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Daily News Blog

18
Feb

Implications for Human Health: Chronic Inhalation of Paraquat in Low-Doses Disrupts Sense of Smell

(Beyond Pesticides, February 18, 2021) New research published in the journal Toxicological Sciences finds extended inhalation of the common herbicide paraquat causes male mice to lose some sense of smell, even at low doses. This study highlights the significance of understanding how specific chemical exposure routes can influence disease development. Olfactory (relating to the sense of smell) impairment is a precursory feature of Parkinson’s disease (PD), and studies connect paraquat poisoning to PD risk. Hence, future pesticide management policies should assess specific disease risks with bodily chemical concentration from low-dose, chronic neurotoxic chemical exposure. The study’s researchers note, “These data support the importance of route of exposure in the determination of safety estimates for neurotoxic pesticides, such as [paraquat]. Accurate estimation of the relationship between exposure and internal dose is critical for risk assessment and public health protection.” Despite evidence demonstrating that olfactory  nerve cells transport toxic airborne particles and solutes to the brain upon inhalation, the possibility of olfactory impairment (damage) from paraquat inhalation lacks adequate assessment.

To assess the impact paraquat has on olfactory function, researchers exposed a cohort of adult female and male mice to paraquat aerosols in an inhalation chamber for four hours a day, five days a week, for four weeks. Researchers investigated paraquat concentrations in several brain regions (olfactory bulb, striatum, midbrain, and cerebellum), lung, and kidney—during and after exposure—using mass spectrometry. Lastly, researchers examined olfaction (sense of smell) by employing the olfactory discrimination paradigm.

The study results find paraquat inhalation causes a significant burden to all regions in the brain, with the olfactory bulb harboring the highest impact. Although paraquat concentrations are detectable in the lung (highest) and kidney (lowest) tissue, concentration levels subside to the control range within four weeks after exposure. However, olfactory impairments persist for months following the last exposure. Furthermore, paraquat inhalation induces male-specific differences in olfaction, not observable in females.

Many studies find an association between pesticide exposure and an alteration in the senses when pesticides enter the body. Research links pesticide exposure to blurred vision (vision loss), change in taste receptors (taste loss), loss of sensory reception (touch), loss of olfactory function (smell), and loss of auditory function (hearing). Although pesticides can enter the body via various exposure routes (i.e., dermal [skin]/contact absorption, inhalation of particles, ingestion of contaminants), inhalation is most daunting, offering a direct pathway to the brain. Upon inhaling a pesticide, the particles enter the nose and travel to the brain via the olfactory neurons (nerve cells) responsible for smelling. Moreover, the mouth/throat, ear, and nose connect. Therefore, pesticide particles have the potential to permeate the entire body and bloodstream and accumulate in fatty tissues, causing disease-inducing issues like oxidative stress and endocrine disruption. The lack of research on pesticide exposure through inhalation has implications for human health, especially since past studies indicate different exposure routes still result in similar disease outcomes. 

One of the most notorious pesticides associated with PD development is paraquat, indicative of PD pathology. Scientific literature comprehensively documents the neurotoxicant properties of paraquat as laboratory experiments reproduce features of Parkinson’s in the brain of animals. A preceding study finds a 2.5-fold increase in PD risk among users of paraquat in comparison to non-users. Paraquat exposure can increase the production of specific proteins in the brain that damage cells producing dopamine, causing motor problems and muscle tremors. Although many countries, including Europe and Canada, ban the use of both chemicals due to concerns about links to Parkinson’s, the U.S. merely restricts use. 

In the U.S., the agency restricts paraquat application to certified applicators, allowing chemical-use to rise over the decade, with 2018 seeing a 100 percent increase in paraquat use in wildlife refuges. Even more concerning is that some personal protection equipment (PPE) may not adequately protect certified pesticide applicators from chemical exposure during application. Moreover, paraquat drift from refuges and other pesticide-treated areas presents a risk for PD development to nearby residents. A Louisiana State University study finds that residents living adjacent to a pesticide-treated (2,4-D, paraquat, and chlorpyrifos) pasture and forest from the agriculture and timber industry have higher PD incident rates. Pesticide residues present additional residential pesticide exposure, increasing the risk for PD via ingestion of contaminated water or food. Nevertheless, both direct occupational and indirect nonoccupational exposure to pesticides, especially paraquat, can increase the risk of PD

This study’s results demonstrate that extensive, low-level exposure to paraquat through the nasal cavity is consistent with olfactory bulb stress that may have implications for PD risk. Furthermore, the study reveals paraquat inhalation outcomes, including a sex-specific change in the sense of smell, are consistent with the onset symptoms of PD. Prior studies indicate that a pathological (disease-causing) agent, like pesticides, may infiltrate the nervous system via the olfactory bulb, gut, or both and circulate throughout the nervous system to increase PD risk. 

Research demonstrates that numerous pesticides belonging to various pesticide classes and differing in modes of action present a risk of developing PD. Advocates say that government officials must evaluate all health effects related to chemical exposure equally regardless of chemical composition. Only a small percentage of PD incidences are genetic, and PD is quickly becoming “the world’s fastest-growing brain disease.” Therefore, research like this is vital for examining how various pesticides and their exposure routes present potential risk factors for developing diseases like Parkinson’s. Authors of the study conclude, “Inhalation of nonvolatile pesticides is understudied because people think the likelihood of inhaling them is low. However, pesticides can become airborne when sprayed, creating the opportunity for inhalation. […]From a broader perspective, we need to recognize that numerous studies have documented the presence of various pesticides in ambient air, and it is important to consider what health consequences might arise from those exposures.”

Human senses are integral to everyday human activities, and it is vital to understand how chronic pesticide exposure can limit the body’s ability to function normally. Advocates are calling for policies that enforce stricter pesticide regulations and increase research on the long-term impacts of pesticide exposure. There are several limitations in defining real-world poisoning as captured by epidemiologic studies in Beyond Pesticides’ Pesticide-Induced Diseases Database (PIDD). The adverse health effects of pesticides, exposure, and the aggregate risk of pesticides showcase a need for more extensive research on occupational and nonoccupational pesticide exposure, especially in agriculture. For more information on the multiple harms of pesticides, see PIDD pages on the brain and nervous system disordersendocrine disruption, cancer, and other diseases. Furthermore, see Beyond Pesticides’ Parkinson’s Disease article from the Spring 2008 issue of Pesticides and You.

Proper prevention practices like buyinggrowing, and supporting organics can eliminate exposure to toxic pesticides. Organic agriculture has many health and environmental benefits, which curtail the need for chemical-intensive agricultural practices. Regenerative organic agriculture nurtures soil health through organic carbon sequestration while preventing pests and generating a higher return than chemical-intensive agriculture. For more information on how organic the right choice, see Beyond Pesticides webpage, Health Benefits of Organic Agriculture

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: National Institute of Environmental Health Science (NIEHS), Toxicological Sciences

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