The Age-1 vs Age-2 Finding
In NHANES, one-year-olds consistently have higher geometric mean blood lead than two-year-olds. Across all 8 survey rounds from 1976 to 2023, age-1 GM BLL is at or above age-2. This is the opposite of what NYC and UK screening programs show.
The difference is sample design. NHANES is a national probability sample. It captures ambient and prenatal lead exposure that front-loads BLL in the first year of life, before the hand-to-mouth peak at 24 months. NYC/UK screening programs draw from paint-heavy urban contexts where the 24-month hand-to-mouth pattern dominates.
Both are real. Which one you see depends entirely on who you're measuring. The age matrix PDF above maps out all 8 NHANES rounds, the Rochester cohort from Canfield NEJM 2003, and the NYC/UK screening literature side by side.
The prenatal exposure window is the policy implication. If ambient and in-utero routes front-load BLL before a child turns one, screening at housing turnover or move-in, before exposure occurs, is the lever that actually moves outcomes.