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* From the Department of Community Health and Epidemiology (Dr. Pahwa), and Institute of Agricultural Rural and Environmental Health (Drs. McDuffie and Dosman), University of Saskatchewan, Saskatoon, SK, Canada.
Correspondence to: Punam Pahwa, PhD, FCCP, Institute of Agricultural Rural and Environmental Health, University of Saskatchewan, Royal University Hospital, 103 Hospital Dr, Saskatoon, SK, S7N OW8, Canada.
Abstract
Objectives: To determine longitudinal changes in the prevalence of chronic respiratory symptoms among Canadian grain workers.
Design: Data on respiratory symptoms, smoking status, and pulmonary function were obtained approximately every 3 years (termed cycle) over 15 years beginning in 1978 from five regions of Canada.
Participants: The number of grain workers participating in each cycle were as follows: cycle 1 (n = 5,702); cycle 2 (n = 5,491); cycle 3 (n = 3,713); cycle 4 (n = 2,847); and cycle 5 (n = 3,079).
Results: A procedure based on generalized estimating equations (PROC GENMOD; SAS Institute; Cary, NC) was used to fit marginal models to determine risk factors influencing the prevalence of chronic respiratory symptoms (wheeze, dyspnea, sputum, and cough). The prevalence (predicted probability based on the final model) of chronic respiratory symptoms had an increasing trend with increasing number of years in the grain industry from cycle 1 to cycle 3 (before dust control) for all three smoking categories (current smokers, ex-smokers, and nonsmokers). For cycle 4 and cycle 5 (after dust control), there was a reduction in the prevalence of these respiratory symptoms. For example, in cycle 1, the prevalence of chronic wheeze among current smoking grain workers increased from 12% (for those in the industry for < 5 years) to 44% (for those in the industry for > 35 years); in cycle 5, the prevalence of chronic wheeze among current smoking grain workers increased from 9% (for those in the industry for < 5 years) to 28% (for those in the industry for > 35 years). Similar trends were observed for ex-smokers and nonsmokers and for other chronic respiratory symptoms.
Conclusions: Our results indicate that grain dust control was effective in reducing the prevalence of chronic respiratory symptoms among grain workers in all smoking and exposure categories.
Key Words: generalized estimating equations • grain dust • longitudinal studies • respiratory symptoms
It is well known that exposure to grain dust is a major contributor to the high prevalence of respiratory symptoms in farmers, grain workers, and feed mill workers. A high prevalence of work-related symptoms in relation to grain dust exposure has been reported by several researchers.12345 A number of cross-sectional and cohort studies conducted in different countries have shown the association between increased prevalence of chronic respiratory symptoms and long-term exposure to grain dust. Studies12345678910 based on Canadian and US cohorts have shown that grain workers are significantly at higher risk for respiratory symptoms and reduced lung function compared to unexposed workers. A few cross-sectional studies891011 have shown a higher prevalence of respiratory symptoms among grain handlers compared with workers not exposed to grain dust. Some of the studies1567121314 have investigated the contribution of smoking, ethnicity, layoff, duration of employment, and allergic status to the prevalence of respiratory symptoms among grain workers.
As a result of a symposium in 1976, recommendations were made and preventative measures were taken in Canada in the late 1970s until the mid-1980s to reduce the 8-h time-weighted average grain dust exposure in grain elevators to 10 mg/m3. In a report to the grain industry by Dosman et al15 and by Becklake et al,16 recommendations were made to reduce this dust further to 5 mg/m3.
The purpose of this article was to assess the effectiveness of the preventative measures taken in the grain industry in terms of the longitudinal changes in the prevalence of respiratory symptoms in a Canadian cohort of grain elevator workers with long-term exposure to grain dust in the workplace. The database used for the analysis was based on a Grain Dust Medical Surveillance Program.67 This program included a follow-up of pulmonary function measurements and respiratory symptoms among Canadian grain workers employed in primary and terminal grain elevators across Canada.
Materials and Methods
Details of the Grain Dust Medical Surveillance Program are given elsewhere.67 Briefly, eight provinces and territories participated in the environmental monitoring and medical surveillance programs. These were divided into five regions: Atlantic (East of Quebec); St. Lawrence (Quebec only); Great Lakes (Ontario; East of Thunder Bay); Central (Ontario; Thunder Bay and Westward; Manitoba; and Saskatchewan); and Mountain (Alberta; British Columbia; Yukon; and North West Territories). The data were collected in intervals termed cycles. The periods of cycles and the number of grain workers that participated in each cycle were as follows: October 1978 to September 1981, cycle 1 (n = 5,702); October 1981 to September 1984, cycle 2 (n = 5,491); October 1984 to September 1987, cycle 3 (n = 3,713); October 1987 to September 1990, cycle 4 (n = 2,847); and October 1990 to September 1993, cycle 5 (n = 3,079).
