Table 2.

Outcome and quality assessment of studies, limitations and benefits of Articles

Author ,Year, County	Strengths and Limitations of Study	Quality
Chiodo, 2004, U.S.	Strengths: Controlled for various factors (maternal alcohol/drug use; socioeconomic status; maternal age, marital status, education, household; child sex), quality control measures taken for BLL measurements. Limitations: Only one-time blood lead measured which may not be true indicator of lifetime lead exposure; sample population only included children of African American mothers.	High
Braun, 2006, Mexican Americans, and non-Hispanic blacks	Strengths: Large sample size, large number of confounders determined (gender, child IQ, maternal education, home environment, maternal smoking, income); recent studies indicate that concurrent blood lead level is a stronger predictor of lead-associated IQ decrements than blood lead measured during early childhood, although some might argue that concurrent blood lead tests are not an adequate biomarker of a child’s lifetime exposure Limitations: the cross-sectional nature of the data makes it difficult to infer a causal relationship from observed associations; sample population only included children of Mexican Americans, and non-Hispanic blacks.	High
Min, 2007, South Korea	Strengths: External quality control measured for blood draws. Limitations: Confounding variable not included (SES, nutrition, environment of household). Study population recruited from a common district office which included all white collared workers. Only 1 blood measurement taken which may not be an indication of lifetime blood lead levels. Very small sample size.	High
Chiodo, 2007, U.S.	Strengths: Many control variables taken into account (prenatal alcohol and drug use, socioeconomic status, home environment, maternal IQ); internal and external quality control measures taken for BLL measurements; large sample size. Limitations: Only one-time blood lead measured which may not be true indicator of lifetime lead exposure; only African-American children were in the sample population and is not representative of population in the U.S.	High
Nigg, 2008, U.S	Strengths: Blood draws were analyzed twice to ensure accuracy, clinical diagnosis of ADHD types were known. Limitations: BLL measured only 1 time which may not be a good indicator of lifetime lead exposure; Hyperactive children may ingest more lead and therefore have higher BLL; not a random population sample.	High
Author ,Year, County	Strengths and Limitations of Study	Quality
Chandra mouli, 2009, U.K.	Strengths: Large sample size, large number of confounders determined (gender, child IQ, maternal education, home environment, maternal smoking, income); quality control methods for BLL measurements were validated; Limitations: Selection bias due to higher income and higher education level of participants families; Only had lead data from 30 months which may or may not be representative of the child's BLL during critical age of development.	High
Wang, 2008, China	Strengths: Large sample size. Used ADHD diagnosed children and compared to non- ADHD children to compare BLL. Many covariates and confounders identified such as sex, low SES, age, family history of ADHD, maternal drinking, parent education level. Internal quality control measured taken for BLL measurement method. Limitations: Only one BLL measurement taken which may not be an accurate indication of lifetime BLL.	High
Froehlich, 2009, U.S	Strengths: Large sample size, large number of confounders determined gender, household income, age, race/ethnicity, postnatal tobacco exposure); used DSM-IV defined ADHD criteria. Limitations: Only 1 BLL was taken which may not reflect lifetime BLL exposure. No quality control measures for BLL measurements were stated.	Medium
Ha, 2009, South Korea	Strengths: Large sample population used. Large numbers of confounding factors were identified (education level of parents, household income, maternal smoking and alcohol consumption during pregnancy). Limitations: Only one BLL measured which may not be an accurate measurement of lifetime BLL. No internal/external quality control measures were stated.	High
Cho, 2010, Korean cities	Strengths: adjusted for age of the child, gender, paternal education level, maternal IQ, child’s IQ, and birth weight of the child Limitations: First, the cross-sectional nature of our data limits our ability to assess direct causal associations between environmental lead and tobacco smoke exposure and inattentive and hyperactive symptoms and neurocognitive performance. Second, a single measure of blood lead level may make it difficult to disentangle the effects of earlier childhood lead levels from current levels.	High
Author ,Year, County	Strengths and Limitations of Study	Quality
Kim, 2010,	Strengths: they checked SES, parental education, ETS, smoking during pregnancy, and child gender Limitations: this study included the fact that the results may be confounded by unmeasured early neurodevelopmental patterns and the family history of ADHD in these children and limited information about these children's gestations and the home environment	High
Nicolescu, 2010,	Strengths: Various methods of measuring ADHD symptoms were used. Limitations: Possible confounders not controlled (home environment, maternal IQ), only 1 BLL measurement obtained which may not be a good indicator of lifetime BLL in child.	High
Nigg, 2010, U.S	Strengths: Clinical diagnoses of ADHD types were known; measured ADHD symptoms via multiple methods. Limitations: Only 1 blood lead measurement used which may not represent lifetime BLL;Not random population samples (sampling bias); hyperactive behavior may cause children to ingest more lead.	High
Darugar, 2010, Iran	Strengths: Differences in parents education and outcome of them are cofounding Limitations: Uncertainty of the reliability of the results, Only one BLL measured which may not be an accurate measurement of lifetime BLL.	Low
Boucher, 2012, Montreal	Strengths: The strengths of this study include ability to control for confounding by other contaminants present in seafood—specifically for confounding of the association between child Pb and outcomes by cord Hg. Limitations: this study is that the maternal report of substance use during pregnancy was obtained in many cases about a decade after delivery. Another limitation is that do not have formal diagnoses for ADHD.	High
Kim, 2013, USA	Strengths: relationship between lead (Pb) exposure and medically diagnosed attention deficit hyperactivity disorder (ADHD) in children. The role of mercury (Hg) and cadmium (Cd) exposures in ADHD development is even less clear. Limitations: small sample size, had difficulty matching ADHD cases with non-ADHD controls. Sapling was for African American and cucasian.	High