Some classes of pesticides are hazardous to human health because the life process they control in pests works the same way in humans. For example, rodenticides and insecticides that interfere with the enzyme cholinesterase in pests can interfere with the same enzyme in people causing uncontrolled twitching and loss of nerve control. In contrast, most herbicides are designed to disrupt processes in plants that are not present in humans, such as interfering with root or shoot growth. Most herbicides are not readily absorbed through the skin. Most are rapidly broken down and excreted in urine and feces. Properly trained applicators should always take precautions and follow label instructions to minimize exposure since, for most products, the level of toxicity determines the amount of safety equipment that must be worn.

Exposure to herbicides occurs in two primary ways: acute and chronic exposure.

Acute exposure is a single dose of a large amount of herbicide through ingestion, or eye, lung, or skin contact such as could occur during mixing. It can also include spilling or spraying the product onto clothing or onto the face or body as a result of broken or leaking equipment. Symptoms of acute poisoning usually begin within minutes or hours after exposure and are easily traced to a specific exposure. Acute exposure can also include irritation and burns.

Acute oral exposure refers to a single dose taken by mouth (ingestion). Acute dermal exposure refers to a single dose directly contacting the skin (skin absorption). Acute inhalation exposure refers to a single dose exposure through breathing or inhaling in an enclosed area. Few herbicides are highly toxic to humans, and few herbicides are readily absorbed through the skin. Since most rights-of-way herbicide mixing usually occurs outdoors, inhalation is generally not a problem. However, always take precautions to avoid breathing herbicide vapors, mists, or dusts.

Chronic exposure is the result of repeated contact to low levels of a pesticide over a long period of time. Chronic effects may appear after months or even years. Chronic exposure is often the result of inadequate or improper use of protective clothing or equipment. It can occur from wearing the same contaminated clothes without proper laundering or by not washing or bathing after working with a herbicide product.

The results of chronic exposure response might include allergies, cancer, birth defects, and organ damage. However, several toxicology tests are required before a herbicide can be registered. These include effects on reproduction (teratogenic), ability to cause cancer (carcinogenic), ability to cause genetic mutation (mutagenic), and organ toxicity. A properly trained applicator should understand the long-term hazards (if any) associated with the products they work with.

The terms toxicity and hazard do not mean the same thing. Toxicity is a measure of the ability of a chemical to cause harm; injury, illness, or death. Hazard is the possibility that injury can occur and includes two factors -- toxicity of the product or mixture and the amount of exposure. A herbicide diluted in water has the same toxicity as the concentrated form, but presents less of a hazard because the applicator must be exposed to a greater quantity of solution to receive the same amount of herbicide or toxicant. Applicators should be particularly knowledgeable of the hazards of handling concentrated forms of herbicides before they are mixed for application.

The terms LD50 and LC50 are used to express the level of acute toxicity of herbicides. LD is an abbreviation for "lethal dose." The LD50 is a single dose of a chemical that, when fed to a group of test animals or applied dermally, will kill 50% of the animals. LC means "lethal concentration." LC50 is the concentration of the chemical in the air or water that will kill 50% of the test animals with a single exposure. In most cases, herbicide acute toxicity measurements are determined on both male and female rats. LD amounts are expressed in milligrams of product per kilogram of body weight (mg/kg). LC amounts are expressed in terms of milligrams of chemical mist, or dust, per liter of air (mg/l) or as milligrams of herbicide per liter volume of solution (mg/l), usually water. Milligrams per liter is equivalent to parts per million (ppm). The higher the LD50 or LC50 value the less toxic the herbicide. This may seem, at first glance, to be incorrect. The higher the LD50 means it takes more of the herbicide to kill half of the test animals; therefore, the product is considered less toxic than those herbicides which kill half the animals at lower doses. A herbicide with an acute oral LD50 of 5,000 mg/kg requires almost 0.1 ounce of the herbicide per pound of body weight to reach the LD50 value. LD50 and LC50 values can be measured for both the active herbicidal compound or for the entire herbicide formulation, which can include the inert ingredients and adjuvants.

The LD50 and LC50 values are normally found on the Material Safety Data Sheets (MSDS) for each product. Pesticide labels will generally not provide the LD50 or LC50 values. Instead these numbers are provided to the applicator through a signal word found on each pesticide label. The signal words are CAUTION (high LD50 or LC50 values), WARNING (moderate LD50 or LC50 values), and DANGER (low LD50 or LC50 values). Generally speaking, DANGER/POISON labeled products require greater protective equipment than those products classified as WARNING or CAUTION. Likewise, a product with the signal word WARNING may require additional safety equipment than those herbicides with a CAUTION signal word. It is important to note that the signal words provide a quick overview of the product's acute toxicity to the person handling the product.

No Observable Effect Level (NOEL) is the highest exposure dose that gave no effect on the animals in a particular toxicological test. It is an important measure because LD50 and LC50 values do not indicate the toxicity of a compound over a range of herbicide concentrations. For example, two compounds may have the same LD50 value, but one is much more toxic at a lower dosage that the other. Not only are NOEL values useful in acute and chronic toxicology studies, but also are critical in determining maximum tolerated doses (MTD) based on results of reproductive, teratogenic, mutagenic and carcinogenic studies. From the NOEL, a safety factor of 100 to 1,000 times greater dose is often built into MTDs to account for different sensitivity levels that are possible between experimental test animals and other animals or humans.

Most herbicides have low toxicity (high LD50). Exposure levels that can cause illness are rare. However, herbicides with low toxicity can irritate the nose, throat, eyes, and skin of sensitive applicators. Know how to protect yourself, your workers, and other people from the possible effects due to exposure of the herbicides being mixed or applied.

The manner by which the herbicide contacts the body plays a critical role in how the body responds to the herbicide. Herbicides can enter the body through the mouth (oral), through the skin and eyes (dermal), and through the lungs (inhalation).

Oral exposure can occur by not washing your hands before eating, drinking, smoking, chewing, or by wiping your hand across your mouth.

Dermal exposure can occur by not washing your hands after handling herbicides or their containers, not wearing gloves while mixing and loading and handling containers, directly splashing the product on the skin, the spray mist contacting unprotected skin or eyes, wearing herbicide contaminated clothing (including hats, boots and gloves), wearing inadequate or improperly cleaned protective clothing and safety equipment, handling contaminated equipment during mixing or application, and not washing gloves and boots before removing them.

Inhalation exposure can occur by breathing vapor, dust or mist during mixing or application, and by not wearing appropriate protective equipment.