Acrylamide, a chemical found in baked and fried foods, does not increase the risk of developing breast cancer as had been feared, scientists claim.
Acrylamide is found in chips
Harvard School of Public Health and the Karolinska Institute researchers found women who ate more of foods containing the chemical were at no higher risk.
The chemical is found in foods including chips, crisps and coffee.
The study, of 43,000 women, is published in the Journal of the American Medical Association.
Acrylamide was first discovered in commonly eaten foods in 2002.
The World Health Organization has said lab experiments suggest it is carcinogenic, or cancer causing.
It appears to form as a result of a reaction between specific amino acids and sugars found in foods when heated to high temperatures.
Tests exposing rats to acrylamide have resulted in large mammary gland tumours forming. But the animals were exposed to levels of the chemical 1,000 to 100,000 times greater than levels humans are exposed to through diet.
The US and Swedish researchers set out to discover if the amount of the chemical which would be ingested as part of an ordinary human diet posed a cancer risk.
The women studied were part of the Swedish Women's Lifestyle and Health Cohort.
They were first assessed in 1991, when they were asked about their diets and how much of certain foods they ate.
The women were then reassessed in 2002, when it was found 667 of them had developed breast cancer.
The researchers used data on whether the women ate more or less of the foods now known to contain acrylamid to determine exactly how much acrylamide the women were eating.
It was found the women who had the lowest daily acrylamide intake had no higher breast cancer risk than those whose intake was higher.
In addition, there was no increased risk of breast cancer among women who ate a higher quantity of specific foods known to contain acrylamide.
The average daily acrylamide intake among the participants was 25.9 micrograms (mcgs) per day.
The foods that contributed the most to the chemical intake were coffee (54%), fried potatoes (12%) and crisp bread, (9%).
Writing in JAMA, the researchers said: "We found no evidence of an association between the amount of acrylamide consumed by these Swedish women and risk of breast cancer.
Lorelei Mucci, from the Harvard School of Public Health, who led the study, said: "It's reassuring to see that the study suggests that the amount of acrylamide consumed in the Swedish diet is not associated with an excess risk of breast cancer.
"Given the widespread public health implications of acrylamide, however, it is important to examine the risk associated with other cancers as well as neurological conditions"
Henry Scowcroft, science information officer at Cancer Research UK, said: "Previous research has suggested that acrylamide can cause several types of cancer in animals, and several studies have shown that it is present in small amounts in baked and fried foods.
"However, the levels found in foods are minuscule compared to the amount that causes cancer in animals, and so several groups around the world have been looking at whether acrylamide in food is actually a health hazard."
He added: "We welcome the results of this study, which suggest that acrylamide in food has no effect on human breast cancer rates.
"The work confirms the same group's 2003 findings that the chemical had no effect on peoples' risk of kidney, bladder or large bowel cancers.
"Further work now needs to be done to confirm these findings in other populations.
"It is worth pointing out that the best way to reduce your risk of cancer by eating sensibly is with a balanced diet that is low in fat and red meat and rich in fruit, vegetables and fibre."