Long-term exposure to arsenic in drinking water can cause cancer among other health risks, according to the World Health Organization (WHO).
Cornwall Council confirmed it was responsible for regulating supplies.
A private water supply is one not provided by a water company and can present a risk to health due to the quality of the water, according to the government.
Out of the 127 people whose urine was tested, about 20 people had levels of arsenic in their water supply above the 10 parts per billion (PBB) threshold recommended by the WHO. One volunteer was found to have 20 times this amount in their private water supply.
10 May 2016
Toxic arsenic found in Cornwall private water supplies