Washington DC Public Schools Ensures Lead Levels Are 20 Times Less Than PGCPS

Washington, DC public schools recently lowered their own “action level” for lead in water to 1 ppm (parts per billion).  Yet Prince George's County uses a lead level of 20?

Clearly PGCPS Should be using an action level of 1ppb , just as Washington DC Schools are doing.
Why are Washington DC Schools using 1 ppb and Prince George's County using 20 ppb?

Read about it here http://www.washingtoncitypaper.com/news/article/20782859/dc-tested-public-schools-water-for-lead-more-than-60-had-high-levels