Factors Associated with the Seasonality of Blood Lead Levels Among Preschool Wisconsin Children

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1 Factors Associated with the Seasonality of Blood Lead Levels Among Preschool Wisconsin Children Jeff Havlena, MS; Marty S. Kanarek, PhD, MPH; Margie Coons, RN, MS ABSTRACT Context: Aggregate blood lead testing data for Wisconsin children younger than age 6 exhibit seasonal trends in both average blood lead levels and in the percent of those tested and found to have blood lead levels in excess of the 10 mcg/dl threshold for poisoning. Blood lead levels and poisoning rates typically peak during the late summer and early fall months, and are at their minimum during the late winter. Method: Blood test data was analyzed to determine variations by month and age. Results: Seasonal variations are evident even among the very young: infants younger than 10 months who likely have limited opportunity to encounter lead hazards within their home or in the outdoor soil. Seasonal periodicity is most evident among infants who reside in very urban and very rural communities. The observed seasonal periodicity might be associated with the seasonal availability of lead within the children s environment. Particulate matter data measured at several ambient air quality monitoring stations exhibit a similar periodic seasonality, suggesting a possible relationship between blood lead levels and the availability of dust and airborne particulates during the summer months. Conclusion: Clinicians should consider the seasonality of blood lead levels when scheduling tests and interpreting test results. Author Affiliations: Wisconsin Childhood Lead Poisoning Prevention Program (WCLPP), Wisconsin Department of Health Services (DHS), Madison, Wis (Havlena, Coons); Department of Population Health Sciences, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wis (Havlena, Kanarek, Coons); Gaylord Nelson Institute for Environmental Studies, University of Wisconsin-Madison, Madison, Wis (Kanarek). Corresponding Author: Jeffrey Havlena, Surveillance Manager, Wisconsin Childhood Lead Poisoning Prevention Program, 1 W Wilson St, Room 145, Madison, WI 53702; phone ; jeffrey.havlena@wisconsin.gov or jahavlena@wisc.edu. INTRODUCTION Blood lead levels among preschool age children have long been known to follow seasonal trends. As early as 1925, researchers have documented that maximum blood lead levels occur during the summer and late summer months. 1 More recently, Laidlaw, Filippelli and other researchers found a distinct seasonal variation of blood lead levels and the incidence of lead poisoning, primarily among children who live in urban areas. 2-3 The observed seasonality has been attributed to (1) airborne dust originating from dry summer soils being blown into open windows; (2) children playing outdoors in lead-contaminated soil; (3) lead-contaminated soil tracked into living spaces on shoes and clothing; and (4) seasonally varying levels of vitamin D and calcium in children s blood, and the physiologic effects on the absorption and retention of lead within children s blood. The seasonal component of childhood blood lead is so widely acknowledged that Hunter referred to childhood lead poisoning as the summer disease. 4 Seasonal differences in blood lead levels were well documented during the 1970s and early 1980s by Hunter and others. 4-6 These researchers found a distinct seasonal influence on blood lead levels, with the highest levels occurring during the late summer, and lowest levels during late winter and early spring. They postulated that seasonal differences could be caused by various factors such as air pollution and physiological factors that include seasonal variations in the uptake and retention of vitamin D and calcium as well as exposure to soil and dust associated with outdoor play activities and open windows during the warmer months. Research conducted during the Childhood Lead Exposure Assessment and Reduction Study concluded that some of the seasonal variability in blood lead levels is probably due to increased exposure to lead in dust and soil. 7 The authors used atomic absorption spectrophotometry and mass spectroscopy on samples of indoor dust and outdoor soil at a number of loca- 151

