May 30, 2019: EPI investigator Jeffrey Bloomquist reviewed the insect repellent DEET's safety record, and finds it poses little to no risk when used in compliance with product label directions. In fact, he says, it's a useful tool to prevent insect-borne diseases when utilized intermittently.
Warm and sunny days mean that people who venture out-of-doors are encountering the mosquitoes, ticks, and biting flies that dwell in our gardens, forests, lakes, glades and beaches.
A spritz of insect repellant can hedge against the risk of contracting any number of insect-borne diseases such as Zika, West Nile virus, St. Louis encephalitis, Lyme disease or Rocky Mountain spotted fever.
Many arboviruses cause diseases which invade the nervous system and produce encephalitis, aseptic meningitis, or muscle and joint pain. Pregnant women who contract an arbovirus may also place their fetus at risk of complications such as microcephaly, which can occur with Zika virus.
Given the grave risks, EPI investigator Jeffrey Bloomquist, a professor of insecticide toxicology, wants the public to be fully informed about their available choices. But which insect repellant is most effective while being safest to use?
“DEET has been widely used in repellents for decades,” says Bloomquist, who is also affiliated with UF’s department of entomology and nematology in the Institute of Food and Agricultural Sciences. “Yet there is still a lot of misperception that it’s a dangerous neurotoxin even though the evidence for that is very weak. People think it works like a nerve gas, but that is simply not true.”
Given the many positive outcomes of using DEET in terms of protecting people from contracting an insect-borne disease, Bloomquist says it’s worth taking a second look at DEET’s safety.
Long viewed as the gold standard in insect deterrence, DEET-based repellants are among the most effective and longest lasting on the market. A 2002 study found that a 23.8 percent DEET formulation can efficiently repel mosquitoes for 5 hours. This is extremely long lasting compared to other products that were effective for only 20-90 minutes.
But it turns out that the ingredient that best repels skeeters, DEET, may also scare off people in the market for a great repellent. DEET began losing popularity in the 1980s due to fears about irreversible harm to the human central nervous systems based on a few high-profile intentional poisonings where people drank the chemical and died. Later, in 2009, a paper was published claiming that the chemical was neurotoxic to mammals and warned about use by people.
Bloomquist decided to investigate the scientific literature on the epidemiology of DEET’s use and its safety profile. Working with colleague Daniel Swale, an entomologist at Louisiana State University, the pair also examined the current state of understanding of the mechanisms of DEET in suspected neuro- and cardiac toxicity. Their findings are in press with the journal Pest Management Science. (Disclosure: Bloomquist is the journal’s executive editor, but the paper underwent independent peer review.)
According to their research, there have been nine deaths attributed primarily to DEET over a 60-year time period. Of these, four people intentionally ingested it. Autopsies revealed that some of the ingestion cases also involved other chemical substances, including drugs and alcohol, making the specific cause of death difficult to pinpoint. The other five cases involved skin exposure to extremely high concentrations that are atypical of normal use.
When DEET is used according to its product instructions, a small number of people may experience skin irritation, a burning sensation, blisters and, in extreme cases, localized tissue death. Reported brain injuries from acute or chronic exposure include tremors, coma, hypertonia, and seizures. Low blood pressure, and slow heart beat have also been reported in association with DEET use. However, all of the adverse reactions listed above appear to be reversible when use of DEET is stopped, with no long-term or lingering side effects.
Bloomquist and Swale also reviewed several papers which examined DEET’s mode of action, looking particularly at studies that tested whether or not DEET inhibits an enzyme called acetylcholinesterase. This enzyme acts to return a neuron to its resting phase after the neuron is activated.
It has long been questioned to what degree DEET acts on this enzyme, and if its effects are short lived or permanent. Swale and Bloomquist previously published a study in 2014 to further examine this line of inquiry. They found that DEET worked only as a very weak inhibitor of acetylcholinesterase, and that it was not effective enough to be toxic except in extremely high concentrations. Their new review paper identified several studies in addition to their earlier work which found DEET to be a “weak and reversible inhibitor” of acetylcholinesterase. In other words, whereas neurotoxic warfare agents are irreversible because they bind and react with acetylcholinesterase, DEET’s effects are not only weak but also reversible – they cease entirely once the chemical is metabolized and excreted.
Bloomquist says this means that any adverse effect of DEET on people can’t be attributed to it blocking acetylcholinesterase from doing its job. Instead, he and Swale suspect that DEET may block certain ion channels which may account for the skin irritation and even numbness that some people experience when using DEET repellants and errantly swipe some on their lips.
“We take many acceptable risks in our daily activities, like driving a car, and we tend to weigh these risks against the benefits,” Bloomquist says. “The fact is, when used as the label indicates, DEET poses very little risk to people’s health. There are much bigger risks to consider in contracting a debilitating illness like Lyme’s disease."
Given how climate change is shifting the range of mosquitoes and other biting insects, using a repellant safely is an important tool to prevent the acquisition, and spread, of an insect-borne disease. DEET is intended for intermittent use with outdoor activities. It’s not meant to be applied heavily daily or sprayed on a home’s walls, for example.
While it remains an open question as to whether some people may have a heightened sensitivity to DEET, the science is closing in on determining that DEET is not itself a neurotoxin that poses a serious threat to people when used according to product label instructions.
So, get ahead of the biting bugs and spritz away this summer. (After, of course, reading your insect repellant’s label on how to apply it safely.)
Written by DeLene Beeland; Top image: Deer ticks are a common vector for Lyme disease, courtesy of the CDC Public Health Image Library; Deet chemical structure graphic courtesy of Jeff Bloomquist.
Read Jeffrey Bloomquists EPI profile here.