A collaborative study has lent more force to the suggestion that water pollution is triggering male fertility problems.
The study involving researchers from Brunel University, the Universities of Exeter and Reading and the Centre for Ecology and Hydrology has revealed that a group of testosterone-blocking chemicals is finding its way into UK rivers, affecting wildlife and potentially humans.
Supported by the Natural Environment Research Council, the study has led to the identification of a new group of chemicals that act as 'anti-androgens', which means that they inhibit the function of the male hormone, testosterone, reducing male fertility.
Some of them, according to a research article published in the journal Environmental Health Perspectives, are contained in medicines, including cancer treatments, pharmaceutical treatments, and pesticides used in agriculture.
The researchers think that when these chemicals get into the water system, they may play a pivotal role in causing feminising effects in male fish.
Previous studies conducted by Brunel University and the University of Exeter scientists have already confirmed that female sex hormones (estrogens), and chemicals that mimic estrogens, are leading to 'feminisation' of male fish.
These chemicals are found in some industrial chemicals and the contraceptive pill, and enter rivers via sewage treatment works.
The researchers say that this causes reproductive problems by reducing fish breeding capability, and, in some cases, can lead to male fish changing sex.
A link between this phenomenon and the increase in human male fertility problems caused by testicular dysgenesis syndrome has also been suggested by several other studies.
However, scientists could not fully believe in the existence of any such link because the list of suspects causing effects in fish was limited to estrogenic chemicals, whilst testicular dysgenesis is known to be caused by exposure to a range of anti-androgens.
Lead author on the research paper, Dr Susan Jobling at Brunel University's Institute for the Environment, said: "We have been working intensively in this field for over ten years.
The new research findings illustrate the complexities in unravelling chemical causation of adverse health effects in wildlife populations and re-open the possibility of a human - wildlife connection in which effects seen in wild fish and in humans are caused by similar combinations of chemicals.
We have identified a new group of chemicals in our study on fish, but do not know where they are coming from. A principal aim of our work is now to identify the source of these pollutants and work with regulators and relevant industry to test the effects of a mixture of these chemicals and the already known environmental estrogens and help protect environmental health."
Senior author Professor Charles Tyler of the University of Exeter said: "Our research shows that a much wider range of chemicals than we previously thought is leading to hormone disruption in fish. This means that the pollutants causing these problems are likely to be coming from a wide variety of sources.
Our findings also strengthen the argument for the cocktail of chemicals in our water leading to hormone disruption in fish, and contributing to the rise in male reproductive problems. There are likely to be many reasons behind the rise in male fertility problems in humans, but these findings could reveal one, previously unknown, factor."
The researchers are currently focusing on identifying the source of anti-androgenic chemicals, as well as continuing to study their impact on reproductive health in wildlife and humans.