June 10, 2010

Atrazine: What is the safety limit?

Anything can be a hazard at the wrong level.

A cup of coffee is fine. An injection of caffeine (or massive consumption) can send you into a paranoid delusional fit (or death). Oxalic acid in spinach is harmless in an amount someone would normally eat. But if you chow down on 10-20 pounds at once, your liver (and you) could be toast.

But how can you learn what the risk is for caffeine or oxalic acid? Or for another compound like the herbicide atrazine? When is it safe and when is it unsafe?

Risk = Exposure x Toxicity
Exposure comes from food, water or direct contact. Toxicity involves things like cancer, reproduction problems, birth defects, “acute” effects (severe symptoms from a sudden high exposure), nervous system damage and so on.

For caffeine, exposure comes from food and drinks. For atrazine, because it does not exist as food residue, exposure comes from water.

Knowing that atrazine exposure comes from water, researchers then need to figure out how much material causes toxicity. This involves studying animals exposed to different doses – very low to very high – and for short term (known as acute) and long term (known as chronic) exposure. They want to find out possible responses at every level – and regulators like the Environmental Protection Agency require standard lab practices and open data that can be verified and checked.

What the researchers need to find is the level at which there is an effect, and then drill down to a level where there is no-effect – the smallest amount that has no toxicity in the short or long term.

Then the EPA adds safety factors – layers of safety – to that no-effect limit.

For atrazine, EPA adds a 10x “uncertainty” factor. Then another 10x factor to ensure children are safe. And then, just to be sure, a final 10x safety factor.

So what does that mean?

A mile away from the fire
Let’s say you’re at a bonfire and want to keep people safe. You determine that a 5-foot safety ring is an acceptable safety factor.

If the EPA got involved, it would take that times 10 to allow for an uncertainty factor. To keep small children safe, another 10x safety factor would be added. And just to be sure, you better add another 10x.

After adding in all of EPA’s safety factors, you’d end up 5,000 feet away from the bonfire! (That’s 5 x 10 x 10 x 10 – about a mile away.)

For atrazine the “mile away” safety limit is 298 parts per billion in a single day. For children, it is 100 ppb in a single day. (FYI: 1 part per billion is like a single blade of grass on a football field.)

I italicized "single day" because there is some confusion about that because the EPA has set a limit of only 3 ppb for an annual average in a drinking water system. (See this EPA page on drinking water limits.)

In other words, if a water system’s annual average (based on a multitude of samples) is 1 ppb but a single test showed 10 ppb, it is well within EPA’s safety guideline because the daily safety limit is 298 ppb (and the max daily test in this case was 10 ppb) and annual average is 3 ppb (the average in this example is 1 ppb).

To make sure atrazine isn’t showing up in water systems at levels higher than the “mile away” safety limits, dozens of community water supply systems are sampled multiple times during the year – and more frequently during the use season for atrazine. And these water systems are in areas where atrazine is used (it’s generally used in small amounts in branded premixed herbicides) and in  watersheds that are more vulnerable due to soil types, slope, rainfall, etc. We’re talking thousands of tested samples annually.

The good news is, of the samples that do test positive, a majority are less than 1 ppb. A few may reach 10 ppb, while others are at 0.1 ppb. Over the last several years, no water system has averaged 3 ppb or more – and no test has ever come near the 298 ppb daily limit.

Interestingly enough, some 6,000 studies are available on atrazine. That’s a lot; it’s probably the most-studied chemical on the planet. New products usually are backed by 100-150 studies. (Drug companies start human trials well before that level of knowledge!)

Data on atrazine test results in water is available from EPA.

Additional background is available at AgSense.org.

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