Table 2.
Relation of Covariates to Lifetime Average Blood Lead Concentration and Mean IQ Score at Five Years of Age.*
| Covariate† | No. of Children | Lifetime Average Blood Lead |
IQ |
|---|---|---|---|
| µg/dl | |||
| Mothers | |||
| Education level | |||
| <12 yr | 56 | 8.9±4.6 | 85.4±9.4 |
| 12 yr | 50 | 6.4±3.5 | 91.2±12.4 |
| >12 yr | 48 | 6.6±4.1 | 93.4±10.8 |
| Race‡ | |||
| Nonwhite | 113 | 8.2±4.4 | 87.5±9.5 |
| White | 41 | 4.9±2.6 | 96.1±13.6 |
| Income level | |||
| $6,000 | 37 | 8.8±3.8 | 83.8±9.3 |
| $6,001-$20,000 | 80 | 7.4±4.2 | 89.2±9.8 |
| >$20,000 | 37 | 5.8±4.4 | 97.0±12.7 |
| HOME total score§ | |||
| Low (<20) | 24 | 10.1±3.2 | 85.8±8.1 |
| Middle (20–30) | 76 | 7.6±4.8 | 87.9±9.7 |
| High (>30) | 54 | 5.8±2.9 | 94.2±13.3 |
| Prenatal smoking | |||
| No | 122 | 7.3±4.4 | 90.2±12.0 |
| Yes | 32 | 7.6±3.9 | 88.3±8.5 |
| IQ¶ | |||
| Low (<75) | 52 | 8.6±4.1 | 85.7±8.8 |
| Middle (75–85) | 45 | 7.7±5.0 | 86.9±8.5 |
| High (>85) | 57 | 5.9±3.3 | 95.9±12.8 |
| Children | |||
| Birth weight | |||
| <3500 g | 106 | 7.6±4.3 | 88.9±10.8 |
| ≥3500 g | 48 | 6.9±4.1 | 91.8±12.3 |
| Sex | |||
| Male | 73 | 7.6±3.9 | 88.3±12.5 |
| Female | 81 | 7.2±4.5 | 91.2±10.1 |
| Transferrin saturation | |||
| <20% | 60 | 7.0±4.2 | 89.5±8.5 |
| ≥20% | 94 | 7.6±4.3 | 90.0±12.9 |
The lifetime average blood lead concentration was estimated at the ages of 3 and 5 years by computing the area under the blood lead curve (AUC) from 6 through 36 months and from 6 through 60 months, respectively, and then dividing the AUC by its corresponding age span to yield an average on the micro-gram-per-deciliter scale. Data obtained at the age of three years were similar to the data obtained at five years of age and are not shown. Plus–minus values are means ±SD. To convert values for lead to micromoles per liter, multiply by 0.0483.
Some continuous variables were categorized for this analysis.
Race was self-assigned as white or nonwhite.
The Home Observation for Measurement of Environment Inventory (HOME) is an index that reflects the quality and quantity of emotional and cognitive stimulation in the home environment. The total score is the sum of 39 items, each scored as present (1) or absent (0), in six categories (maternal responsivi-ty, acceptance of child, organization of the home environment, provision of play materials, maternal involvement with the child, and the variety of stimulation).
The Stanford–Binet Intelligence Scale, fourth edition (abbreviated), was used to assess IQ.