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Why we compare your water with 0.5 ng/L of PFAS, and not 4

Published 4 September 2026 · pfasnearme

Look up your town on this site and you will see the water compared with 0.5 ng/L for the four most harmful PFAS. Sweden's legal limit is 4 ng/L. The EU's is 100 ng/L for a wider group. So ours is eight times stricter than the strictest law in Europe. People ask, fairly, where that number comes from and why they should trust a website over a food agency.

The short answer is that we did the exact same calculation the food agencies did. We just made the cautious choice at the two points where they made the comfortable one. This post walks through it. You can check every step.

Everyone starts from the same number

In 2020 the European food safety authority, EFSA, published how much of these four PFAS a person can take in over a lifetime without expected harm. The four are PFOA, PFOS, PFNA and PFHxS. The number is 4.4 nanograms per kilogram of body weight per week. That is 0.63 ng per kilo per day. For a 70 kg adult it works out to about 44 ng a day, from everything they eat and drink combined [1].

The evidence behind it is not abstract. Children with more PFAS in their blood made fewer antibodies after routine vaccinations. That is the effect EFSA built the budget around.

Sweden, Denmark and the Netherlands all built their drinking water limits on this one number. So did we. Nobody in this story is using different science.

A water limit is two decisions, not one

Here is the part most people never get told. EFSA's number is a budget per person. To turn it into a concentration in water you have to decide two things.

First, who are you protecting. An adult drinks about 2 litres a day, roughly 0.03 litres per kilo of body weight. A bottle-fed baby drinks about 0.15 litres per kilo. That is five times more water for its size, so the same water fills a baby's budget five times faster.

Second, how much of the budget may water use. Food already contains PFAS, so water can only get a share. The World Health Organization's default is 20% [2]. The older and stricter convention is 10%.

That is actually all there is to it. Pick a person, pick a share, divide.

Same maths, four different answers

Look at that table for a moment. None of these numbers is "what the science says". The science gives you the budget. Everything after that is a choice about who you are willing to leave outside the safety margin. The Netherlands and Sweden chose a healthy adult and a generous share. Denmark chose a small child and a tight share and landed on 2. We chose the most exposed realistic person and a tight share and landed on 0.5.

Why we went with the cautious choice

We feel strongly about this for three reasons.

First of all, food already fills most of the budget. Sweden's own food agency, Livsmedelsverket, estimated that a typical adult gets about half of the tolerable amount from food alone. For 4-year-olds it is worse. 43% of them exceed the whole budget from food alone, before a single glass of tap water [4]. Denmark's food institute looked at the same picture and limited water to 10% for exactly this reason [5]. So the 20% share is a bit of a fiction for anyone who eats normally.

Secondly, Sweden's own scientists get numbers close to ours when they look at children. The scientific report behind Sweden's 4 ng/L limit contains a table for a 4-year-old. After typical food intake, that child has room left for about 0.3 ng/L in water. Give water a 10% share instead and the number is 0.7 ng/L [4]. Our 0.5 sits right between those two. So when someone tells me 0.5 is extreme, I point at Sweden's own table. The 4 ng/L in the law is the adult row. The children's rows were left in the report.

Finally, the EFSA number itself may not be cautious enough. A 2023 reanalysis of the same vaccine data found effects at blood levels of 1.5 to 4 ng/mL, well below the 17.5 ng/mL EFSA used [6]. The US health goal for PFOA and PFOS is zero [7]. PFOA is classified as carcinogenic to humans, IARC Group 1 [8]. If the budget gets cut later, every limit built on it moves down. Ours has more room to absorb that.

The arithmetic, so you can check it

0.63 ng per kilo per day, times a 10 to 15% share for water, divided by 0.15 litres per kilo per day. That gives 0.42 to 0.63 ng/L. We rounded to 0.5.

Here is what different concentrations do to the budget, from water alone:

One honest caveat. EFSA built its budget for lifetime intake, not literally for babies. A baby's exposure through pregnancy and breastfeeding is modelled separately. We use the bottle-fed baby as the most exposed realistic person, to show how little room 4 ng/L leaves. Even read that way, the table makes the point. At 4 ng/L a bottle-fed baby uses up nearly the whole budget from water alone, before any food is counted.

What 0.5 ng/L does not mean

It does not mean water at 1 or 2 ng/L is dangerous. There is no acute danger at those levels and nobody gets sick from a glass. It is an avoidable contribution to a body burden that builds up over years, because these four substances stay in blood for years. PFHxS has a half-life of around 5 years.

It does not mean the water is illegal. Our benchmark is stricter than every current law, so water "above our benchmark" can be perfectly legal. Both statuses are shown on every city page.

And it is meant as an annual average. We compare with the latest public check because that is what exists. Substances too small to measure count as zero, so our totals are lower bounds, never inflated.

Why bother with a stricter number at all

Because the legal limits are lagging the science, and they always have. The EU set 100 ng/L without taking EFSA's 2020 assessment into account. Sweden then went to 4. Denmark to 2. Each step down was the same number with a more careful choice. We just took the next step and wrote down why.

So if your town shows up above 0.5 ng/L, the point is not to panic. The point is that a bottle-fed baby in that town is using a large part of a health budget on tap water alone, and that is worth knowing.

Frequently asked questions

Is 4 ng/L of PFAS in drinking water safe?

It is legal in Sweden since 2026. For an adult it uses 18% of EFSA's health budget from water alone. For a bottle-fed baby it uses 95%. We treat 4 ng/L as an action threshold, not a safety target.

What is the EFSA limit for PFAS?

4.4 ng per kilogram of body weight per week for the sum of PFOA, PFOS, PFNA and PFHxS, which is 0.63 ng per kilo per day. It is not a water limit. It is a total budget, food and drink combined, that each country then turns into a water limit.

Which country has the strictest PFAS drinking water limit?

Denmark, at 2 ng/L for the sum of the four PFAS, a legal requirement since 2021. Sweden applies 4 ng/L since 2026. The Netherlands recommends 4.4 ng/L as PFOA equivalents. The US enforces 4 ng/L each for PFOA and PFOS, with a health goal of zero.

Why 0.5 ng/L and not zero?

Zero is the right direction for PFOA and PFOS, and we say so. But a benchmark has to be something a lab can measure and a water utility can aim at. 0.5 ng/L is the strictest number that still comes out of the standard method with defensible choices.

Sources

1. EFSA (2020): Risk to human health related to the presence of perfluoroalkyl substances in food
2. WHO Guidelines for drinking-water quality, chapter 8: allocation of the daily intake to drinking water
3. RIVM (2021): Advies drinkwaterrichtwaarde EFSA-4 PFAS (derivation of the Dutch 4.4 ng/L)
4. Livsmedelsverket (2021): Vetenskapligt underlag för PFAS i dricksvatten (the report behind Sweden's 4 ng/L; tables 3, 5 and 6)
5. DTU Fødevareinstituttet (2021): note behind Denmark's 2 ng/L drinking-water criterion
6. Budtz-Jørgensen & Grandjean (2023): benchmark dose calculations for PFAS immunotoxicity
7. US EPA: PFOA and PFOS, MCL 4.0 ng/L, MCLG zero
8. IARC (2023): PFOA Group 1, PFOS Group 2B

All sources retrieved September 2026. The full method is on the methodology page. The calculations and research synthesis were prepared with the help of AI, then checked against the primary sources.