How do you check PFAS in your local tap water? Reading the national tables and working out “what percent of the TDI”
The national survey tables show “each water supplier’s highest value in each fiscal year (PFOS + PFOA combined).” We cover how to read the sampling points, nd, “to be done within the fiscal year” and suppliers that buy all their water, how to find what city water bureaus publish themselves, testing from April 2026, and how to work out what percent of the tolerable daily intake your area’s value represents.
Short answer
The national table (published December 25, 2025) shows, for each water supplier, the highest value in each fiscal year from FY2020 to FY2025 (to the end of August 2025). Values are PFOS and PFOA combined. Urayomi’s prefecture pages list this table supplier by supplier.
A supplier that “buys all its water” has no water source of its own and buys its water from a bulk water supplier (such as a water supply authority). The key to reading the table is to look at the bulk supplier’s value too.
What percent of the tolerable daily intake (TDI) your value represents can be found with concentration × amount drunk ÷ body weight ÷ 20 × 100. For 2 L a day and 50 kg body weight, it is concentration × 0.2%. Exactly at the 50 ng/L standard, it is 10% of the TDI.
Put it in terms of your own intake
- Intake from water
- — ng/kg/day
- Compared with Japan’s tolerable daily intake (PFOS and PFOA, 20 ng/kg bw/day each)
- —
- Compared with EFSA’s tolerable weekly intake (4 PFAS combined, 4.4 ng/kg bw/week ≈ 0.63 a day)
- —
- Compared with intake from 37 foods (MAFF estimate: PFOS 0.13 + PFOA 0.09 = 0.22 ng/kg bw/day; the 37 foods make up about half of what people eat by amount)
- —
The combined value is treated as a single substance (in reality it is split between PFOS and PFOA, so the share of Japan’s TDI would be smaller). EFSA’s value is for the sum of PFOA, PFNA, PFHxS and PFOS; tap-water values here do not include PFNA or PFHxS. This calculation shows how the same intake reads differently against different yardsticks; it does not judge health effects.
Reading the national survey tables
MLIT and the Ministry of the Environment have twice published values for water suppliers nationwide: on November 29, 2024 (FY2020 to the end of September 2024) and in the follow-up survey of December 25, 2025 (FY2025 up to the end of August 2025). The tables are published as Excel files.
| Item in the table | Meaning |
|---|---|
| Highest value in the fiscal year | The highest value the supplier measured in that fiscal year. Even suppliers with several treatment plants are rolled into one value. A note in the national table also says values “differ within the supply area depending on the water source and treatment plant system.” It is not necessarily the value at your own tap |
| Combined | PFOS plus PFOA. The breakdown is not in the table |
| Sampling point | In principle, the tap. For suppliers that did not measure at the tap, the value is for water at the treatment plant outlet or for raw water (water before treatment at the plant). The 2024 table has columns such as “tap water only” and “raw water only,” but the 2025 table does not |
| nd | Below the detection limit or the quantification limit. It means “measured, but lower than the lowest level that can be measured” |
| To be done within the fiscal year | Not a value but a note meaning “testing is planned within that fiscal year” (190 entries) |
| Blank | No report for that fiscal year |
- Raw water values are not tap water values. There are cases where raw water exceeds 50 but is lowered at the treatment plant before being supplied. Urayomi marks raw water values with “raw water.”
- In the December 2025 table, 3,201 of the 3,550 water supply and bulk water supply operations have test results. Most of the operations tested are nd (up to the end of September 2024, 1,413 of 1,745).
- Between the 2024 and 2025 tables, 714 rows show different values for the same fiscal year. Most are reports added later. Urayomi treats the 2025 table as correct.
- As of November 2024, 98.2% of the population served was confirmed to receive water at 50 ng/L or less (1.5% unconfirmed, 0.3% private water supplies).
If the supplier buys all its water, trace it back to the source
Some water suppliers have their own sources (rivers, dams, wells), and some buy all their water from a bulk water supplier (such as a prefectural water supply authority). For suppliers that buy all their water, the water source and treatment plant belong to the bulk supplier.
