While the use of one toxic chemical—on our foods, lawns, and elsewhere—has its inherent risks, scientists warn that the combination of two or more such ingredients in common pesticides could have an even more noxious impact, one which is commonly overlooked.

In fact, a investigation released Tuesday by the environmental watchdog Center for Biological Diversity (CBD) found that over the past six years the U.S. Environmental Protection Agency (EPA) has approved nearly 100 pesticide products that contain these so-called "synergistic" compounds, effectively "increasing the dangers to imperiled pollinators and rare plants."

As CBD explains, "[s]ynergy occurs when two or more chemicals interact to enhance their toxic effects," turning "what would normally be considered a safe level of exposure into one that results in considerable harm."

"The EPA is supposed to be the cop on the beat, protecting people and the environment from the dangers of pesticides. With these synergistic pesticides, the EPA has decided to look the other way, and guess who's left paying the price?" asked Nathan Donley, a scientist with the Center and author of the report, Toxic Concoctions: How the EPA Ignores the Dangers of Pesticide Cocktails (pdf).

One toxic cocktail that has gotten some attention is Dow AgroScience's Enlist Duo, which contains two of the most commonly used pesticides in the nation: 2,4-D and glyphosate. The EPA approved the product in October 2014 but revoked the license after discovering a patent application in the U.S. Patent and Trademark Office Database that warned of synergistic toxicity to plants.

Following the lead of the EPA, Donley analyzed the patent database for other recent pesticide products approved by the EPA for agrochemical giants, Bayer, Dow, Monsanto, and Syngenta.

According to Donley, among the key findings are:

69 percent of these products (96 out of 140) had at least one patent application that claimed or demonstrated synergy between the active ingredients in the product;

72 percent of the identified patent applications that claimed or demonstrated synergy involved some of the most highly used pesticides in the United States, including glyphosate, atrazine, 2,4-D, dicamba and the neonicotinoids thiamethoxam, imidacloprid and clothianidin, among others.

As the research notes, another example of a common pesticide that has proven synergy but that the EPA has failed to cross-examine for compounded impacts are bee-harming neonicotinoids.

"It's alarming to see just how common it's been for the EPA to ignore how these chemical mixtures might endanger the health of our environment," Donley said.

"It's pretty clear that chemical companies knew about these potential dangers, but the EPA never bothered to demand this information from them or dig a little deeper to find it for themselves," he added.

Andre Leu, an organic farmer based in Australia and president of the International Federation of Organic Agricultural Movements (IFOAM), has done extensive research on the subject of synergistic compounds.

In Leu's 2014 report The Myths of Safe Pesticides (pdf), he states unequivocally that it is a "myth" that pesticide formulations are "rigorously tested."

Leu writes: "Given that the other chemical ingredients are chemically active as they are added to the formations to make the active ingredient work more effectively, the assumption that they are inert and will not increase the toxicity of thew hole formulation lacks scientific credibility. The limited scientific testing of formulated pesticide products shows that they can be hundreds of times more toxic to humans than the pure single active ingredient."

Donley said that "the EPA has turned a blind eye for far too long to the reality that pesticide blends can have dangerous synergistic effects. Now that we know about all the data that are out there, the EPA must take action to ensure that wildlife and the environment are protected from these chemical cocktails."