Lead paint in older homes is the primary source of childhood lead poisoning.  In the past 15 years, the number of Cambridge children with elevated blood lead levels has declined dramatically.  In fiscal year 2005, fewer than five children had elevated lead levels (greater than 15 micrograms per deciliter).  Local experts believe this decline is largely due to widespread screening, family education, and an increase in the number of renovated and de-leaded residences in Cambridge.