The Environmental Surveillance Program ran parallel to the Grain Dust Medical Surveillance Program. Dust concentrations in workplaces were monitored. The specifications of this program are given elsewhere.17
The number of observations contributed by Canadian grain workers stratified by region and cycle are given in Table 1 . Data were collected on the following: company; province; region; type of grain elevator; age; height; weight; smoking information; lung function measurements (FEV1, FVC, mid-maximum flow rate, and FEV1/FVC ratio); respiratory symptoms (chronic cough, chronic sputum, chronic wheeze, and chronic dyspnea); and physician. Chronic symptoms were defined17 as follows: chronic cough, with cough in the morning or during the day or night for > 3 months a year for 2 years; chronic sputum, with phlegm in the morning or during the days for > 3 months a year for 2 years; chronic wheeze, with chest wheezing or whistling most days or nights; and chronic dyspnea, with shortness of breath on effort on the level or walking up a slight hill.
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2 test was used for comparisons of the categorical variables. Marginal logistic regression models based on the generalized estimating equations approach1819 were fitted using a procedure (PROC GENMOD; SAS Institute; Cary, NC)20 to determine significant predictors and then predict the probability of respiratory symptoms developing over time.
The generalized estimating equations approach, which incorporates within-subject correlation, was used to fit the multivariable model in order to determine the significant predictors of the respiratory symptoms. Marginal models for each of the respiratory symptoms (chronic wheeze, chronic dyspnea, chronic cough, and chronic sputum) were used to assess the longitudinal changes in the prevalence of respiratory symptoms among the Canadian cohort of male grain workers. In these models, the covariates used were: baseline height, current weight, current exposure (years in industry), smoking status, region, cycle, and an interaction term between years in industry and smoking status. These covariates for the multivariable model were selected based on standard model building strategies.21
Goodness of Fit
Initially four multivariable marginal models were fitted using different within-subject covariance structures for each of the respiratory symptoms and the within-subject covariance structure that provided minimum SEs for most of the covariates was selected in order to fit the final model (shown in Appendix).22 The exchangeable within-subject covariance structure was selected for model building because it provided the minimum standard errors for most of the covariates. Significant predictors were determined by using the Wald statistic.22
Results
Tables 2, 3 display the demographic description of the study population. The means and SEs for the demographic variables (age, height, and weight) and years in the grain industry are presented in Table 2. Grain workers in the Central region were the youngest in all the cycles. Age increased approximately 3 years only for the St. Lawrence and the Mountain regions between cycle 3 and cycle 4 and for the Central region between cycle 2 and cycle 3. For other regions, between any two consecutive cycles, the mean age either increased or decreased by > 3 years, indicating that workers were entering and leaving the industry. St. Lawrence workers were shorter in height and lighter in weight compared to the grain workers in other regions. Table 3 shows the maximum proportion of current smokers (60.8% at cycle 1) were in the St. Lawrence region, which declined to 26% by cycle 5.
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10 mg/m3, and 60% of all dust level measurements were 5 mg/m3.15 Based on this information, the results in Table 5
are presented based on before and after dust control. The most common reported symptom was chronic cough (27.8%) among St. Lawrence grain workers followed by chronic sputum (20.9%) before dust control. The least reported chronic respiratory symptom was chronic dyspnea (9.9%) among grain workers in the Great Lakes region. After dust control, the most common reported symptom was chronic dyspnea (16.1%) among St. Lawrence grain workers followed by chronic sputum (15.0%) among grain workers in the Mountain region. After dust control, the least reported symptom was chronic wheeze (3.6%) among St. Lawrence grain workers.
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Our results show that there is a positive association between long-term exposure to grain dust and increased prevalence of respiratory symptoms. Our findings suggest a dose-response relationship between years worked in the grain industry and the prevalence of chronic respiratory symptoms. Based on the ongoing intermediate cross-sectional analysis of these data, actions were taken from the early 1970s to the mid-1980s to reduce the maximum allowable grain dust in the Canadian grain industries. Seventy percent of grain elevators had achieved dust control by 1986 (cycle 3 ended in 1987). Based on the longitudinal analysis of these data, we observed that implementation of dust control preventative measures in grain elevators was effective and contributed to the reduction of respiratory symptoms. The prevalence (predicted probability) of chronic respiratory symptoms increased for all three smoking categories with increasing number of years in the grain industry, but the increase was much lower after dust control measures were widely implemented. These prevalences were highest for current smokers. For chronic wheeze, chronic cough, and chronic sputum, these prevalences were very similar for ex-smokers and nonsmokers.
Several controlled epidemiologic studies have demonstrated that grain dust cannot be considered as a nuisance or inert dust5; grain dust has a range of biological activities mediated through different cell types.23 Pahwa et al7 evaluated the relationship between the long-term effects of grain dust and decline in the lung function measurements among grain elevator workers in Saskatchewan, Canada, studied over a 15-year period. The authors7 reported that the yearly loss in lung function test variables increased with increasing years in the industry among nonsmoking, ex-smoking, and smoking grain workers; and that the decline in lung function during the first three cycles was much faster compared to the decline in cycle 4 and cycle 5. However, in this article, longitudinal changes in the chronic respiratory symptoms were not reported.
