Thorhallur Ingi Halldorsson, professor and researcher at the Faculty of Food Science and Nutrition, School of Health Sciences, University of Iceland ↗, has dedicated his work to understanding how diet and environmental contaminants intersect to affect human health. With a strong background in nutrition and toxicology, his research has increasingly focused on per- and polyfluoroalkyl substances (PFAS), a group of highly persistent chemicals often referred to as “forever chemicals”. Through his work, Halldorsson seeks to clarify how these substances accumulate in the human body, what health risks they pose, and how scientific evidence can inform policy to protect public health. As PARC’s Chemical Leader on PFAS and human health, he plays a key role in bridging research, monitoring, and risk assessment across Europe.

Could you briefly share your journey into nutrition and environmental health research, and how PFAS became a focus of your work?

After graduating in 2004, I got a job at the Statens Serum Institute in Copenhagen and became involved in a research group responsible for the dietary component of the newly formed Danish National Birth Cohort, which had recruited approximately 100,000 pregnant women. Shortly after that, I enrolled as a PhD student, and my area of interest and research focused on the interlink between nutrition and exposure to food-borne contaminants. At the time, PFAS were emerging as chemicals of concern, and this quickly became an area of interest to me.

PFAS are often called “forever chemicals”. Why?

Because of the strong carbon–fluorine bond (C–F), which makes them extremely resistant to biodegradation. The long-chain PFAS are structurally similar to fatty acids, which means renal reabsorption is high, and since they are poorly metabolised, they have a long elimination half-life in humans.

But do you think the general public knows how they accumulate in the human body and what makes them such a health concern? 

No, I would expect that the general public has limited knowledge of this, and I think that is very understandable. 

What does the current evidence tell us about the main routes of PFAS exposure for the general population?

Diet is the main source of exposure in the general population, but which dietary sources are the main contributors can vary depending on the environment and local food production. Unfortunately, there are also many identified and unidentified PFAS hotspots around Europe, and in those cases, contaminated drinking water can be a major route of exposure.

Can you specify the most concerning compounds from a health perspective?

At the moment, the long-chain PFAS (carbon length >6 to 7, depending on the compound) are of most concern, given their long elimination half-life and effects on the immune system. However, for the short-chain PFAS and many other emerging PFAS, we have limited toxicological data and understanding of environmental levels.

What long-term health outcomes have been associated with PFAS exposure, particularly among vulnerable groups like children or pregnant women?

Effects on the immune system seem to be the most sensitive health endpoint, but associations with lower birth weight, blood lipids, liver enzymes and effects on mammary gland development are outcomes that have been judged to be of concern.

In your talks, you often mention some incoherencies when we talk about sustainability, where it seems that to eat healthily, we need to make unsustainable choices. Is it possible to achieve a positive path for both health and sustainability?

I think it is quite clear that health and sustainability are strongly linked. Take, for example, plant-based foods, which have much lower environmental footprints compared to foods of animal origin, or the use of biodegradable materials, which in the long term will have fewer negative consequences for the environment. However, we often justify our choices (for instance, high consumption of animal foods) by referring to possible health benefits while ignoring sustainability issues. In short, I think health and sustainability most often go hand in hand.

How is the research done within PARC contributing to the understanding of PFAS-related health risks?

PARC covers a very broad range of research when it comes to PFAS, ranging from ecotoxicity to human health, along with human biomonitoring studies and the use of advanced methods for exposure assessment. In short, it is a long and impressive list of projects that will contribute substantially to future evaluations of these substances.

Could the things you mentioned be used to inform new regulatory health benchmarks?

Yes, I think that is exactly the purpose of the PARC initiatives on PFAS, and we are in close contact with stakeholders like EFSA to ensure that the work meets their needs.

What should be the next big step in PFAS-related public health research or risk communication?

I think we need to move beyond the legacy long-chain PFAS, which have mostly been phased out (while still very much present in the environment). The use of other PFAS alternatives will, in the long term, result in increased human exposure, and it would be a shame to wake up with limited knowledge about these substances in the future. As such, we need to be a bit strategic in what we study and perhaps not overfocus on the legacy compounds. Regarding risk communication, I think it is important to put the potential health effects into perspective, so as not to scare the public. PFAS are definitely a health concern, but the effects expected at low-level exposure, although important, are not of the nature that the general public should be overly worried about, as current policies are very much aimed at reducing exposure to the long-chain legacy PFAS as much as possible.

What are PARC’s priorities for the next phase of research and policy action on PFAS, and how do you see your role as a Chemical Leader?

There are many priorities, but further work on the immune effects of PFAS, exploring the use of relative potency factors, human biomonitoring, and many other projects are in the pipeline. I think my role is mostly to have an overview of what is going on and to facilitate the flow of information between projects, as such information exchange generally results in better research.