Because of the health risks posed by lead contamination, the federal government requires water systems across the country to test samples from household taps for the toxic metal. Most water systems in Illinois have found lead at least once in the past decade; the state has more lead pipes connecting homes to water mains than any other. But a water system is not required to take action unless more than 10 percent of the samples taken exceed 15 parts per billion during a full testing cycle, often three years. To examine residents' potential exposure more closely, the Tribune analyzed test results by calendar year and found that more than 170 water systems in Illinois have exceeded the federal lead standard at least once in the last dozen years. Those systems serve hundreds of thousands of people in all. Look up test results for your own water system below.

Read the full story about testing for lead in Illinois water systems.

Water systems where lead testing found high levels The dots on the map below represent water systems where the Tribune found lead test results exceeded the federal standard in at least one year since 2004. (The Environmental Protection Agency does not consider a water system to be in violation unless test results exceed standards in a full testing cycle, which is often three years.) Scroll over the map to see the names of the water systems and other details.

Lead test results for local water systems Local water systems typically are not required to notify homeowners about lead levels or take other action unless test results exceed federal standards during a full testing cycle, which may be three years long. To provide a more detailed picture of residents' potential exposure, the Tribune analyzed lead results by calendar year. The EPA's procedures involve identifying the 90th percentile for the test scores and comparing that figure to the standard of 15 parts per billion. To carry out that calculation, the Tribune used a common mathematical formula that allowed for a consistent comparison. The EPA directs officials to use one of several alternative methods that may produce different results. Find results for your water system by typing its name in the "Search" input below. In many cases, your water system is the name of your city or town, such as Chicago, Marengo or Galesburg. Years in which the Tribune found the system exceeded the federal standard are highlighted in red. The Illinois EPA did not provide detailed information for some water systems; many are no longer operating. The EPA says it does not consider scores between 15.1 and 15.4 to be in violation, because it rounds to the nearest whole number for compliance purposes. Water system Test year Lead concentration at 90th percentile (ppb) Primary county Number of people served Number of tests