PESTICIDES

You have heard it.   Pesticides are poisons.  Pesticides can be converted to chemical warfare agents.  The pesticides themselves can be used as chemical warfare agents.  The same technology (e.g. crop dusters) for dispersing pesticides can be used to disperse chemical warfare agents.

These statements are a mixture of truths, exaggerations, or are mostly false.  Let us take a look at these chemicals and what they can do.

What is a Pesticide?

A pesticide is an agent used to destroy pests.   Pesticides include insecticides (destroys insects), herbicides (destroys plants), arachnicides (destroys spiders, mites, ticks), larvacides (destroys immature stages of insects such as grubs and caterpillars), fungicides (destroys fungus), rodenticides (kills mice, rats), molluscocides (destroys clams, snails, etc.) and fumigants.  Sometimes insect repellents (such as DEET) and plant growth regulators are lumped in with discussions of pesticides.   The agent is usually a chemical that destroys the unwanted pest, but the agent could also be a naturally occurring plant extract or bacteria that inflict a fatal disease on the targeted pest.

Much effort is spent developing new pesticides that are specific to the targeted pest but do not harm other life including humans.  After the pesticide has accomplished its intended purpose, it should not persist in the environment.   Some of the older pesticides developed decades ago have been since found to harm birds and other life, or were very poisonous to humans, and have been outlawed for use in the United States.   Some may be produced or used in other countries for special use, such as use of DDT for control of mosquitoes where malaria is prevalent.

Pesticides may be applied to crops, to soils where crops are grown, buildings were crops are stored, ponds, forested areas, grasslands, applied topically to animals including humans, or put in cattle feed.

In lieu of pesticides, environmentally conscious people may advocate (1) control practices such as drainage of containers or ditches where mosquitoes may breed, (2) encouragement or addition of natural predators to control the unwanted pests, (3) crop rotation and (4) inclusion of “wild areas” within agriculture land.  The wild areas provide a home for natural predators.

Pesticide Classifications

Pesticide formulations as used by the consumer contain one or more active ingredients plus various “inert” ingredients.   The inert ingredients are there to give the formulation the desired physical characteristics so the pesticide can be easily and safely applied.   Sometimes the formulations are mixed with fertilizers.    Most formulations sold to the public are pellets, which are relatively dust free and release the active ingredient slowly.   However some are applied as dusts or as aerosolized liquid droplets.  Some are fumigants and are released as vapors or gases.   Some are poison bait and are used for rodents, birds, cockroaches, etc..

Pesticides are classified into either chemical classes or use classes according to the active ingredient.   The U.S. Environmental Protection Agency recognizes 1000+ active ingredients and almost 90,000 different brand name formulations.   Roughly 40 new active ingredients are approved each year.  Some active ingredients have been withdrawn from use in the United States because of harm to people or the environment.  A complete list is at http://www.epa.gov/pesticides/science/models_db.htm#databases The PEAC tool lists approximately 250 active ingredients including a few that have been withdrawn from use.  The approximately 250 selected for display in the PEAC tool are or were either widely used or are relatively toxic.

Pesticide names (for active ingredients) are assigned by the International Organization for Standardization (England).   The names assigned are unique and internationally recognized.   The U.S. Environmental Protection Agency and California Department of Pesticide Regulation maintains a listing of brand name formulations.

Examples of Pesticide Use Classes

• Insecticides and arachnicides
• Herbicides
• Fungicides
• Molluscicides
• Avicides
• Rodenticides
• Growth hormones

Examples of Chemical Classes

• Carbamates
• Organophosphates
• Chlorinated hydrocarbons (Organochlorine pesticide)
• Bromated hydrocarbons
• Botanicals (natural pesticides derived from plants)
• Triazine and Triazoles
• Ureas
• Thiocarbamates
• Dithiocarbamates
• Dinitrophenols
• Substituted amides
• Bipyridilium pesticides
• Phenoxys
• Pyridines
• Inorganic

A more complete listing of classifications with specific pesticide examples may be found at the EXTOXNET website developed by several state cooperative extension offices and maintained by Oregon State University, at http://extoxnet.orst.edu/tibs/pestgrp.htm .

The U.S. Department of Transportation classifies pesticides by chemical classes.  UN (United Nations) shipping numbers reflect this.   However the UN numbers also take into account the “inert ingredients” because they affect whether the pesticide is shipped as a liquid or solid, and the flammability of the liquid.   The following pesticide classes and corresponding UN#s are listed in the 2004 Emergency Response Guidebook:

Specific Listings

• Aluminum phosphide pesticide, UN# 3048
• Methyl parathion, liquid, UN#3018
• Methyl parathion, solid, UN#2783
• Parathion, UN#2783
• Parathion and compressed gas mixture, UN#1967
• Zinc phosphide, UN# 1714

Table 1.  Pesticide classes and UN Shipping Numbers

There are also generic listings for insecticide gases.

• Insecticide gas, n.o.s., UN# 1968
• Insecticide gas, toxic, flammable, n.o.s., UN#3355
• Insecticide gas, toxic, n.o.s., UN#1967
• Insecticide gas, flammable, n.o.s., UN#1954 or UN#3354

The words “toxic” and “poisonous” are synonymous in the 2004 Emergency Response Guidebook, and n.o.s. means “not otherwise specified”.      However, “Liquid, toxic”, “Liquid, toxic, flammable” and “Liquid, flammable, toxic” have different meanings as defined by DOT:

• Liquid, flammable, toxic:  flash point less than 23^oC  [23^oC = 73.4^oF]
• Liquid, toxic, flammable:  flash point between 23^oC and 61^oC
• Liquid, toxic:  flash point over 61^oC  [141.8^oF]

Parathion and methyl parathion also fall into the general classification of organophosphorous pesticides.

How Toxic are Pesticides to Humans?

It depends upon the particular pesticide.   Some are highly toxic and some are practically nontoxic.    Some may be acutely toxic but display no long-lasting effects if and when the person recovers.   Exposure to other pesticides may increase the risk of cancer or motor impairment later in life.

The U.S. National Library of Medicine maintains a data base on toxicity of chemicals including pesticides.   To access this, go to http://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?HSDB, and enter the pesticide name or its CAS number.

Acute toxicity is measured in terms of exposure of the chemical to a test animal, usually a rat.  The dosage (mg of chemical per kilogram of body weight) which kills 50% of the test animals is called LD₅₀ .   Exposure may be by ingestion, skin contact (a patch containing the chemical is attached to the animal), or by inhalation.   LC₅₀ is the concentration of chemical in the air that kills 50% of the animals over a specified time period, usually 1 or 4 hours.   The inference is made that the animal toxicity data can be transferred to humans.    Pregnant females and the very young are often more susceptible. 

Table 2.  Toxicity data for seven of the more toxic pesticides: