that BLLs in childhood are a significant predictor of: 1) IQ loss, measured with standardized test scores; 2) increased likelihood of low-IQ outcomes in exposure levels; and 3) increased abnormal risk response across a series of situations involving uncertain outcomes. The results presented in this paper illustrate the significance and persistent affect of early in life lead exposure. An underappreciated medium of child exposure to flow and legacy sources of lead is in-utero transmission of lead from mother to infant. Transmission of lead to the fetus occurs via diffusion across the placental barrier over the course of a pregnancy. Chapter three estimates the causal effect of maternal lead exposure on birth outcomes during the initial period of the phase out. Results show consistent evidence that fetal exposure to lead through the maternal blood lead pathway significantly depresses infant health. Our findings suggest that an increase in maternal blood lead: 1) decreases infant birthweight; 2) increases the risk of low and very low birthweight; 3) shortens gestation length; 4) increases the risk of prematurity; and 5) increases the risk of a low APGAR score. A back of the envelope calculation of the economic benefits of the phase-out of leaded gasoline through the reduction of healthcare-related costs involved in treating low birthweight infants, are in the tens of billions annually. It might be tempting to assume that lead exposure is a rear-view problem, at least in the United States, as BLLs in children have fallen since the 1990s, coincident with a series of actions that banned lead from paint, plumbing, food cans and automotive gasoline. However, the flow of lead into the environment continues from various point source polluters as well emissions from aviation gasoline used by an estimated 160,000 piston-engine aircraft (Kessler, 2013). Though the benefits to public health attributable to national policies are immense, the stock of legacy lead and present day flow sources of environmental lead remain a persistent threat to public health

Structure and Composition of Old-Growth and Unmanaged Second-Growth Riparian Forests at Redwood National Park, USA

Restoration of second-growth riparian stands has become an important issue for managers of redwood (Sequoia sempervirens [D. Don] Endl.) forest reserves. Identifying differences between old-growth and second-growth forest vegetation is a necessary step in evaluating restoration needs and targets. The objective of this study was to characterize and contrast vegetation structure and composition in old-growth and unmanaged second-growth riparian forests in adjacent, geomorphologically similar watersheds at Redwood National Park. In the old-growth, redwood was the dominant overstory species in terms of stem density, basal area, and importance values. Second-growth was dominated by red alder (Alnus rubra Bong.), Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco), and redwood. Understory species were similar in both forests, with several key differences: Oxalis oregana Nutt. and Trillium ovatum Pursh had greater importance values in the old-growth, and Vaccinium parvifolium Sm., Dryopteris spp. and sedges Carex spp. had greater importance values in the second-growth. Notable differences in structure and composition suggest that restoration practices such as thinning could expedite the acquisition of old-growth characteristics in second-growth riparian forests

Early-Stage Thinning for the Restoration of Young Redwood--Douglas-Fir Forests in Northern Coastal California, USA

Among forested parks and reserves of the Pacific Coast of the United States, the restoration of late-successional conditions to second-growth stands is a management priority. Some traditional silvicultural treatments may help achieve this objective. We evaluated early-stage thinning as a restoration treatment to facilitate the growth and development of young (33- to 45-year old), homogeneous, and second-growth stands of coast redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii). Targeting both stand-level responses and dominant (focal) tree responses for analysis, we compared structural attributes of adjacent thinned and unthinned stands, 12–17 years after thinning. Thinned stands displayed enhanced metrics of tree vigor, growth, and mechanical stability, thereby improving response to future restoration treatments and broadening the range of potential stand conditions. We conclude that early-stage thinning has been successful as a preliminary restoration treatment because it accomplished many initial goals of forest restoration, while retaining sufficient tree numbers to buffer against possible attrition from future disturbances

Recruitment of ponderosa pine seedlings beneath partial overstories

The objective of this project was to investigate the fate of ponderosa pine regeneration from seed to established seedling, and to determine the relative influences of several important stand elements on those fates. The project was carried out in a series of observational and experimental recruitment studies in central Oregon. In an analysis of the temporal and spatial patterns of seedfall during one season, viable seeds accounted for 62-73 percent of all seeds, and seed density averaged more than 1 million per hectare. More than half of all viable seeds fell within the first 30 days. Seedfall occurred more rapidly at the more mesic site. The less mesic site produced more seed, and the denser stand within each site yielded more seed. Seedfall patterns within each stand were not spatially uniform. At the two stands of moderate densities, the relationship of seed density to distance from trees followed a negative binomial distribution. In an observational study of germinant emergence and survival, about 3 percent of viable seed resulted in emergents. A small fraction of germinants survived the first summer, with most mortality occurring immediately after emergence. Most emergents were from caches, and germinants from caches remained a substantial portion of the cohort after two seasons. Mineral soil substrates produced fewer emergents and were associated with higher mortality rates than litter substrates. Understory cover produced fewer germinants than open locations, but mortality rates were lower. After one year, most survivors were beneath understory cover. In a seed-sowing experiment, less than 30 percent of seeds sown in fall resulted in live germinants the following spring, and just 5 percent resulted in live germinants by the following fall. Shrubs had no effect on germinant emergence rates, but germinants under shrubs succumbed to desiccation more slowly. Nearly 4 of 5 germinants that survived the first summer were beneath shrubs. Shrubs reduced solar radiation and reduced soil temperatures, especially during midday, but did not affect relative humidity or air temperature. Shallow burying of seeds to mimic rodent caching greatly improved emergence rates, with nearly 90 percent of all germinants emerging from buried seed

Adiposity, Cardiometabolic Risk, and Vitamin D Status: The Framingham Heart Study

OBJECTIVE: Because vitamin D deficiency is associated with a variety of chronic diseases, understanding the characteristics that promote vitamin D deficiency in otherwise healthy adults could have important clinical implications. Few studies relating vitamin D deficiency to obesity have included direct measures of adiposity. Furthermore, the degree to which vitamin D is associated with metabolic traits after adjusting for adiposity measures is unclear. RESEARCH DESIGN AND METHODS: We investigated the relations of serum 25-hydroxyvitamin D (25[OH]D) concentrations with indexes of cardiometabolic risk in 3,890 nondiabetic individuals; 1,882 had subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) volumes measured by multidetector computed tomography (CT). RESULTS: In multivariable-adjusted regression models, 25(OH)D was inversely associated with winter season, waist circumference, and serum insulin (P < 0.005 for all). In models further adjusted for CT measures, 25(OH)D was inversely related to SAT (−1.1 ng/ml per SD increment in SAT, P = 0.016) and VAT (−2.3 ng/ml per SD, P < 0.0001). The association of 25(OH)D with insulin resistance measures became nonsignificant after adjustment for VAT. Higher adiposity volumes were correlated with lower 25(OH)D across different categories of BMI, including in lean individuals (BMI <25 kg/m2). The prevalence of vitamin D deficiency (25[OH]D <20 ng/ml) was threefold higher in those with high SAT and high VAT than in those with low SAT and low VAT (P < 0.0001). CONCLUSIONS: Vitamin D status is strongly associated with variation in subcutaneous and especially visceral adiposity. The mechanisms by which adiposity promotes vitamin D deficiency warrant further study.National Institutes of Health's National Heart, Lung, and Blood Institute (N01-HC-25195, R01-DK-80739): American Heart Associatio