We are often told environmental chemicals are safe because we are exposed to concentrations “too low” to cause harm.  Toxicologists call such a low concentration the no-observed-effect-concentration or NOEC. [Some experts use the acronym NOEL for no-observed-effect-level.]

The counter argument is that we are not exposed to one chemical at a time.  Instead, we are constantly exposed to mixtures of chemicals.  Each chemical by itself might not cause harm – that is, each chemical might be present below its NOEC/ NOEL – but the overall effect of the chemical mixture comes from the combination of individual chemicals acting together.

A good metaphor is how adding salt to a recipe makes you taste the sugar more.  Salt changes how our bodies respond to the sugar.

Until recently, the importance of chemical mixtures has been debated without much evidence.

New Information on Chemical Mixtures

Recently, Professor Philippa Darbre and her colleagues at the University of Reading near London published results showing how mixtures of common chemicals called parabens work on cancer cells in the laboratory. [click here for the abstract]

Parabens are Present in Most Human Breast Tissue

Parabens* are absorbed through the skin from over-the-counter personal care products we buy and use every day.  Parabens accumulate and are stored in breast tissue.  Professor Darbre’s group measured the concentrations in breast tissue that had been removed by mastectomy to treat cancer for 40 different women.  [click here for the abstract]

The levels of parabens varied among the different women and in different parts of the breast (There were higher levels in breast tissue that had been nearer the axilla or armpit), and 40 percent of samples had at least one paraben at a level above the NOEC/ NOEL for that individual paraben.

Effects of Parabens can add up

When they tested the effects on cancer cells of a seven day exposure to each of the five parabens individually, at NOEC/ NOEL concentrations, none of the chemicals had an effect by itself

However, seven days of exposure to a combination of all five chemicals in the same NOEC/ NOEL for each of the parabens showed a trend for increased growth. 

When they exposed the cells for two weeks instead of one week to the same combination of very low levels of the five parabens – with each paraben still at a level too low to have an effect by itself – the combination of parabens made cancer cells grow significantly more.

Cancer Cells and the Chemical Mixtures measured in Individual Women

Next they exposed cancer cells to the exact mixtures of the parabens that were measured in specific individual women. In several, but not all cases, they observed enhanced growth of breast cancer cells after exposure to the chemical mixtures. 

Longer Testing finds Additional Effects

Darbre and her colleagues also exposed cancer cells for four rather than two weeks reasoning that parabens accumulate in tissue so human breast tissue is actually exposed a lot longer than two weeks.

After a longer, four-week exposure, cancer cells grew more than after the shorter, two-week test exposure. 

It’s more Convincing that Not Every Combination caused Cancer Cells to Grow

If every woman’s chemical mixture caused cancer cells to grow, it would not match what happens in real life, and we would wonder if the study was designed incorrectly.

The concentrations measured in some women caused cancer cells to grow, but the concentrations measured in some other, different women did not cause cancer cells to grow, even in mixtures.  

This variation is like real women. Not every woman gets cancer.  

Exposure to chemical mixtures may be like radiation exposure.  Only a portion of women exposed to radiation get cancer from it, but we limit radiation exposure as much as possible.  Similarly, exposure to chemical mixtures is unnecessary, so we should also try to limit exposure to chemical mixtures. 

Cancer, Chemical Mixtures and the Precautionary Principle

Is Professor Darbre’s study the proverbial “smoking gun” linking cancer and chemical mixtures?  Not yet.  Do we have reason to be concerned?  I believe we do

We clearly have reason to question tests that conclude that chemicals are benign when they have been tested as individual chemicals. Human breast tissue is exposed to chemical mixtures all the time.  To determine what effects chemicals might have, it is necessary to test chemical mixtures.  This is rarely done.

Possible Precautions for Now

Until we have better information, I personally try to find paraben-free shampoos and soaps.  It’s difficult, but in my opinion, it is the safest thing to do for the present.  This is called the Precautionary Principle.  Be careful as a precaution. 


* They tested methylparaben, ethylparaben, propylparaben, butylparaben, and isobutylparaben.  You can look in the fine print on the label of your shampoo, cosmetics, skin creams, etc to find out whether you are exposed to these chemicals.