How Toxic Are Crop Herbicides?

How Toxic Are Crop Herbicides?

The single biggest concern people have about herbicides and pesticides being used on their food is the toxicity of those products. But most people don’t actually have a clear understanding of what toxic means or how toxicity is determined. And to make things more complicated, there is a mind-boggling amount of hyperbolic and misleading information about the toxicity of crop herbicides.

So first, let’s make sure we’re all on the same page about what makes something toxic. In simple terms, everything is toxic, and everything is also not toxic. How can that be so? It all comes down to dose – the amount of a substance determines whether it is toxic or not. So for practically every substance on earth, there will be a dose that is low enough to be nontoxic, and one that is high enough to be toxic. This is true irrespective of whether we consider that substance to be “good”, like water, or “bad”, like a synthetic herbicide.

How to calculate the toxicity of a crop herbicide (or any substance)

Whenever scientists need to measure how acutely toxic a substance is, they experiment to find something called the LD50. This is used for anything from vitamins to medicines, “natural” herbicides to synthetic ones. The LD50 represents the amount of that substance that is needed to kill 50% of a test population when administered as a one-time dose. This is usually tested in rats, but in the case of herbicides and pesticides an LD50 is usually determined for at least a few different animals to avoid poisoning wild birds, fish and vertebrate animals. Obviously, research ethics prevent us from directly determining an LD50 for humans, but rat physiology is so similar to ours that they are widely used in medical research.

The LD50 is given in mg of substance per kg of the animal/person’s body weight. So for instance, the crop herbicide glyphosate (active ingredient in RoundUp) has an LD50 of 5600 mg/kg (taken orally in rats), meaning if you had a bunch of rats in a cage, you would have to feed all of them 5600 mg/kg of their body weight in order to have half of the rats die from acute poisoning. Let’s assume all the rats weigh 1 kg (about 2.2 lbs) – all the rats would need to be fed 5600 mg of glyphosate, and we would expect about half of them to die.

Let’s take another example: caffeine. Caffeine’s LD50 in rats is 192 mg/kg, meaning it would only take 192 mg of caffeine to kill half of those rats weighing 1 kg, compared to 5600 mg of glyphosate.

As a matter of fact, several synthetic crop herbicides (including the active ingredients in Bravo, Treflan, Pendulum, Arylex, and Admire) are less acutely toxic than several naturally derived herbicides used in organic farming (including copper hydroxide, lime sulfur, and rotenone). For perspective, all of those synthetic pesticides are less acutely toxic than table salt!

Long-term exposure to crop herbicides

But acute toxicity isn’t the whole story. What about small, incremental exposure to crop herbicides over the course of a lifetime? There’s a way of determining that, too. It’s called NOEL: no observable effect limit. The NOEL for the most sensitive mammal species tested is used by the EPA to establish maximum levels of herbicide residue that are considered acceptable. To determine NOEL, animals are given a prescribed amount of the substance every day. Then, the NOEL is effectively reduced to 1% of the actual value to account for highly sensitive portions of the human population. This gives the EPA its “tolerances” for herbicide residue on crops. For glyphosate, this number is 2 mg per kg body weight per day.

As an example using glyphosate and the foods with the highest EPA tolerances (around 5 parts per million, although most crops are 0.5 ppm or less), let’s say you weigh 65 kg (143 lbs). To get “too much” glyphosate, at 2 mg/kg, you would need to eat 2*65= 130 mg per day. Since 5 ppm = 5 mg/kg, we can figure out how much produce you’d have to eat to reach the maximum daily tolerate. 130 mg divided by 5 mg on each kg of produce means you would need to eat 26 kg of produce each day to reach the limit. That’s over 57 pounds of food every day!

So, how toxic are crop herbicides? It depends on which one you’re talking about, but in most cases, the answer is: not as toxic as you think. Or as the EPA says, “practically nontoxic”.