Researchers who played a critical role in the discovery of lead in Flint, Michigan, drinking water say tests reveal another rise in water quality.

Virginia Tech says lead wasn't detected in 57 percent of 154 Flint homes tested in November — up from 44 percent in July.

Flint has a lead problem because it used water from the Flint River for 18 months without treating it to prevent pipe corrosion. As a result, the water caused lead to leach from old pipes and into homes.

But since fall 2015, the city has been tapped into a regional water system that uses corrosion controls. That's why the testing results have improved.

Nevertheless, researcher Marc Edwards emphasized again that residents still should keep a lead filter on their kitchen taps.