The insecticide, Naled, which was sprayed over an area of Miami-Dade County twice last week and is due to be sprayed again this weekend in an effort to stop the spread of the Zika virus, is a potent neurotoxin that kills adult mosquitoes on contact. The protesters who disagree with the Centers for Disease Control and Prevention and local governments’ assurances that it is safe, are justified in their concerns — ask the European Union, which banned the use of this organophosphate in 2012; the beekeeper in South Carolina who recently lost more than 2 million bees after aerial spraying of Naled; or the governor of Puerto Rico who, despite an escalating outbreak of Zika, refused to use it and sent supplies back to the mainland. He was right to err on the side of caution.

The CDC says on its website that the use of Naled is safe because the amount used is small and because it has been used extensively in the United States since the 1950s. Having seen what chlorpyrifos, another organophosphate, did to human health, I take little comfort from the latter assertion. (Chlorpyrifos was phased out for indoor use in the United States, but is still widely used in agriculture.)

But let’s deal with the former assertion: that it is safe for all of us because only two tablespoons are used per acre sprayed.

I am a physician and research scientist. I have spent three decades studying the effects of chemicals, including organophosphates, on human health. I was the environmental consultant for the Department of Veteran Affairs on the “unexplained illnesses” of Gulf War Veterans. These veterans were exposed to a huge range of chemicals including organophosphate pesticides (OPs); my studies established a strong link between these chemical exposures and their chronic health problems. Congress invited me to testify about this on several occasions. My extensive work on OPs has led me to understand that dose alone does not equal the poison and that, in short, there is a subset of individuals who are particularly vulnerable to the effects of these toxicants. “Dose plus host” determines toxicity.

OP insecticides are cholinesterase inhibitors, which are toxic to the nervous system. They are highly toxic by all routes of exposure. The fact that these Naled exposures are so-called “low level” and the pesticide is rapidly degraded outdoors is misleading because even low levels of OPs rapidly enter the brain and central nervous system via inhalation, ingestion and/or skin contact. They do their damage and are quickly eliminated, leaving no footprints behind — in contrast to heavy metals like lead or mercury, and chlorinated pesticides like DDT that can be detected in the body, even decades later. This rapid exit of OPs from the body makes them especially difficult to study.

Meanwhile, the amount and capacity to detoxify OPs varies enormously, and we do not know in advance who the genetically susceptible adults or children (including unborn children) are — not until they become ill from this class of chemicals days or weeks or even years (in the case of children) after their exposure. They often have subtle flu-like symptoms, like fatigue, headaches, memory and concentration problems that can disable some people for a lifetime — from a single exposure.