This week, Risk Bites is back to looking at dose-response models to
estimate risk.
To recap: a dose-response model describes the relationship between how
much of something gets to a specific part of your body, and the magnitude of
the effect that its presence causes.
Today, we'll look at two of the simplest models around. These are still used by risk
assessment experts, and offer tremendous insight into the risks
presented by different substances and how they can be managed.
To start with, we have the threshold model.
Imagine
you breathe in a lungful of carbon particles from the back end of your car.
Thanks to millions of years of evolution, your lungs are pretty good
at handling inhaled particles.
So your body gets rid of the particles, repairs any damage that they caused, and
you remain hale and hearty.
That is, until the amount of stuff you inhale overcomes your lungs' ability to
handle it.
Once you pass this threshold, the more particles you inhale, the more damage is caused,
and the greater the risk is to your health.
This type of response - and it isn't limited to particles and lungs - can be modeled
with a "threshold" dose-response model.
At low doses, the body can handle the stuff you are exposed to and there is negligible risk,
but above a threshold dose the body's defenses begin to struggle,
and increasing dose leads to increasing risk.
According to the threshold model if exposure is kept well below a certain
level, risk is minimal.
But what if you are exposed to something that causes irreparable
damage?
It used to be thought that this was the case with substances that cause cancer.
Here's how the thinking goes: imagine a chemical gets into the nucleus of the
cell and messes with your DNA. Now imagine that messed-up DNA is
replicated every time the cell divides. So rather than your body getting
rid of the problem, it makes it worse. And at some point that messed up DNA
could potentially lead to cancer.
You now have a situation where a higher dose means greater risk, but
there is always some risk associated with dose, no matter how
small that dose is.
In other words, there is no
threshold.
This "no-threshold" dose-response model is still used to estimate the risk
associated with exposure to cancer-causing agents.
Of course, both of these models are wrong. Just like all models, they have their
limitations. But they are useful.
We now know that DNA damage and repair are more complex than previously
thought, and so the no-threshold model is over-simplistic.
We also know that people respond to non-carcinogens in complex ways, and
so the threshold model is also over- simplistic. But each model still provides
risk assessment experts with valuable insight into the harm that
exposure to a substance might
cause.
And they provide a powerful starting point for thinking about the relationship
between dose and response and what this means for understanding and managing
risk.
Risk Bites will be looking at increasingly sophisticated dose-response
models in later episodes but until then, stay safe.