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Daily News Blog

30
Jul

Long-Term Pesticide Exposure Linked to Hearing Loss in Farmworkers

(Beyond Pesticides, July 30, 2020) Simultaneous exposure to pesticides and noise from agricultural machinery increases farmworkers, risk of hearing loss, according to the study, “Hearing Loss in Agricultural Workers Exposed to Pesticides and Noise,” published in the journal Annals of Work Exposures and Health. Hearing loss is the 3rd most common health issue in the U.S., affecting eight million Americans. Although specific conditions like age, illness, and genetics, can mediate hearing loss, research suggests other factors can induce auricle (ear) damage, including medications, exposure to toxic chemicals (including pesticides), and loud, ongoing noise. Past studies find an association between hearing loss and pesticide exposure or noise exposure, alone. However, this study is one of the first to associate hearing loss with the additive effect to concurrent, persistent pesticide exposure, and noise.

This research is significant as 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, for farmers and everyone alike. Researchers in the study note, “[I]t is necessary to understand what work-related factors are contributing to this high prevalence of hearing loss in [Thai] agricultural workers in order to develop effective interventions and policies.”

In agriculture, farmworkers and their children suffer elevated rates of injuries and illnesses, including hearing loss, various cancers, neurological disorders, and respiratory diseases. Additionally, research suggests that chemical-intensive agriculture’s high pesticide use puts farmers and farmworkers at greater risk of pesticide exposure than other occupations. With agricultural workers experiencing a higher frequency of hearing loss than other professions, an understanding of the etiology of non-age-related hearing loss associated with pesticide exposure is important to adequate pesticide regulation. 

To calculate the metrics for pesticide exposure (intensity x duration x frequency) and cumulative noise exposure for farmers, researchers examined 163 chemical-intensive conventional and 172 organic farmers using a survey. The survey contained data about demographic characteristics, noise-related activities, and agricultural machinery use. Additionally, conventional farmers diarized maintained a diary of their pesticide use every day for a year. Hearing assessments employed pure tone audiometric testing for all participants, using a mean hearing threshold in the low-frequency band (0.5–2 kHz) and high-frequency band (3–6 kHz) for analysis. Lastly, researchers compared pesticide and noise metrics to the average hearing threshold in each frequency band using linear regression models for each ear to determine association.

After adjusting for confounding variables, this research finds that years of exposure to pesticides in conventional farming and noise from agricultural machinery increases the average hearing threshold (> 25 dB HL threshold) in the high-frequency band. An increase in threshold in high-frequencies results in a higher prevalence of abnormal hearing (clinical hearing loss) as 93 and 78 percent of those with high-frequency hearing loss also have a low-frequency hearing loss in the right and left ear, respectively. Specifically, the highest level of cumulative pesticide exposure stems from organophosphates (OP)—derived from World War II nerve agents)—which are significantly associated with hearing loss in the high-frequency band, according to the study. Based on the regression models, the results support the study’s hypothesis that that pesticide and noise have an additive effect on hearing.

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 acutely and chronically affect most senses, loss of auditory function is unique as it is solely a chronic side effect of pesticide exposure. Previous occupational (work-related) studies suggest that organophosphate insecticides increase the risk of vision or hearing loss. A 2018 study finds that pesticides can increase the risk of occupational hearing loss in farmworkers. Tobacco growers exposed to pesticides display symptoms of central auditory dysfunction, according to a 2016 study. However, not all pesticides-related hearing loss is occupational and has implications for the health of children and pregnant women. Prenatal exposure to the OP chlorpyrifos disrupts hearing and vision maturation in infants. Furthermore, research finds that legacy organochlorine pesticides (OCP) (banned for use) have ototoxic (toxic to the ears) effects, as prenatal OCP exposure causes hearing deficits in infants.

In addition to understanding how pesticides impact the five senses, it is vital to consider the ways that pesticides can enter the body and cause harm. Three commons ways pesticides enter the body are via contact (on the skin, eyes), ingestion (mouth), and inhalation (mouth, nose), with the most common route of exposure via absorption through contact with the skin. Pesticide inhalation from vapors and fine particles in spray solution can severely damage the nose, throat, and lung tissue as the rapid absorption of pesticides increases the risk of respiratory problems. Oral exposure to pesticides via ingestion can be most severe as symptoms include damage to the gastrointestinal tract, repository depression, and even death. The chemical state of the pesticide (i.e., solid, liquid, or gas) affects the probability of bodily penetration as pesticides in liquid or gas formulas can enter the body through all exposure routes (inhalation, ingestion, contact). Although solid formula pesticides have a lower probability of entering the body, they can still enter the body via the same manner as gas, through volatilization, and liquid; they have to be small enough and remain on the skin long enough to do so. Once these chemicals enter the body, the bloodstream can readily absorb them, causing disease-inducing issues like oxidative stress and endocrine disruption.

This study adds to the expanding volume of research linking pesticide exposure to ototoxicity as statistic models demonstrate that cumulative pesticide exposure via increased pesticide use is a risk factor for hearing loss. Some research explains the underlying mechanism of pesticide-induced hearing loss because of injury to the stereocilia (inner ear cilia like organelles) in the organ of Corti, or the organ responsible for hearing. Additional studies suggest pesticides, like OPs, induce damage to the cochlea (which houses the organ of Corti) by generating reactive oxygen species (ROS) that contribute to oxidative stress. Although this study suggests pesticides and noise exposure on hearing is additive (two independent factors with the same effect), other studies find these factors to be synergistic, with more vigorous combined consequences, as the amount of time to induce hearing loss decreases upon exposure. With evidence suggesting chronic pesticide exposure and use causes a decline in the functionality of the five senses, advocates say that stricter policy regulations are vital to mitigate the adverse impacts not only on the senses both on the entire human body.

Hearing loss is an increasingly common health issue, plaguing people around the globe. Although genetics, other ailments, and prolonged noise exposure contribute to the decline in auditory function, pesticides, independently and in conjunction with other factors, exacerbate the decline in hearing function. Studies related to pesticides and auditory function can aid in future research to understand the underlying mechanisms that cause hearing loss, especially in essential occupations where hearing loss is more common (e.g., agriculture). With the Trump administration dismantling many environmental regulations, private sector understanding of how exposure to environmental pollutants, like pesticides, can increase the risk of developing chronic health issues, is critical to corporate action—especially if regulatory rollbacks increase the persistence of environmental pollutants.

Policies should enforce stricter pesticide regulations and increase research on the long-term impacts of pesticide exposure. Beyond Pesticides tracks the most recent studies related to pesticide exposure through our Pesticide Induced Diseases Database (PIDD). This database supports the clear need for strategic action to shift away from pesticide dependency. For more information on the multiple harms, pesticides can cause, see PIDD pages on sexual and reproductive dysfunctionendocrine disruption, cancer, and other diseases. Furthermore, buyinggrowing, and supporting organic can help eliminate the extensive use of pesticides in the environment. 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 is the right choice for both consumers and the farmworkers who grow our food, see Beyond Pesticides webpage, Health Benefits of Organic Agriculture

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

Source: Annals of Work Exposures and Health

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