The agency's current rules permit lead levels of 50 parts per billion or lower in water.

Others said the conclusions could support maximum levels of less than three parts per billion.

Where those standards are being met, the average first-draw water probably has a level of less than 5 parts.

The hole appeared after chlorine levels increased to the present level of just 3 parts per billion.

After the pool has been properly shocked, he said, the chlorine content should be maintained at a level of 1 to 2 parts per million.

It could measure molecules present at a level of only a few parts per billion.

In 1985, state officials found the additive near the home of a neighborhood woman at the extraordinary level of 100,000 parts per billion, they said.

A level of 100 parts per million is generally considered dangerous enough to prompt evacuation.

"We couldn't find it at levels of 200 parts per billion," he said.

But even if a level of 10 parts was found, the worst houses could have had water with far more than 15 parts.