2 Figure 1. Mean monthly blood lead; Wisconsin children younger than 6 tested tions in Jersey City, New Jersey. Comparison of the samples indicated that approximately two-thirds of the dust found inside the homes was from sources outside the homes. The remaining one-third was from deteriorating lead-based paint inside the homes. By comparison, Laidlaw et al cite a study from the Bunker Hill Superfund site in Idaho that found approximately 40% of blood lead levels are likely due to house dust, with the remaining 60% split evenly between communitywide soils and soils in the immediate neighborhood surrounding the houses. 2 Yiin et al postulated that the outdoor sources of lead serve as the primary cause of the observed seasonality, and become more available during the warm summer months when windows are open and there is increased foot traffic and exchange between the indoor and outdoor environments. 7 Laidlaw et al proposed a conceptual model for seasonality that incorporates climate and soil physical characteristics to explain the processes through which leadbearing soil particles predominantly fine, clay-sized particles (PM 10 and finer) that are most susceptible to windblown transport are mobilized and transported preferentially during the summer months. 2 This model extends the large body of research on wind erosion and transport of agricultural soils, and links PM 10 with transport of lead and availability in a child s home environment. Clausnitzer and Singer found that the amount of fine particulate matter entrained and concentration of respirable dust both increased linearly as air temperature increased, and decreased as a power function as soil moisture increased. They also found that maximum production and transport of respirable dust occurs during hot, dry, and windy summer months when fine soil particles are more freely available to become mobilized and transported sub aerially. 8 These are the very conditions when houses are most permeable, being open to the exchange between indoor and outdoor environments. Potential sources of anthropogenic lead in the soil include residual lead from leaded gasoline, lead from industrial processes, and lead particulate from leadbased paint on the exterior of houses and buildings. Laidlaw et al concluded that in many urban settings, residual automotive lead continues to be a significant component of the overall concentration of lead in the soil and outdoor environment. 2 Filippelli et al measured the concentration of lead in surface soils adjacent to Indianapolis streets. 3 They determined that although the concentration of lead decreased exponentially with distance from the roadways, the low solubility of lead and the presence of grasses (as in front yards and parks) could lead to a concentration of lead in the near-surface soils, where it might be available for transport to children. Physical partitioning of anthropogenic lead into carbonate, iron, and manganese minerals, and the finer, clay-sized soil fractions leads to increased transport and bioavailability. As a result, anthropogenic lead in dust from such soils is considered to be more problematic and pose a greater risk than naturally occurring lead 3. This paper explores whether the seasonal behavior and apparent relationship to seasonal environmental influences observed by Laidlaw and others applies to Wisconsin. METHODS The Wisconsin Childhood Lead Poisoning Prevention Program (WCLPPP) within the state Division of Public Health has tracked childhood lead poisoning in Wisconsin for more than a decade. 9 Since the 1994 enactment of State Statutes and , all blood lead levels for tests done on children are reported to the state Division of Public Health, and the results and associated test information entered into a comprehensive database. The WCLPPP maintains all childhood blood lead testing data in the STELLAR database, which includes information on the child, the child s address, his or her attending physician, and test date as well as sample type and result for all tests conducted on Wisconsin children. Among the data collected are test and blood lead level information, along with data about the child s address and age at the time of the test. The WCLPPP lead testing data were used to develop analyses of the temporal distribution of mean blood lead levels. Blood lead test results were analyzed for children younger than age 6 (N=676,928 children and 1,168,298 tests; mean monthly number of children tested range 152

3 from 6263 for December to 9159 for September), with particular emphasis on infants younger than 10 months of age (n=100,599 infants and 102,616 tests; mean monthly number of infants tested range from 597 for December to 697 for March). Data for infants younger than 10 months was selected because: (1) they were presumed to be too young to walk and consequently not likely to come in direct contact with lead dust on windowsills or outdoor soil, and (2) their blood lead testing histories likely were short and the data not biased by multiple follow-up tests for poisoned children; only 1884 (1.9%) of the tested infants had more than 1 test previously. Infants are assumed to be relatively immobile and not likely to come in direct contact with lead at windows or in the outside soil; however, many likely come in contact with dust on floors and other accessible surfaces. Nearly 80% of the infants included in this study were tested at 8 or 9 months of age (mean=8.5 months; median=8.2 months), possibly associated with their 9-month well child visit. Although the American Academy of Pediatrics guidelines recommend that children be screened at 9-12 months, then again at 24 months, most children receive their 1-year test during their 12-month well child visit. 