The low dose hypothesis - why small doses of toxins can have a big impact

Posted by Liz Cook on

It is a common misconception that causes us to unwittingly engage in habits that are potentially harmful to us - the notion that a small dose of a bad thing is not harmful. 

The phrase,"The dose makes the poison", has been taken to mean that a substance is only harmful once it reaches a certain biological level, giving us a false sense of security around the toxins we are exposed to every day. Low dose exposures - typically measured in parts per billion - have been assumed, even by science, to be inconsequential, or even safe.

To date, regulations around environmental exposures to certain chemicals have been based on this concept of toxicology and risk assessment. But this notion is now being challenged by the mounting evidence of the low-dose hypothesis.

In the 1990s, a group of scientists postulated that EDCs (endocrine-disrupting chemicals) have effects at low doses, especially on reproductive and developmental end points, and that humans and animals exposed to EDCs will be affected by low doses. Since that time, work done in laboratories around the world has focused on addressing this hypothesis, and some scientists believe this research has advanced to the point where this concept should no longer be considered a hypothesis

The low dose hypothesis speculates that extremely low-dose exposures to certain substances actually cause a more significant biological reaction than has been observed at higher doses - doses well below those levels previously tested and determined to be safe by regulatory authorities. 

Moreover, this effect is magnified when these chemicals are combined with other chemicals, potentiating their harm. In fact, some chemicals which can be seen as harmless on their own in low doses may actually become harmful at the same dose when combined with other chemicals.

Linda Birnbaum - head of the National Institute of Environmental Health Sciences and the National Toxicology Program- says the issue of additive or low dose effects is complex; human hormone levels vary from person to person as a result of inherent genetic differences, differences in genetic expression as well as different environmental exposures.

"Endocrine disruptors are a subtle challenge. What is normal for me may not be normal for you. We all have our own normal." 

The results of a series of studies on some of the most extensively studied toxic chemicals and pollutants, which are contrary to the way the United States Environmental Protection Agency (EPA) and other regulatory agencies assess the risk of chemicals, indicate that we have underestimated the impact of toxic chemicals on disease.

Of particular concern are lead, airborne particles, benzene and tobacco, and the more ubiquitous modern chemicals including endocrine-disrupting chemicals (EDCs), such as bisphenol-A (BPA) found in plastics, phthalates (found in perfumes and soft plastics) and atrazine (a common herbicide). The health and environmental impacts of these ubiquitous chemicals are vast and detrimental, and include heart disease, diabetes, obesity, depression, some cancers, asthma, liver failure, early-onset of puberty and other reproductive and developmental harms in humans and animals.

Exposure to these chemicals during early brain development is of particular concern; in many cases, there is no apparent threshold or safe level at any developmental stage, let alone during gestation, early childhood or adolescence.

What has become clear is that we don't give enough weight to the impact of everyday chemicals in our lives, and their effect on our hormones, which can lead to detrimental health impacts.

Sandra Steingraber, a biologist at Ithaca College in New York, states that our hormones should be considered "a tuning fork that responds to messages streaming in from the environment. Small chemical doses can interfere with hormone levels."

And that needs to be taken more seriously if our end-goal is to live as well as possible. 




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