A Tokyo citizens' group said Monday it has detected high concentrations of potentially harmful substances known as PFAS in some western Tokyo residents' blood, suggesting it could be due to contamination of the local water supply by U.S. military base activity.

Despite the discovery of high levels of PFAS, or per- and polyfluoroalkyl substances, Koji Harada, an associate professor of public health at Kyoto University, who headed the testing efforts, said it was "highly unlikely to cause acute health effects."

Photo taken on Jan. 30, 2023, shows a western Tokyo citizens' group holding a press conference over the detection of potentially harmful substances in people's blood, believed to originate from contaminated potable water. (Kyodo)

PFAS is a general term for a group of artificial chemicals, such as PFOS, or perfluorooctanesulfonic acid, and PFOA, or perfluorooctanoic acid. As Japan has no guidelines for measuring their concentration in the blood, Harada said residents were tested under German standards.

Photo taken on Jan. 30, 2023, shows Koji Harada, an associate professor of public health at Kyoto University, who headed efforts to test western Tokyo residents for potentially harmful substances in their bloodstreams. (Kyodo)

Speaking at a press conference, the group said PFAS are found in foam extinguishers used on U.S. military bases but said it was unclear whether the findings had any relation to Yokota Air Base, a U.S. Air Force installation located in the Tama area of western Tokyo.

The group began carrying out tests in November to determine the source of the contamination. The tests involved 87 people, mostly residents of Kokubunji and some from other cities, such as Kodaira, Koganei and Musashino.

According to its interim results, 21 of them were found to have exceeded acceptable levels of PFOS in their bloodstream. Six of the 21 also had unacceptable levels of PFOA.

The highest detected concentration of PFOS was 35.8 nanograms and 18.6 nanograms for PFOA. The German standard per milliliter for each chemical is 20 nanograms and 10 nanograms, respectively.

"The concentration is evidently high for some residents," Harada said. "That leads one to think potable water is the main cause of it."

Harada had also previously detected PFAS in residents' bloodstreams in the vicinity of a U.S. base in Okinawa Prefecture in Japan's south.

Out of 387 people tested, Harada found 27 exceeded the German standard, with the highest example being 41.6 nanograms of PFOS in a person.

"The state and local governments must deal with the issue from the perspective of what to do with the source of contamination," Harada told reporters.

In the past, the Tokyo metropolitan government has detected high levels of PFOS and PFOA in wells in parts of the Tama area, leading some purification facilities in Kokubunji and Fuchu to halt drawing water from affected wells.