Common chemicals we encounter every day may combine in the human body to cause the development of cancer, scientists say.
The startling findings from a task force of around 174 scientists from 28 countries, published today tackles long-standing concerns that there are links between mixtures of commonly encountered chemicals and the development of cancer.
From the thousands of chemicals to which people are routinely exposed, the scientists selected 85 prototypic chemicals that were not considered to be carcinogenic to humans, and they reviewed their effects against a long list of mechanisms that are important for cancer development.
They found 50 of those chemicals supported key cancer-related mechanisms at levels which humans are regularly exposed.
The findings supported the idea that chemicals may be capable of acting in concert with one another to cause cancer, even though low-level exposures to these chemicals individually might not be carcinogenic.
It was the first time the issue has ever been considered by interdisciplinary teams that could fully interpret the full spectrum of cancer biology and incorporate what is now known about low-dose chemical effects.
"Since so many chemicals that are unavoidable in the environment can produce low-dose effects that are directly related to carcinogenesis, the way we've been testing chemicals, one at a time, is really quite out of date," said study lead author William Goodson III, a senior scientist at the California Pacific Medical Center in San Francisco.
"Every day we are exposed to an environmental chemical soup, so we need testing that evaluates the effects of our ongoing exposure to these chemical mixtures."
Associate Professor Andrea 't Mannetje, of Massey University's Centre for Public Health Research, said the chemicals included in the review were all very common and widespread in our environment and diet.
"Exposure to very low levels of these compounds cannot be avoided and also New Zealanders are exposed to these in their daily lives," she said.
Some of these chemicals remained in our environment and bodies for a very long time, even long after the chemical is banned.
"An example of that is the insecticide DDT - a study we did showed that all New Zealanders have detectible levels of DDT compounds in their blood, even though DDT has not been used in New Zealand for several decades."
Other pesticides were still widely used in New Zealand, such as Diazinon, even though in some cases other countries have stopped using them, she said.
"Most chemicals do not stay in our bodies for a long time, but we are continuously exposed to them."
Dr 't Mannetje said triclosan, an antiseptic agent used in many soaps, and Bisphenol A, which could leach into food from plastic that contained it, were both known as endocrine disruptors.
"We are currently conducting a study in the general population to determine what the body burdens of New Zealanders are for these compounds."
The report, published in leading journal Carcinogenesis, found common assessment methods may be "underestimating cancer-related risks".
In light of the new evidence, the task force called for an increased emphasis and support for research on low-dose exposures to mixtures of environmental chemicals.
Otago University reproductive biologist Dr Linda Gulliver was the only Kiwi scientist selected to join the so-called Environmental Mixtures task force which contributed to the study.
She was part of a team that looked at one of the ten established hallmarks of cancer cells - their ability to grow and multiply in an uncontrolled manner that is prevented in normally functioning cells.
Her group found that chemicals that act as environmental oestrogens and androgens played important roles in the activation of the cancer hallmark of "Sustained Proliferative Signalling," as well as the cross-activation of several of the other cancer hallmarks.
Dr Gulliver told the Herald how present assessment models employed globally were based on the premise that higher levels of chemicals had a worse effect, which could mean a greater potential for cancer.
These models assumed a threshold exists beyond which the chemical would start producing it effects - up to a maximum effect - yet in many of the chemicals tested, low dose effects were seen and in still more the threshold was unknown.
"We therefore need a lot more research - and the funding to drive it - on low levels of chemical mixtures in the environment and their potential for cancer causation," she said.
"This information can then be used to derive better risk assessment models that can test mixtures of chemicals at environmentally relevant levels and determine what combinations of chemicals we may consider avoiding, and perhaps manufacturers modifying the make up of certain compounds to reflect more accurate safety regulations.
"Occupational risk may also be revised in light of us knowing more about the chemical mixtures we are exposed to."
Another member of the task force, Dr David Carpenter, director of the Institute for Health and the Environment of the University at Albany in New York, said our understanding of the issue was changing rapidly.
"Although we know a lot about the individual effects of chemicals, we know very little about the combined and additive effects of the many chemicals that we encounter every day in the air, in our water and in our food," he said.
Current estimates suggested that as many as one in five cancers may be due to chemical exposures in the environment that were not related to personal lifestyle choices, making it vital to better understand the effects of exposures to mixtures of commonly encountered chemicals.
Professor Ian Shaw, a toxicologist at Canterbury University, said people were already, and most definitely, exposed to a cocktail of carcinogens, some of which might work in concert by affecting cells in such a way that their effects add up.
"If this is the case it means that two chemicals at low doses - perhaps below the dose that causes cancer - might add up to make a combined dose that could cause cancer."