A chemical found in cleaning materials, textiles and plastics pose a breast cancer threat, scientists from Texas and Southern Carolina believe.
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Experts have suspected for some time that hormone-disrupting substances in the environment may pose a threat.
Now the Journal of Applied Toxicology reports one chemical, 4-nonylphenol, triggers breast cancer in mice.
But cancer experts said people should not panic and that work was needed to check whether people were also at risk.
The UK government's Department for Environment, Food and Rural Affairs has already been looking at nonylphenol and a similar chemical, bisphenol A, which mimic hormones.
For nonylphenol, the assessment has shown that measures to reduce the risk are necessary, which are currently being finalised.
Many breast cancers are linked to hormone levels.
Nonylphenol mimics female sex hormone oestrogen once in the body. In the liver, it stimulates an enzyme system that in turn increases the production of another similar hormone estriol.
Both oestrogen and estriol have been linked with breast cancer.
It also binds to oestrogen receptors in the breast that can trigger cancer growth to a greater extent than oestrogen.
Dr William Baldwin and his team, at the University of Texas at El Paso and Clemson University in South Carolina, compared the effects of 4-nonylphenol (4-NP) and oestrogen in mice.
They gave a variety of doses of 4-NP or oestrogen to mice genetically engineered to readily develop breast cancer.
Many of the mice given 4-NP developed breast cancer over the next 32 weeks, while mice given equivalent doses of oestrogen (based on the relative binding affinities of 4-NP and estradiol for their receptor) did not.
When they looked at why this might be they found that 4-NP did indeed stimulate estriol production in the liver.
However, this did not lead to increased levels of estriol in the blood stream.
Therefore, as well as stimulating enzymes that produce estriol, it must also inhibit estriol production in some way, and the mechanisms involved in triggering cancer growth must be complex, the researchers believe.
Dr Baldwin said: "Long term exposure to 4-NP could leave individuals at a significantly increased risk of developing breast cancer."
However, Dr Sarah Rawlings, Breakthrough Breast Cancer's head of policy, cautioned: "This is a very small study and much more research is needed before any meaningful conclusions can be drawn about the link between this chemical and breast cancer in women."
Henry Scowcroft of Cancer Research UK said: "Some research has implicated certain chemicals in the environment in the rising incidence of breast cancer in the West.
"This paper is no cause for alarm, however. It is unlikely that weak oestrogens such as 4-nonylphenol, which are found in very low concentrations in the environment, play a major role in breast cancer incidence.
"While the results of this work are interesting, it is difficult to conclude that because a chemical causes cancer at a given concentration in mice, it has the same effect on humans at a much lower dose."
Liz Carroll, Breast Cancer Care's Head of Clinical Services, said: "It will be many years before we will know the answer but if people are concerned, they can contact the Breast Cancer Care helpline 0808 800 6000. Meanwhile new research is needed before a definitive link can be established."