This map and database include tests for more than 6,100 D.C. homes conducted from March 2003 to January 2004. The map reflects lead readings from the second draw of water tested, which means a sample is taken between one and two minutes after the tap is first turned on in the morning. In most cities, this second reading is expected to typically show lower lead levels, but results in D.C. over the last year pointed to a greater problem.
Errors can occur in the readings for a range of reasons. The water samples, drawn by resident volunteers, can be difficult to take correctly. Errors in data entry and illegible writing can also affect accuracy.
The Post database is based on lead-testing results provided by Dorothy Brizill, a city activist who operates www.dcwatch.com, an independent Web site monitoring D.C. government. The Post matched the results against property records to obtain more detail on the homes. Post reporters also verified many of the individual results with residents who had received official letters from the D.C. Water and Sewer Authority.
The scanning and matching process can introduce another potential source of error. Although every effort has been made to check the resulting database against those documents, WASA has refused to release the original results through the D.C. Freedom of Information Act, making further error checking impossible.
The map displays the data in three ranges: 0 to 15 parts per billion, which is considered a safe level; 15.1 to 150 ppb, which is considered unsafe; and above 150 ppb, which is considered unusually and dangerously high. The third range was chosen because experts are concerned that water filters have not been tested at levels above 150 ppb and may be ineffective in removing enough lead from the water to make it safe to drink.