10 It is likely that many of the infants included in the current analysis were tested early because they were perceived to be at risk of exposure to lead in their home. RESULTS Figure 1 shows the composite mean monthly blood lead levels for Wisconsin children younger than age 6 for the period 1996 through The seasonal periodicity is evident, with a classic 6-month annual period between the month with the minimum mean blood lead level (March) and the month with the peak (September). The September peak of 4.96 mcg/dl is 15.9% greater than the March minimum of 4.28 mcg/dl. The mean value for the 6-month peak period from June through November is 9% greater than the mean value for the minimum period December through May. These are composite values, aggregated over the multi-year period from 1996 through Individual years show greater variability (Figure 2, which is for infants younger than 10 months), as do values for individual children. Figure 2 shows the time series of monthly mean blood lead levels for Wisconsin infants younger than 10 months for the years 1996 through This figure shows: (1) a generally downward long-term trend, and (2) an apparent seasonal trend for most years, with peak values typically occurring during late summer, and minimum levels during the late winter. Figure 2. Monthly mean blood lead levels for Wisconsin infants younger than 10 months, tested from Milwaukee is represented in black, the rest of Wisconsin in gray. Figure 3. Composite monthly mean blood lead levels and number of Wisconsin infants younger than 10 months tested from Figure 4. Composite monthly mean blood lead levels by census tract percent of urban Wisconsin infants younger than 10 months tested from

4 Figure 5. Composite monthly mean blood lead levels by census tract percent of urban Wisconsin infants younger than 10 months, tested from Figure 6. Mean monthly maximum one-hour PM 2.5 at ambient air quality monitoring period of record: Dodge ( ); Kenosha ( ); Milwaukee ( ); Waukesha ( ). Figure 7. Mean monthly maximum 1-hour PM 2.5 at Milwaukee Station 26 during the period and composite monthly mean blood levels for Milwaukee infants younger than 10 months, tested from Figure 3 shows the statewide, composite monthly mean blood lead levels, along with the number of children tested. This graph shows that there does not appear to be a bias in the mean blood lead levels, in that there is no apparent relationship between the blood lead levels and the number of children tested. The magnitude and period of the seasonal trends appear to be approximately similar for each geographic region. However, further geographic stratification shows a distinct difference in seasonality between children who live in rural, suburban, and urban settings. The most distinct seasonal trend occurs for census tracts identified by the Census Bureau as being 80%-100% urban (Figure 4). 11 Tracts that fall within this category typically lie within the core central areas of large- and moderate-sized cities. The next most distinct seasonal trend occurs among children who live in census tracts that are considered 0%-20% urban. These tracts are spread throughout the state, comprise the bulk of the land area of Wisconsin, and are typically agricultural areas away from city centers (Figure 5). The intermediate tracts (20%-50% urban and 50%-80% urban) typically lay outside cities, and may be considered to be suburban tracts. The oldest and lowest valued housing occurs most frequently within the 0%-20% and 80%- 100% urban tracts, and the highest value and newest housing occurs predominantly in the intermediate (20%-80% urban) tracts. The monthly maximum 1-hour values for particulate matter smaller than 2.4 microns (PM 2.5 ) were examined in order to examine the relationship between windborne dust and the observed trends in blood lead levels. 12 Monthly maximum 1-hour PM 2.5 values are appropriate for this comparison in that they represent a maximum potential for exposure, rather than an average value. The transport of windblown, leadcontaining dust into homes is dependent upon, among other factors, the intensity of the wind and the amount of suspended particulate, rather than the duration of the wind. A short-lived, but intense event could lead to significant transport of particulates into the interior of buildings, especially if the doors and windows were open or in use at the time. Figure 6 shows the mean monthly daily max 1-hour PM 2.5 values for 4 air quality monitoring stations: Dodge County, Kenosha County, Waukesha County, and Milwaukee County. The seasonal trends are remarkably similar to each other, and to the seasonal trends of mean blood lead levels (Figure 7). 154