- Example: Sakai City (堺市) buys all its water from the Osaka Water Supply Authority (大阪広域水道企業団) (its source is the Yodo River). In the national table, Sakai’s values are 12 ng/L for FY2024 and 10 ng/L for FY2025. At the outlets of the authority’s Murano (村野), Niwakubo (庭窪) and Mishima (三島) treatment plants, values were 10–13 ng/L in May 2026.
- In the 2024 survey, some suppliers that buy all their water did not test “because they buy all their water” (105 operations). In that case, the bulk supplier’s value is the clue.
- A single city can have a mix of its own municipal water supply, small water supply systems and partial purchases from a water supply authority. Urayomi’s prefecture pages note the bulk supplier where known.
Finding what cities and water bureaus publish themselves
The national table has only one value per fiscal year. Values by treatment plant, by tap and by season are sometimes published by water bureaus and water supply authorities themselves.
| Publisher | What they publish | Recent values (ng/L) |
|---|---|---|
| Sakai City Waterworks and Sewerage Bureau (堺市上下水道局) | 12 tap sites in the city. Four times a year from FY2025 | FY2026 (May–August) 7–10 |
| Osaka Water Supply Authority | Inlets (raw water) and outlets of three treatment plants, and branch points to the municipalities it supplies | Outlets in May 2026: 10–13 |
| Osaka City Waterworks Bureau (大阪市水道局) | Raw and treated water at three treatment plants. PFOS and PFOA separately (FY2023 research survey) | Treated water: PFOA 5–11, PFOS below 1–3 |
| Tokyo Metropolitan Waterworks Bureau | 131 tap sites (84 in Tama, 47 in the 23 wards) and treatment facilities, quarterly | Highest at Tama taps in FY2025: 14 |
| Okinawa Prefectural Enterprise Bureau | Sources, inlets and outlets of the Chatan treatment plant (北谷浄水場) and others. Weekly to monthly | FY2026 (April–August) Chatan outlet: max 6 |
- How to search: look for the water bureau’s page with “municipality name + 水道 + PFOS” or “water bureau name + 水質検査結果 (water quality test results).” Some prefectures collect links to the pages of suppliers in the prefecture (Tochigi, for example).
- Even for the same fiscal year, the national table and the city’s release can differ. For Sakai in FY2025, the national table shows 10, while the city’s test on August 5, 2025 showed a maximum of 12. Differences in cut-off dates or sampling points are possible, but the reason has not been confirmed.
- Tables of overseas standards on local government pages may show outdated values. For overseas values, see the guide to the standards.
Testing and publication from April 2026
From April 1, 2026, PFOS and PFOA became a water quality standard, and water suppliers are required to test for them.
- As a rule, testing is roughly once every three months or more. Even for municipal water supplies, if there is little risk of major changes in raw water quality (no change in water source or treatment method in the past three years) and all results over the past three years were at or below one-fifth of the standard (10 ng/L), testing can be reduced to once a year, and if at or below one-tenth (5 ng/L), to once every three years.
- Small water supply systems and private water supplies can test every six months or once a year if past results show little risk of detection, and once every three years if, under the same conditions as above, all results over the past three years were 5 ng/L or less. If a value exceeded 10 ng/L in the past year, testing goes back to once every three months.
- A supplier that buys all its water from a bulk supplier can skip its own testing if the bulk supplier’s results are 10 ng/L or less and it is confirmed that the concentration does not rise during delivery. Even then, it checks the bulk supplier’s results, and if they exceed 10, it tests on its own once every three months.
- Samples are taken at the tap as a rule. If it is clear that the concentration does not rise during transmission and distribution, the treatment plant outlet or similar points are also acceptable.
- Now that it is a standard item, PFOS and PFOA are expected to appear in each supplier’s water quality test results. However, as of October 3, 2026, the national government has not yet released supplier-by-supplier values since the April 2026 standard. Until then, suppliers’ own pages are the most up-to-date source.
