Pesticides and Asthma: Challenges for Epidemiology

DO PESTICIDES CAUSE ASTHMA? Pesticides are chemical or biological agents used to kill or incapacitate unwanted organisms both indoors and outdoors. In and around the home, these agents are commonly and increasingly used to control rats, ants, cockroaches, flies, moths, mites, weeds, and molds. Pesticides are usually grouped into families not only depending on the organism they target (e.g., fungicides for fungi, insecticides for insects) but also according to their chemical characteristics (e.g., organophosphates, carbamates, pyrethroids). Asthma is a common chronic inflammatory disease of the airways characterized by variable and recurring respiratory symptoms (wheezing, breathlessness, chest tightness, and dry cough), airflow obstruction, and increased bronchial responsiveness (1, 2). However, it is increasingly recognized that this is not a homogeneous disease since multiple subtypes (e.g., early-onset allergic, late-onset eosinophilic, and exercise-induced asthma) have been described (3). Some of the factors that have been linked to asthma include allergies, tobacco smoking, air pollution, viral and bacterial infections, changes in sex hormone levels, obesity, and occupational exposures (2–5). Variants in several genes (e.g., CHI3L1, IL6R, DENND1B, IL1RL1– IL18R1, PDE4D, RAD50–IL13, HLA-DQ, IL33, SMAD3, ORMDL3–GSDMB, IL2RB, RORA, TSLP, and PYHIN1) have also been associated with increased susceptibility to this disease (2). Asthma affects more than 300 million people worldwide (6), and in the last decades, its prevalence has been increasing especially in low and middle income countries (7–9). Over the same period, the use of multiple chemicals such as pesticides for crop protection and home disinfestation has also increased (10–12). Associations between pesticide exposure and asthma in children, adults, and occupational groups have been reported, but these have not yet been shown to be causal and the biological explanation tying the two remains unclear – it could lie in the mechanisms such as irritation (13), inflammation (14), immunosuppression (15, 16), endocrine disruption (17), or a combination of these. So far, it is also unknown how pesticides interact with genes that increase susceptibility to asthma.