5 DISCUSSION There appears to be a strong degree of seasonality of blood lead values across Wisconsin. However, for infants >10 months, this trend appears to be most noticeable within very-urban areas and rural settings, and least noticeable in suburban areas. PM 2.5 data for several stations appears to correlate with the seasonal blood lead trends. These results appear to be in keeping with the findings of Laidlaw et al who assert that blood lead levels are associated with lead-containing dust, a significant fraction of which arrives from outside the child s house. 2 Further, the apparent dependence on geographic location could be due to the quality of housing in each of the census areas. Farm houses in the rural tracts and older houses in the city-core census tracts that contain a disproportionate fraction of low-valuation, older houses are most likely to be poorly maintained, lack weatherization, and not have air conditioning. All these factors could contribute to the observed seasonal trends, and should be the subject of further study. In the meantime, parents, health care professionals, public health officials, and others with an interest in reducing the impacts of childhood lead poisoning in Wisconsin should be aware of the seasonal nature of blood lead levels, and consider retesting children who have blood lead results that approach the 10 mcg/dl threshold level during an off-peak period. For example, if a child received his or her 12-month blood lead test in May with a result of 9.0 mcg/dl, the clinician should consider retesting that child during a higher risk period, especially if the child resides in a central city or very rural area. Furthermore, the geographic variation in seasonality and the trends of blood and PM 2.5 point to the potential importance of off-site sources for lead, and the need to consider airborne sources when evaluating the overall risk to children. If further investigation identifies outdoor sources of lead in soil and wind blown dust, appropriate measures should be taken. These can include methods to increase the ground cover, such as landscaping with grasses or other vegetation (most effectively using plants that are known to remove lead from the soil via phytoremediation), as well as improvements to the child s house and adjacent structures to reduce the infiltration of dust into the child s house. Funding/Support: Research support for this manuscript provided by the National Cetner for Environmental Health Centers for Disease Control and Prevention, Childhood Lead Poisoning Prevention Program. Financial Disclosures: None declared. References 1 Aub JC, Fairhall LT, Minot AS, Reznikoff P, Lead poisoning. LEAD Action News. 1996;4(1): Laidlaw MAS, Mielke HW, Filippelli GM, Johnson DL, Gonzales CR. Seasonality and children s blood lead levels: developing a predictive model using climatic variables and blood lead data from Indianapolis, Indiana, Syracuse, New York, and New Orleans, Louisiana. Environ Health Perspect. 2005;113(6): Filippelli GG, Laidlaw MAS, Latimer JC, Raftis R. Urban lead poisoning and medical geology: an unfinished story. GSA Today. 2005;15(1): Hunter JM. The summer disease: an integrative model of the seasonality aspects of childhood lead poisoning. Soc Sci Med. 1977;11: Billick IH, Curran AS, Shier DR. Analysis of pediatric blood lead levels in New York city for Environ Health Perspect. 1979;31: Stark AD, Quah RF, Meigs JW, de Louis ER. Season as a factor in variability of blood-lead levels in children. Conn Med. 1980;44(7): Yiin LM, Rhoads GC, Lioy PJ. Seasonal influences on childhood lead exposure. Environ Health Perspect. 2000:108; Clausnitzer H, Singer MJ. Environmental influences on respirable dust production from agricultural operations in California. Atmos Environ. 2000:34(11); Department of Health and Family Services. Wisconsin Childhood Lead Poisoning Elimination Plan Madison, WI: Wisconsin Childhood Lead Poisoning Prevention Program; 2004, Updated American Academy of Pediatrics: Screening for Elevated Blood Lead Levels. Pediatrics. 2006:101(6); US Census Bureau, AmericanFactFinder. ( Accessed May 5, US EPA Air-Quality System. aqsdb.html. Accessed May 5,

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