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A recent report found that when a person is exposed to a little bit (or low doses) of a lot of different commonly encountered chemicals, than the combinations over time may cause changes that increase the risk of cancer (they may initiate cancer). Think about it - we are not exposed to just one chemical at a time (which is how chemicals are tested), but to mixtures or a "chemical soup". It is almost impossible to avoid them. As one of the researchers said: "We urgently need to focus more resources to research the effect of low dose exposure to mixtures of chemicals in the food we eat, air we breathe and water we drink." Testing mixtures of chemicals is currently not required by law.

The effects may be synergistic  - an enhanced effect that is more than the sum of the individual chemicals. And some chemicals may have bigger effects at smaller doses (typical of endocrine or hormone disruptors), than at larger doses - which is not how chemicals are typically viewed (typical view: the more you are exposed to a chemical, the greater the effect). A global task force of 174 scientists looked at 85 common everyday chemicals (at everyday low doses), and 50 were found to support cancer-related mechanisms (processes essential to cancer development). Examples of problematic chemicals that are common in everyday life: triclosan (in many soaps and personal care products), bisphenol A (in many plastics, including can linings), and atrazine (common herbicide or weedkiller). From the LA Times:

Combinations of 'safe' chemicals may increase cancer risk, study suggests

Lots of chemicals are considered safe in low doses. But what happens when you ingest a little bit of a lot of different chemicals over time? In some cases, these combinations may conspire to increase your risk of cancer, according to a new report. “Many [chemicals] have the possibility, when they are combined, to cause the initiation of cancer,” said Hemad Yasaei, a cancer biologist at Brunel University in England, one of the authors of the report. “They could have a synergistic or enhanced effect.”

This is not the way regulators typically think about cancer risk when they evaluate a compound’s safety.Normally, they test an individual chemical on laboratory animals, exposing them to progressively smaller amounts until it no longer causes malignant tumors to grow. Then they take that dose, determine the equivalent for humans, and apply what is called a “margin of safety” by declaring that some small fraction of that low dose is safe for people.

The big assumption driving the margin of safety is that a smaller amount of a chemical is less dangerous than a larger amount. (Think of the familiar axiom, “The dose makes the poison.”) But that’s not true for all chemicals, experts say. Some chemicals, such as those that mimic hormones, may actually be more dangerous at lower doses because the human body is exquisitely attuned to respond to minute amounts of natural hormones such as estrogen and testosterone.

And regulators haven’t required testing of mixtures of chemicals at all...Leroy Lowe, president of Canadian nonprofit Getting to Know Cancer and leader of the report published this week by the journal Carcinogenesis. The new report raises questions about whether this approach is adequate...Humans are exposed to about 80,000 man-made chemicals over their lifetimes, experts say. These chemicals are in the foods we eat, the water we drink and the air we breathe. "We live in a chemical soup,” said toxicologist Linda Birnbaum, director of the National Institute of Environmental Health Sciences, who was not involved in the new study.

The research team — a coalition of 174 researchers from 28 countries — set out to determine whether mixtures of these chemicals, at the very tiny concentrations found in the environment, could plausibly trigger the formation of cancerous tumors. They focused on 85 particular chemicals that were impossible to avoid in modern life, that were likely to disturb biological function and were not thought to pose cancer risks at the very low doses that people tend to ingest them.

The researchers scoured the scientific literature to understand how each of these chemicals could affect 10 important processes that are essential to cancer development. Among them: tumor-promoting inflammation, resistance to cell death and the formation of new blood vessels to feed malignant cells. In addition, they categorized whether each of the chemicals exerted biological effects at very low doses to which humans are ubiquitously exposed. (These doses are so small that they tend to be measured in parts per million or parts per billion.)
Of the 85 chemicals researchers examined, 50 were found to affect cancer-causing processes in the body, even at very low doses.

These 50 everyday chemicals included bisphenol A (used in manufacturing plastics), triclosan (often found in hand sanitizer and anti-bacterial soap) and atrazine (a commonly used herbicide). Since each of these chemicals affects different processes that could lead to cancer — bisphenol A makes cells less sensitive to signals to stop reproducing, for example, while atrazine encourages inflammation — it’s plausible that consuming mixtures of these chemicals is riskier than consuming any one individually.

More details about this report. From Science Daily: Cocktail of common chemicals may trigger cancer