The influence of occupational exposures on respiratory health with a specific focus on agriculture in low- and middle-income countries

Abstract

Chronic respiratory disease, particularly chronic obstructive pulmonary disease (COPD) is reported to be a major cause of morbidity and mortality worldwide. While smoking is the main cause of the disease, previous studies have shown associations between COPD and socioeconomic status and occupations. Many of the industries historically associated with occupational respiratory diseases have been transferred to low- and middle-income countries (LMICs) where workers’ health protection is generally limited. As farming is a common economic activity in LMICs and has been associated in previous epidemiological studies with adverse respiratory health effects, I hypothesise that this occupation, and the underlying exposures in particular pesticide exposure, could be an important cause of chronic lung disease in these countries. This doctoral thesis aims to examine the relationships between chronic respiratory disease (lung function and chronic respiratory symptoms) and occupational exposure specific to agricultural contexts especially in LMICs. It comprises: first, a systematic review and meta-analysis of pesticide use and lung function; second, a cross-sectional study in Nan province, Thailand examining the relationship between agricultural exposures and respiratory health; and third, a data analysis of the large multinational Burden of Obstructive Lung Disease (BOLD) study focusing on high risk occupational exposures, primarily farming, and respiratory outcomes. The first study is a systematic review and meta-analysis exploring the available literature on the relationship between occupational exposure to pesticides and lung function. Of the 2,356 articles I retrieved, 56 were included in the systematic review and were pooled in meta-analyses of the ratio of forced expiratory volume in the first second to forced vital capacity (FEV1/FVC), FVC and FEV1. There was tentative evidence that exposure to cholinesterase (ChE) inhibiting pesticides reduced FEV1/FVC but no evidence that paraquat exposure affected lung function in farmers. The second study is a cross-sectional survey of adults aged 40 to 65 that I conducted in the agricultural Nan province, Thailand. The aim of this survey was to improve knowledge on the relationship of chronic airflow obstruction and respiratory health problems with several exposures related to farming activities, particularly pesticide use. I found that chronic airflow obstruction was uncommon in the studied villages; as with other studies in developing countries, farming villagers had a low smoking prevalence. Nan farmers had a high percentage of pesticide applicators but farming activities and pesticide use (duration, intensity and lifetime cumulative hours of spraying classified by pesticide types) were unlikely to be a major cause of respiratory problems there. In the final study, I analysed data from the multinational population-based BOLD study of 28,823 adults aged ≥40 years. The aim of this analysis was to examine the relationship between occupational exposures and respiratory health in both high-income countries (HICs) and LMICs. I found that people working in any of three occupational exposure categories (organic dusts; inorganic dusts; and fumes) and 11 high-risk occupations (farming; flour, feed or grain milling; cotton or jute processing; hard-rock mining; coal mining; sandblasting; working with asbestos; chemical or plastics manufacturing; foundry or steel milling; welding; and firefighting) were more likely to report respiratory symptoms than those who did not work in any of those occupations. There were no consistent associations of FEV1/FVC or FVC with any high-risk occupations, particularly farming. The associations between occupational categories and lung function varied by gross national income.