It has become common among environmentalists and human health activists to use 2,4-D as a symbol of, even synonym for, pesticides. There are two major dangers with this. It encourages sprayers to switch from real 2,4-D to more dangerous substitutes. And, it diverts attention from pesticides and solvents that cause far more health problems than 2,4-D ever has.
For example, I often hear that "2,4-D was a component of Agent Orange". This is a factually correct statement, but is misleading to the point of dishonesty. There were indeed significant health problems caused by Agent Orange among populations exposed to it. No reputable investigator blames the 2,4-D component of Agent Orange for these health problems - they were almost certainly caused by the large dioxin content of the other principal component of Agent Orange, 2,4,5-T. (The proportion of dioxins in Agent Orange has been questioned by many chemists as being in excess of that normally associated with production of 2,4,5-T. Despite this, 2,4,5-T is no longer registered in Canada.)
Lawn spray companies use the popular preoccupation with 2,4-D to their advantage. One in Ottawa-Carleton distributed 25 pages of scientific studies that show how little risk 2,4-D poses on a lawn. There was not a word in all that impressive looking writing about any of the other pesticides the company used - every one of which poses a significantly higher risk to human health or to the health of the environment than does 2,4-D.
Other companies now lure customers with advertisements that they do not use 2,4-D. The statement is usually absolutely correct - they have switched to herbicides that are more likely to be dangerous to the environment. 2,4-D acid (the form registered for general use) bonds to soil and plants within minutes. Mecoprop (MCPP), a herbicide chemically very similar to 2,4-D, is designed to stay active in soil for months. Dicamba, another herbicide very similar to 2,4-D, is designed to keep moving through soil - if it hits a plant root, it will get into the plant that way. Of course, if it doesn't hit a 'weed' root, it keeps going down to our tree roots or, failing that, down to ground water and gets into our wells. (Due to its persistence, mecoprop is a significant ground water contaminant in many areas too.) Essentially all lawn herbicide formulations contain mecoprop and dicamba. MCPA, the commonest direct 2,4-D substitute, is in all but law 2,4-D under another name. There is not the slightest reason to believe that MCPA is significantly different with respect to potential health problems than 2,4-D acid. But, it seems to be less effective as a herbicide. So, many herbicide products that 'don't contain 2,4-D' contain a higher proportion of the environmentally more dangerous mecoprop and dicamba than formulations that contain 2,4-D.
The association of 2,4-D with Agent Orange has prompted a great deal of searching for adverse health effects of 2,4-D. Pesticides are designed to kill, they are designed to reduce biodiversity - that's what people buy them for. So, if you search hard enough for adverse biological effects of absolutely any pesticide, especially at extreme doses, you will find them. A hundred studies showing marginal connections of phenoxy herbicides (very few studies actually relate to pure 2,4-D) with health problems does not mean that 2,4-D is more dangerous than other pesticides. It simply means that we have been diverted from more serious dangers to human health - in particular, from insecticides and their solvents. Above all, from insecticides used indoors.
Our indoors, sealed, environment has almost none of the biological mechanisms that degrade pesticides outdoors, almost none of the physical mechanisms that disperse pesticides outdoors. Many chemicals that degrade in days outside survive for a year indoors. Yet, every day, people spray huge quantities of insecticides and toxic solvents indoors, to get rid of insects. Nearly always, north of the termite belt, the insects are cockroaches, which pose little health risk to people other than asthmatics and can be easily and effectively controlled by non-toxic methods (cf.Non-toxic Control of Cockroaches).
After 40 years of involvement with health issues of pesticides, I am certain that there are more significant health problems caused by indoor spraying of insecticides within the tiny Ottawa-Carleton Region than are caused by 2,4-D worldwide. So, if you care about human health, 2,4-D is the wrong target. It's the wrong synonym for danger, it's the wrong symbol of pesticides. There are many hundreds of people in Ottawa-Carleton whose health has been devastated by cockroach sprays 'applied as directed' - diazinon, chlorpyrifos and xylene in particular (cf. Human Reactions to Pesticide Exposure). Elimination of substances strongly toxic to humans should be the goal of anyone who cares for people.
If you care about biodiversity, the health of our biosphere, I submit that 2,4-D is the wrong target too. There is far more atrazine than 2,4-D used in Canada to reduce plant diversity, for example. It is an exceptional urban home lot that boasts as many as 100 species of plants which herbicides kill. Any healthy yard will house more than 1000 species of insects (almost all neutral or beneficial to us, if people only realised it). Insecticides kill a wider variety of living things than herbicides do. They are mostly more unselective and persistent at it too.
Even the origins of 2,4-D are inappropriate for censure. Unlike the many insecticides that did indeed come out of chemical warfare research, 2,4-D is a close analog of the natural plant growth hormone auxin, and was developed by a scientist who was hoping to alleviate world hunger by improving the growth of food plants.
So, whether we care for human health, biodiversity or history, we should forswear colloquial use of the word 2,4-D. It's working against the interests of all of us who care for health and life.
other notes on pesticides