What percent of the TDI is your local value?
The 50 ng/L standard was set so that, for a person weighing 50 kg who keeps drinking 2 L a day, the intake from water is 10% of the tolerable daily intake (TDI). You can convert your local value with the same formula. The calculator on this page uses it.
Intake from water (ng/kg body weight/day) = concentration (ng/L) × amount drunk per day (L) ÷ body weight (kg)
Divide this by the TDI of 20 and multiply by 100 to get the percent of the TDI.
| PFOS + PFOA in tap water | Intake at 2 L, 50 kg | Against the TDI of 20 | Times the 37 food items (0.22) | At 1 L, 50 kg |
|---|---|---|---|---|
| 1 ng/L | 0.04 | 0.2% | 0.2× | 0.1% |
| 5 ng/L | 0.2 | 1.0% | 0.9× | 0.5% |
| 10 ng/L | 0.4 | 2.0% | 1.8× | 1.0% |
| 20 ng/L | 0.8 | 4.0% | 3.6× | 2.0% |
| 50 ng/L (standard) | 2.0 | 10% | 9.1× | 5.0% |
| 100 ng/L | 4.0 | 20% | 18× | 10% |
- The food comparison uses MAFF’s survey of 37 domestic food items (FY2024–FY2025): PFOS 0.13 + PFOA 0.09 = 0.22 ng/kg body weight/day. The 37 items account for about half of total food consumption, and the figure is calculated from the median for each item. At 5 ng/L, water and diet contribute about the same; at 50 ng/L, water is about 9 times the diet (Urayomi’s calculation).
- The TDI% compares PFOS + PFOA combined with the TDI for a single substance (20), so it is on the cautious side. PFOS and PFOA each have their own TDI of 20.
- 2 L is an assumption used to calculate the standard. In this research, we could not confirm any official statistics measuring how much tap water people in Japan actually consume, including in cooking and tea.
- Change the yardstick and the “margin” changes a lot. Using EFSA’s tolerable weekly intake for four substances (about 0.63 ng/kg per day), 5 ng/L is about 32% and 50 ng/L about 320% (EFSA’s value is for the sum of four substances; the water values cover only two).
- Using the fiscal-year highest value gives a higher figure than the usual value. The TDI is an amount assumed over a lifetime, so it is not something to judge from a single value.
Wells and private water supplies
- Private water supplies (in-house water systems for company housing, hospitals, schools, bases and so on) are tallied separately from the national table. In the December 2025 release, 59 of the 4,353 tested exceeded 50 (FY2020 to the end of August 2025). Counts by prefecture are on the prefecture pages.
- Household well water is not part of the water supply system, so it is not in the national tap water tables. For measurements of rivers and groundwater, see the guide to rivers and groundwater.
- If you want to reduce it, the performance of water filters (such as the rated volume for PFOS and PFOA) is compared on the water filter page.
Sources
- Ministry of the Environment press release, “Results of the survey on PFOS and PFOA in water supply (follow-up)” (December 25, 2025)
- Same, list of survey results (Excel)
- Ministry of the Environment, “Results of the survey on PFOS and PFOA in water supply” (November 29, 2024)
- Ministry of the Environment, notice on points to note as PFOS and PFOA become water quality standards (Notice No. 2506305, June 30, 2025)
- Same, enforcement notice (Notice No. 2506301, June 30, 2025)
- Ordinance for Enforcement of the Water Supply Act, Article 15 (frequency of regular water quality testing)
- Sakai City Waterworks and Sewerage Bureau, PFOS and PFOA test results
- Osaka Water Supply Authority, PFOS and PFOA test results
- MAFF, Q&A on PFAS (intake from 37 domestic food items)
- Ministry of the Environment, “Overview of the amendment to the ministerial ordinance on water quality standards”
Last checked 2026.10.04 (against the original government documents and papers, collected and cross-checked in Gemini Notebook). Utility values are as in the national table; Urayomi sorted and summarized them.