Many people hold on-to the belief that pesticides are poisonous, and that they are harmful to humans and bad for the environment. They have come to this belief mostly through unfortunate mistakes made in the past and through generalized broad-brush thinking about a large group of chemical compounds with widely varying characteristics. Under the heading of pesticides, there are several sub-groups, and yes, some are poisonous to animals because they are meant to kill animals (rodenticides and insecticides). Others are poisonous to funguses (fungicides) or lice (pediculicides). Another grouping (biocides) includes antibiotics and antivirals routinely used in modern medicine. Herbicides kill plants by disrupting biologic processes that occur only in plants (more about that later). About the only statement that applies to all of these is that they are man-made chemicals.
In criticizing herbicides, many people have been influenced by our horrible experience in Vietnam with Agent Orange. Containing equal parts of 2,4-D and 2,4,5-T, Agent Orange was used to defoliate the jungles to expose Viet Cong troop movement. Many veterans in the U.S. and many more residents of southeast Asia became sick after the war with illnesses ranging from several types of cancer to various respiratory diseases. After years of inquiries, it was learned that the 2,4,5-T in Agent Orange was contaminated with dioxin, a potent and known carcinogen, as a byproduct of the manufacturing process. Consequently, the production of 2,4,5-T has been banned by the US and numerous other countries. For a detailed discussion of this subject, see http://www.nap.edu/catalog/11242.html#orgs. Over 20 million gallons of herbicides (there we others besides Agent Orange) were spread in Southeast Asia between 1962 and 1972 and we can only speculate at the high concentrations that were probably used during a war-time action. Many soldiers on the ground must have had very high rates of exposure and the persistent nature of dioxin has affected many non-combatants in southeast Asia in the years since the war ended.
DDT is an insecticide that was used heavily during WWII and as late as the early 1970s. The chemical is fat soluble and because of this it bio-accumulates in food chains. This means that the animals at the top of the food chain (including us) get the highest doses. DDT takes more than 15 years to break down in the environment, is a probable human carcinogen and can damage many parts of the human body. It also pushed the Bald Eagle to the brink of extinction. It is really bad stuff and its use was banned in the U.S. in 1972. It's important to note that DDT is an insecticide, not an herbicide.
Dioxins and DDT are included on the EPAs "Dirty Dozen" list of banned Persistent Organic Pollutants (POPs). Many POPs were widely used during the boom in industrial production after World War II, when thousands of synthetic chemicals were introduced into commercial use. Many of these chemicals proved beneficial in pest and disease control, crop production, and industry, but had unforeseen effects on human health and the environment. Their use has been reduced or eliminated through a United Nations treaty agreed to by the U.S. and 90 other countries. For more on this go to http://www.epa.gov/oia/toxics/pop.html#stockholm.