You probably know the feeling. It's been a few days since you last washed your hair, and it feels greasy, heavy, and tangled. You hop in the shower and apply a generous amount of shampoo to your hair.

After you've towelled yourself off and stand in front of the mirror, your hair is soft again, less frizzy, and shines healthily. All thanks to your shampoo.

Human hair is made of keratin. It inherently has a negative charge, but the more worn it is, the greater the negative charge becomes. This makes hair frizzy and dull. Most modern shampoos therefore contain a number of positively charged substances. These softening agents bind to the hair, making it less frizzy and giving it shine.

Previously, it was thought that these softening agents posed no environmental hazards. The substances are large molecules that cannot penetrate animal and plant cells. And normally, only substances that do so are tested toxicologically.

However, new research shows that this is a mistake. The softening agents affect small aquatic animals, such as daphnia, in streams, lakes, and rivers in a way that makes it difficult for them to find food, ultimately leading to their death, explains Senior Researcher Hans Sanderson from the Department of Environmental Science at Aarhus University.

- We have conducted the first study in the world to investigate this. The substances do not penetrate the animals' cells, but rather stick to the surface of the animal. This physically affects the animal, making it unable to move as well, and it impairs its ability to ingest food, he says.

Hans Sanderson and his colleagues have tested a handful of the different softening agents used in personal care products. However, these represent only a small fraction of the 25,000 softening agents used worldwide.

In addition to personal care products, softening agents are also used in contact lens solution and in wastewater treatment plants. When the wastewater needs to be purified, these substances are added because they bind to other molecules. This causes the molecules to clump together, making them easier to filter out of the water.

Softening agents play a crucial role in many products and are part of a billion-dollar industry. But this does not mean they should be exempt from scrutiny, says Hans Sanderson.

- In the EU, lawmakers are starting to realize that we also need to investigate substances that do not penetrate cells. New regulations of these substances are expected. That's why I've conducted these studies at the request of the industry, he says.

Although the experiments were funded by CEFIC, the European chemical industry association, the research is sound and has been published in an international peer-reviewed journal, Hans Sanderson continues.

- I’m well aware of CEFIC’s strong interest in creating the best conditions for its members. That said, I had complete freedom to conduct the experiments as thoroughly as possible. The industry knows that new regulations may be on the way, so they are interested in uncovering the extent of the problems.

In fact, it is standard practice in the EU for CEFIC to support a large part of the research that forms the basis of EU legislation. In other words, they provide a great deal of the knowledge officials need to make the right decisions.

In their experiments with softening agents, Hans Sanderson and his colleagues investigated how the substances affect two species of the daphnia family.

Daphnia are tiny crustaceans that live in freshwater streams or lakes. They eat algae, bacteria and plant matter, and they play an important role in the ecosystem. Both because they purify the water and because they serve as food for a wide range of fish.

But when too many softening agents are present in the water, it harms the daphnia, explains Hans Sanderson.

- We tested two different species. In the first species, the sticky substances attached itself to its body, significantly hindering its movements. This affects the animals survival chances – and therefore has a strong negative effect on it. The second species was slightly less affected.

These findings suggest that regulations should be established in this area and more substances should be investigated, he says.

- These softening agents, which are large molecules, are very difficult to investigate for their harmful effects because they bind to everything. To investigate them as accurately as possible, we need to imitate the conditions in streams and lakes to see the real effect.

- We succeeded in doing so, and we have developed a method to test the environmental effects of many more of these substances.

Hans Sanderson won't speculate on the future consequences of these findings. He does emphasize, though, that the substances are found in most of the products we use – even those with the Nordic Swan Ecolabel.

- Even the Swan-labeled neutral products contain some of these substances. It may be that producers of these products will have to look for a range of other substances in the future, he says.

It should also be reconsidered whether these substances should be used at the country's wastewater treatment plants, where they are currently being used.

- The substances are added to filter out other substances from the wastewater. Therefore we need to address the question of what is worst for the environment: these substances or the substances they help to filter out, he says.