A cross-sectional study8 was conducted in Nigeria to study the occupational health problems of various groups of workers. This study consisted of 91 flour millers, 30 matched internal control subjects from the maintenance unit of the same flour mill factory, and 121 matched external control subjects. Based on this study, it was reported that respiratory symptoms were more prevalent among wheat flour mill workers compared with control subjects. Their study concluded that flour millers in Nigeria, like many grain workers in other countries, were at increased risk for both pulmonary and nonpulmonary symptoms compared with control subjects.
Ige and Awoyemi24 conducted an age- and sex-matched case-control study to assess occupational induced lung impairment as a result of exposure to grain and flour dust. The most frequent respiratory symptoms reported by bakery workers were sneezing, runny nose (53.3%), and periodic breathlessness/chest tightness (23.2%), while 21.5% of workers reported symptoms of cough/phlegm. In our study, before dust control (cycle 1 through cycle 3), ranges for the prevalence of chronic cough and chronic sputum were as follows: 15.8% (reported by Great Lakes grain workers) to 31.1% (reported by Atlantic grain workers) and 14.0% (reported by Great Lakes grain workers) to 20.9% (reported by St. Lawrence grain workers), respectively. Before dust control, ranges for chronic wheeze and chronic dyspnea were 10.4 to 16.6% and 9.9 to 13.7%, respectively. After dust control, ranges for all the chronic respiratory symptoms were reduced: chronic cough, 8.4 to 13.0%; chronic wheeze, 3.6 to 9.4%; chronic dyspnea, 6.1 to 16.1%; and chronic sputum, 5.8 to 15.0%. The prevalences of all chronic respiratory symptoms reduced after dust control.
Based on a study of 373 workers exposed to flour and cereal dusts and 301 nonexposed subjects, Laraqui et al25 reported that the prevalence of clinical respiratory symptoms was 64.1% among exposed subjects and 41.2% among nonexposed subjects. Spirometry was abnormal in 31.6% of exposed workers, and the prevalence of positive skin test results to occupational allergens was higher among those exposed (42.4%) than among those not exposed (9.9%). These results suggests that there is need of implementation of adequate medical and technical prevention, which may help to reduce these higher pulmonary risks among exposed workers.
Pahwa et al6 used first-order autoregressive models to predict the annual loss of lung function among Canadian mail grain elevator workers who participated in the Grain Dust Medical Surveillance Program over a 6-year period involving three observations. Pahwa et al6 reported that exposure to grain dust resulted in declines in pulmonary function test values (FEV1 and FVC) among nonsmoking, ex-smoking, and current smoking grain workers.
A cross-sectional study26 was conducted in the rural areas of two counties in east-central Alberta, Canada, to investigate the relationship between lung health and dust exposure in farmers. This study reported that respiratory symptoms consistent with bronchial responsiveness were significantly positively associated with cumulative dust exposure. A significant positive association was also reported between a physician’s diagnosis of bronchitis and dust exposure. Their study also showed a significant negative association between FEV1 and the ratio of FEV1/FVC and cumulative dust exposure.
There is substantial evidence that workers handling grain acquire allergic respiratory symptoms.27 The mechanism by which these symptoms occur is not yet well understood, but microbiological contaminants are likely to be a significant contributing factor.27 It has been reported that there is a wide range and large numbers of fungi and bacteria to which workers handling grain in the United Kingdom are exposed.27
However, in developing countries, exposure to grain, cotton, tobacco, and tea dusts is still a serious problem. The World Health Organization28 reported that in developing countries, a large number of workers are employed in industries processing agricultural products, which has made the problem of exposure to vegetable dusts more serious in these countries. In these countries, it is important to implement surveillance programs for workers who are exposed to grain dust or other dusts to study the health of workers working n hazardous industries and investigate their respiratory health longitudinally as grain dust preventative measure are being implemented.
Our study showed that control and reduction of grain dust appears critical to preventing chronic respiratory symptoms among grain elevator workers. The model used in the Grain Dust Medical Surveillance Program to reduce/control grain dust, in which grain industry employers, unions, researchers, physicians, and government policy makers cooperated, appears to have worked effectively. We previously observed "positive" changes in the grain industry work environment: a decrease in dust concentrations, improvement in the efficiency of ventilation systems, reduction in respiratory symptoms, and reduction in the annual decline of lung function measurements among these workers.6715 In this analysis, we found that the prevalence of respiratory symptoms decreased after dust control measures were implemented.
Based on the Grain Dust Medical Surveillance Program, the maximum allowable grain dust has been reduced significantly in Canadian grain industries.15 Although the data were collected during from 1978 to 1993, the results of this report could lead to future research in developing countries on the subject of educational programs for employees in the grain industries or in other industries that pose occupational/environmental health hazards, enforcing mask requirements, and reducing the maximum allowable dust in grain industries.
Appendix
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The authors thank the grain handlers who participated in the surveillance program, Labor Canada, physicians, scientists, and a number of occupational health services who collected the information on this project across Canada according to the guidelines established by Labor Canada.
Footnotes
This study was supported by Labor Canada.
This work was performed at the Institute of Agricultural Rural and Environmental Health, University of Saskatchewan, Saskatoon, SK, Canada.
Received for publication October 19, 2005. Accepted for publication December 9, 2005.
References
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