The Lancet article (April 2018) finds:
of 2.3 million deaths every year in the USA, about 400 000 are attributable to lead exposure
Contrary to another answer, the article says:
All models are adjusted for... household income
So it is reasonable to say 1 in 6 based on that article.
Note that this is based upon deaths in the 1988 to 2011 time ...
There is quantitative data in Childhood Lead Poisoning
The City of St. Louis for every year from 1971 through 2009.
Using a 10 micrograms per deciliter standard, the fraction of children testing positive decreased from 31.1% in 2000 and 16.2% in 2001 to 3.2% in 2009.
By 2014, the fraction testing positive decreased to 1.72% according to Missouri
Per Wayne Hall in 2013, "lead exposure in childhood may have played a small role in rising and falling crime rates in the USA but it is unlikely to account for the very high percentage of the decline suggested by the ecological studies. We need more cohort studies in environments where lead exposure remains high, particularly in developing countries. The ...
You ask two questions. @DavePhD addressed the first one. The second is also answerable from the same data source. The total number of children tested was 11,260 in 2000 and 13,522 in 2009. This is not enough to explain a decrease in the positive testing rate from 31.1% in 2000 to 3.2% in 2009.
There is a bit of a problem in choosing the bounds though. ...