Pesticides have become pervasive in the modern-day environment. They are known to act as neurotoxins, meaning they are toxic to the nervous system of insects, and have been shown to accumulate over time in the human body [1]. This, then, raises the question as to whether environmental pesticide exposure poses a risk to the human brain and contributes to age-related cognitive decline.
The long-standing adage that “the dose makes the poison” facilitated the widespread use of pesticides. While exposure to high levels of certain classes of pesticides results in acute signs of nervous system toxicity in people, exposure to low levels typically do not have noticeable effects in the short term [2]. Since insects are so much smaller than humans, the levels that are toxic to insects are generally considered below the threshold for toxicity in people. However, due to the ability of some types of pesticides to accumulate over time within fat tissue, continued exposure may eventually lead to levels that reach the threshold for toxicity.
The risk posed by chronic exposure to low levels of pesticides has been difficult to assess. Different research groups have taken different approaches, which in many cases has led to different conclusions, and has ultimately made the question of pesticide-related health risks a controversial topic. Most people are exposed to pesticides in the environment without realizing it, which makes it difficult to get accurate exposure estimates [2]. Therefore, most studies follow individuals with occupational exposure to pesticides, such as agricultural workers. But because the exposure levels of these workers tend to be much higher than the average person, it is unclear how well these findings apply to the general population.
Numerous studies have found an association between high occupational exposure to pesticides and reduced cognitive performance, and/or an increased risk for Alzheimer’s disease later in life [3; 4; 5]. Notably, the effects were not seen uniformly, as both the composition of the study participants and class of pesticide influenced the findings. In general, women appeared more likely to show exposure-related effects, and exposure to a class of compounds known has organophosphates, which includes DDT, was most associated with cognitive effects [6; 7; 8]. Additional studies measuring pesticide levels in the blood have also shown a link between high levels of DDT and reduced cognitive performance with aging [9; 10]. DDT is known to accumulate in fat tissue and pass into the brain, and women have a higher percentage of fat tissue than men. These associations, then, are consistent with toxicity emerging late in life following chronic cumulative exposure.
The cognitive effects of pesticide exposure appear to follow a similar pattern as what has been seen with cancer. Namely, that the risk for negative health effects for a group of people with similar pesticide exposure will vary from person to person [1]. This is due to a combination of behavioral and genetic effects. Some people may be more thorough in adhering to safety protocols and taking proper precautions, such as wearing personal protective equipment and not bringing contaminated clothing into their living space. Depending on genetics, some people will be more likely to store pesticides in their brain tissue. Individuals can differ in their ability to breakdown the pesticides in the body, and some of these breakdown products are themselves toxic. The age of exposure may also play a role in one’s vulnerability to a particular health effect. For example, the child of a pregnant woman exposed to pesticides may experience a greater impact on brain function relative to the mother. As a result, the threshold for pesticide exposure-related toxicity to the brain will vary from person to person.
Due to its persistence in the environment, pesticide exposure cannot be completely avoided, but the risks can be mitigated. Most pesticide exposure occurs via the food supply, and can be mitigated by thorough washing of fruit and vegetables. Household use of pesticides should be performed by trained professionals whenever possible. It is critical to read the safety labels of pesticides to determine the proper precautions, as their formulation influences their toxicity and the hazards of exposure [11]. Overall, cumulative pesticide exposure is likely to be below the threshold for toxicity in most people, but because some people may be especially sensitive, it is best to take precautions whenever possible in order to reduce lifetime exposures.
Betsy Mills, PhD, is a member of the ADDF's Aging and Alzheimer's Prevention program. She critically evaluates the scientific evidence regarding prospective therapies to promote brain health and/or prevent Alzheimer's disease, and contributes to CognitiveVitality.org. Dr. Mills came to the ADDF from the University of Michigan, where she served as the grant writing manager for a clinical laboratory specializing in neuroautoimmune diseases. She also completed a Postdoctoral fellowship at the University of Michigan, where she worked to uncover genes that could promote retina regeneration. She earned her doctorate in neuroscience at Johns Hopkins University School of Medicine, where she studied the role of glial cells in the optic nerve, and their contribution to neurodegeneration in glaucoma. She obtained her bachelor's degree in biology from the College of the Holy Cross. Dr. Mills has a strong passion for community outreach, and has served as program presenter with the Michigan Great Lakes Chapter of the Alzheimer's Association to promote dementia awareness.
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