toxics Article Insecticide Exposure and Risk of Asthmatic Symptoms: A Systematic Review and Meta-Analysis Jiraporn Chittrakul, Ratana Sapbamrer * and Wachiranun Sirikul   Citation: Chittrakul, J.; Sapbamrer, R.; Sirikul, W. Insecticide Exposure and Risk of Asthmatic Symptoms: A Systematic Review and Meta-Analysis. Toxics 2021, 9, 228. https://doi.org/10.3390/toxics9090228 Academic Editor: Koustav Ganguly Received: 3 September 2021 Accepted: 19 September 2021 Published: 21 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Community Medicine, Faculty of Medicine, Chiang Mai University, Chiang Mai 50200, Thailand; jerasooutch@gmail.com (J.C.); wachiranun.sir@gmail.com (W.S.) * Correspondence: lekratana56@yahoo.com; Tel.: +66-53-935-472 Abstract: Background: The incidence of respiratory disease is increasing. In relation to this, in addition to infection, factors associated with working with chemical insecticides are a cause for concern. Some of the chemicals involved have been shown to affect the respiratory system, and consequentially workers are at increased risk of conditions such as asthma. However, medical opinion around this area is still controversial; therefore, the objective of this study is to investigate the association between exposure to insecticides and asthma by means of a systematic review of the relevant literature. Methods: Relevant literature was identified, and a systematic review was conducted to investigate the association between exposure to insecticides and asthma. Results: A total of five studies (three cross sectional and two cohort) including 45,435 subjects were identified as relevant. The summary odds ratios related to the impact of exposure to specific insecticides on asthma were organophosphates 1.31 (95%CI = 1.17–1.48, I 2 = 27%, p = 0.172), carbamates 1.44 (95%CI 1.08–1.92, I 2 = 56.7%, p = 0.031) and organochlorines 1.31 (95%CI 1.19–1.64, I 2 = 37.3%, p = 0.131). Conclusion: Farmers exposed to certain insecticides may have an increased risk of asthma and asthmatic symptoms, but further research on that issue is urgently needed. Keywords: insecticide; asthmatic symptoms; farmer 1. Introduction Chronic respiratory disease and asthma are major public health issues that cause physical and psychological morbidity and mortality [1]. They also affect the medical and societal costs of patients. The World Health Organization (WHO) has stated that chronic respiratory disease causes the death of approximately >3 million people, and a further 235 million suffer from asthma [2]. There are several risk factors for asthma, including genetics, indoor allergens, respiratory viruses, air pollution, volatile organic compounds, and pesticides [1,3]. The WHO has identified pesticide poisoning as a major public health problem that causes significant morbidity and mortality worldwide [4]. Pesticides are a group of chemicals that are classified according to their target organisms, specifically insecticides, herbicides, and fungicides [5]. Insecticides, for example, are chemicals that kill and repel insects, and, classified by their chemical nature, the most common include: organophosphates, carbamates, organochlorine, pyrethroid, and neonicotinoids [6–8]. The most common chemicals in insecticides used in agriculture are organophosphates, car- bamates, and organochlorine, whereas pyrethroid is commonly used for household and public health purposes [9]. Exposure to insecticides might cause both acute and chronic adverse effects [10,11]. Long-term exposure to insecticides can have chronic effects includ- ing certain types of cancer, Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, diabetes, coronary heart disease, chronic kidney disease, and respiratory diseases [11–13]. Occupational exposure to pesticides is common for farmers, farm workers, and industrial workers involved in pesticide production [12,13]. They can be exposed to the pesticides via three common routes: through the skin (contact), the mouth (ingestion), and the lungs (inhalation) [14]. The inhalation of insecticides can cause serious damage to the nose, the Toxics 2021, 9, 228. https://doi.org/10.3390/toxics9090228 https://www.mdpi.com/journal/toxics