When considering the toxicity of pesticides, we need to address the toxicity to both pests and humans. But first, please note the term pesticide refers to any chemical used to manage pests (insecticide, herbicide, fungicide, etc.), and a pest refers to what is being controlled (insect, weed, disease, etc.). Toxicity to pests and humans needs to be considered separately, as something more toxic to a human does not mean it offers better control of the pest. Also, something highly effective at managing a pest does not have to be highly toxic to humans. We are different organisms so products affect us differently.
Toxicity to pests refers to how well a product controls the pest. This is often referred to as efficacy. Efficacy is a rating on the level of control offered by products labeled for a specific pest. Most of these ratings are a result of research trials conducted by universities. For example, weed scientists will apply several different herbicides in test strips to a field, then observe how many weeds remain in each strip to determine the efficacy rating.
Toxicity to humans is identified by what are called “Signal Words.” These are found on the first page of the label as Caution, Warning or Danger. So, what exactly do they mean and how are they determined?
Signal words are determined by the LD50 rating. This stands for the lethal dose for 50% of the test population. Tests are done to determine what rate (milligram of toxicant/kilogram of body weight) is lethal to 50% of the population. This is then equated to an average adult body weight. The following table shows how much product must be swallowed to cause death in 50% of adults, for each LD50 range.
Lethal dose ratings and consumption
From Table 1, you can see that Caution is the least toxic and Danger, the most. Some products with an LD50 greater than 5,000 may not have a signal word listed because the EPA considers it practically non-toxic. Looking at the amount of product required to be ingested under Caution, one ounce does not seem like a lot but, when you consider using the product, properly mixing it, applying it, storing it, it would be very difficult to accidentally swallow an ounce. It would be easy for a small child to do so, not knowing what the product is, or even an adult if the product is stored in a food container. This is why it is very important to always store pesticides in their original containers and keep them out of reach of children.
When we look at the Danger category, it becomes a lot easier to imagine accidentally ingesting a drop when you are handling and mixing the product. This is where personal protective equipment (PPE) becomes even more critical. Every label has PPE requirements based on the toxicity levels to protect the user. PPE for products in the Caution category are usually long sleeve shirt, pants, shoes plus socks and waterproof or chemical resistant gloves. In the Warning or Danger categories, PPE can include respirators, face shields, chemical resistant clothing, etc. Note that Table 1 refers to the oral toxicity. Other routes of entry include skin absorption, inhalation and ocular. PPE protects against all four areas of exposure.
Common consumer products
Now let’s look at the LD50 values of pesticides and common consumer products.
Remember that the lower an LD50 value, the more toxic it is. This table shows that many of the common consumer products we use every day are more toxic than a lot of the pesticides used. For example, glyphosate (the active ingredient in Roundup®) is the least toxic product listed.
This information enables us to make safer decisions about the chemicals we use. If a person is using Gramoxone Max and sees the toxicity level is much higher than other herbicides, that person can look for an alternative option with the same level of control but is much safer to use.
For those unfamiliar with the rates of pesticides used by farmers, you may be surprised to learn that the per acre rate is often measured in ounces. The rate for dicamba, a common broadleaf control, is 22 oz per acre. Glyphosate is often applied at the same rate. The farmer may be spraying 15 gallons per acre but the majority of that is water.
The glyphosate court cases have added to the confusion of the toxicity of pesticides. As seen in Table 2, glyphosate is one of the least toxic pesticides, but has gotten a lot of attention for allegedly causing cancer. Numerous agencies, including the US EPA and the European Food Safety Authority, have reviewed the research on glyphosate causing cancer and have found it to be non-carcinogenic. One group, the International Agency for Research on Cancer, did say it was a “probable carcinogen.”
This leads into the discussion of the effects of pesticide exposure. This is broken down into two categories: Acute and Chronic. Acute toxicity refers to the immediate effects of exposure such as nausea, headaches, and blurred vision. Chronic toxicity is a result of long-term exposure, typically several years to a lifetime. Examples include cancer, mutations and birth defects in offspring. For all labeled pesticides, this information is available in the safety data sheet. To find labels and safety data sheets visit http://cdms.net. For more information on pesticides, please visit the National Pesticide Information Center at http://npic.orst.edu.
Amanda Douridas is the Champaign Agriculture and Natural Resources Extension Educator, The Ohio State University Extension.