Drinking water at properties in three supplies in Ireland has been found to have at least 10-times the maximum allowable limit of lead.
There were more than 20 positive tests across the country, according to Irish Water's most recent inspections.
The Health Service Executive (HSE) website says lead may harm kidneys, contribute to high blood pressure and cause cancer.
Under the Freedom of Information Act, Irish Water has released details of its most recent test results for the substance.
Water at properties on 23 supplies across the country was found to have above the permitted level.
Lead of 11-times the limit was detected in a property on Dublin City Council's Leixlip supply, which serves much of north and west of the capital city.
Water at a property on Dun Laoghaire/Rathdown County Council's Roundwood Reservoir Supply, which serves places like Leopardstown and Stillorgan village, had 10-times the limit.
And lead of nearly 10-times the threshold was found in a property on the Castlerea Public Water Supply in Co Roscommon.
Environmental consultant Jack O'Sullivan says the results are a major concern.
"You could imagine lead being a little bit above normal, but 10 and 110-times above normal is absolutely horrific.
"The problem with that is lead is water can affect children.
"It's very harmful to young children because they absorb maybe four to five times the amount of lead that an adult would.
"And it's also harmful to the unborn as well".
Irish Water says the 23 exceedences arose from the presence of lead pipework within the properties in question, and is therefore the owners' responsibility.