A group of residents in western Tokyo checking concentrations of potentially harmful PFAS chemicals in their blood have confirmed an exposure that is 2.7 times higher than the national average and at a level that may produce long-term health effects.

According to the survey results released Thursday, which were based on blood samples of 650 residents 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, more than double the average of 8.7 ng/ml among 119 people tested in a 2021 pilot study by the Environment Ministry.

The average age of the 650 people, from 27 municipalities in western Tokyo, was 66.8.

PFAS, also called “forever chemicals” because they are hard to break down in the body and the environment, refers to a class of at least 4,700 synthetic chemicals used in a wide range of products that resist heat, oil and water. The substances may lead to a range of health issues such as kidney or testicular cancer and higher cholesterol levels, as well as increase the risk of lower birth weight and weakened vaccine response in children, according to the U.S. Agency for Toxic Substances and Disease Registry, a branch of the Centers for Disease Control and Prevention.

PFAS levels were high especially among residents in the city of Kokubunji, whose blood concentrations averaged 45 ng/ml, followed by those in Tachikawa (28.6 ng/ml), Musashino (27.2 ng/ml) and Fuchu (23.9 ng/ml).

While the central government sets no limits for PFAS in people's bloodstreams, the U.S. National Academies of Sciences, Engineering and Medicine said in a 2022 report that people with PFAS exposure of between 2 and 20 ng/ml have “potential for adverse health effects,” while those with an exposure of 20 ng/ml or higher have “an increased risk of adverse health effects.”

Koji Harada, an associate professor of environmental health at Kyoto University who was involved in the analysis of the blood, said that higher-than-normal readings are likely attributable to past contamination of drinking water in the area.

Many communities in western Tokyo have for decades used groundwater pumped from the area’s wells as drinking water. The Tokyo Metropolitan Government, however, stopped using groundwater in June 2019 after detecting elevated levels of PFOS and PFOA at some water purification plants.

During a news conference held in Tachikawa on Thursday, Harada called for long-term and more extensive monitoring of people’s blood across the nation by the government, saying that the full extent of PFAS contamination in Japan remains unclear.

“Considering that the levels of contamination are still high despite measures taken by the metropolitan government (to stop supplying polluted water), we need to monitor the health effects over the long term,” he said.

The group 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.

Harada said more investigations are necessary in high-risk communities known to or suspected to have used used such firefighting foam or other PFAS-containing products, such as U.S. military bases, Self-Defense Force bases, petroleum complexes and chrome-plating factories.