By definition, all pesticides are toxic. Their only purpose is to kill living organisms whether they be rodents, weeds, fungi or bacteria. The key for human safety is the level of toxicity and our exposure to these compounds.

Toxicity is usually grouped into two categories: Acute and Chronic.
 

Acute toxicity results from a single exposure such as during a spill or when you get drenched while making an application to the plants.

Acute exposure occurs in three ways. Dermal toxicity results when pesticides come into contact with the skin. Inhalation toxicity occurs when the chemicals are breathed into the lungs. Oral toxicity happens when pesticides are swallowed into the digestive tract.

A common method for expressing acute toxicity is the LD50 rating. This is the amount of a pesticide that was needed to kill half of the test population of animals under laboratory conditions. It is expressed in milligrams per kilogram of body weight. Therefore, the larger the animal ( or person) the higher the quantity needed to cause death.

The key to interpreting the LD50 rating is that the larger the number, the lower the relative toxicity of the pesticide. An LD50 of 500 is ten times more toxic than a product with an LD50 of 5,000.

Pesticide labels contain Signal Words which indicate the relative toxicity of the product based on LD50's. 

1. Danger and Poison with a skull and crossbones will appear on the most toxic pesticides. As little as a few drops could be fatal. These chemicals are called Restricted Use Pesticides (RUP) and may only be purchased by certified applicators.
   
   

2. Danger alone will appear if skin or eye irritation may be severe.
   
   

3. Warning indicates a moderately toxic substance where 1 teaspoonful to 1 ounce could kill a 150 pound person. Most of these products are RUP's although some are available to the general user.
   
   

4. Caution designates the least toxic materials with an acute oral LD50 of over 500 mg/kg. General use pesticides available for use in the home landscape or garden fall into this category.

The more sinister aspect of toxicity in pesticides is their chronic impact. In this case, there are few, if any, immediate symptoms resulting from exposure to pesticides. Rather, these are impacts that build up over time usually due to low level exposures over long periods of times. This may come from getting a little pesticide on your skin several times a year over a period of decades. Nobody knows for sure.

The most typical problem resulting from chronic exposure is, of course, cancer. Probably the best advice is to minimize your exposure to any pesticides by using the proper safety apparel, applying them properly and, most of all, using an alternative control option whenever possible.