The concentrations of the potentially harmful per-and polyfluoroalkyl substances (PFAS) chemicals in the blood of a group of residents in western Tokyo were found 2.7 times higher than Japan's national average, the Japan Times reported on Thursday.

The high level may produce long-term health effects, the Japan Times reported, citing the survey results released on Thursday. The results were based on blood samples of 650 residents in the area taken between last November and March. 

The average exposure to the sum of four PFAS chemicals - PFOS, PFOA, PFHxS and PFNA - was 23.4 nanograms per milliliter (ng/ml), more than twice the average of 8.7 ng/ml among 119 people tested in a 2021 pilot study by Japan's environment ministry, the results showed.

The tap water is the main course of the high PFAS levels, the Asahi Shimbun reported, citing Koji Harada, an associate professor of environmental health at Kyoto University who was involved in the analysis of the blood.

The western Tokyo, also known as the Tama region, partially uses purified groundwater for tap water, and Harada said possible sources of contamination include the U.S. Yokota Air Base, which is located in the upper river basin of the groundwater.

The research team, which conducted the survey, said it plans to check the PFAS levels of 100 wells in areas surrounding Yokota Air Base — where the past use of PFAS-containing firefighting foam has been confirmed — by the end of this month.

Known as "forever chemicals" that are extremely persistent in the environment and human bodies, PFAS could lead to health problems such as liver damage, thyroid disease, obesity, fertility issues and cancer.

PFAS has been detected in various parts of Japan, including many locations around U.S. military bases.

In 2022, blood samples of 387 residents living near U.S. bases in Okinawa Prefecture showed the levels of PFOS and PFOA were 0.8 to 3.1 times the national average, and foam fire-extinguishing agents containing PFOS were leaked into a river from the Naval Air Facility Atsugi, the Asahi Shimbun reported. 

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(With input from agencies)

(Cover: A helicopter is seen at Yokota air Base in Tokyo, Japan, May 22, 2022. /CFP )