Asthma is one of the most common diseases in the United States. In 2010, 8.2% (18.7 million) of adults aged 18 years and older had current asthma.¹ Asthma is associated with considerable morbidity, mortality, and cost.^1–3 In 2007, there were 7.2 million physician office visits, 1.1 million emergency department visits, 299,000 hospitalizations, and 3,262 deaths related to asthma in adults.⁴ Among currently employed adults who had at least one asthma attack in the past 12 months, asthma was associated with an average annual 14.2 million lost work days in 2008.⁴ In 2007, the total cost of asthma to society was estimated at $56 billion.³

Work-related asthma includes occupational asthma (new-onset asthma or the recurrence of previously quiescent asthma induced by exposures at work) and work-exacerbated asthma (preexisting or concurrent asthma worsened by factors in the workplace).⁵ Work-related asthma is associated with serious adverse economic and health consequences.^6–8 Adults with work-related asthma have higher unemployment rates than adults without asthma, higher mean number of days with asthma symptoms, trouble sleeping because of asthma, and activity limitation due to asthma.^7,9 Persons with work-related asthma may need to change or leave employment and thus lose income and benefits. In addition to socioeconomic consequences, work-related asthma has been associated with disability, poor health-related quality of life, and mortality.^8,10,11 A review of population-based studies estimates that 7% to 51% (median 17.6%) of adult-onset asthma is attributable to occupational exposures such as vapors, gas, dust, and fumes.¹² Work-related asthma is associated with over 400 causative agents.¹³

Work on a farm is associated with exposure to a variety of irritants and allergens that can cause or aggravate respiratory symptoms and diseases, including asthma. Organic and inorganic dusts, particulate matter, microbial agents, gasses, volatile organic compounds (VOCs), cleaning agents, fertilizers, and feed additives are frequently present in agricultural settings.¹⁴

In 2007, there were 2.2 million farms and 3.3 million farm operators in the United States.¹⁵ Historical data from the US Third National Health and Nutrition Examination Survey (NHANES III) 1988–1994 indicated that asthma prevalence for the farm-related occupations (i.e., farm operators, managers, and supervisors; and farm and nursery workers) was 3.6%.¹⁶ Based on data collected in 1993 from California farm operators, Schenker et al. documented that 7.8% of operators had current doctor-diagnosed asthma.¹⁷ Using data from the 2006 Farm and Ranch Safety Survey, we previously reported that 4.9% of primary farm operators have current asthma and that 24.8% of primary farm operators with current asthma had asthma that may be related to work on the farm.¹⁸

In 2006, the National Institute for Occupational Safety and Health (NIOSH) sponsored the Farm and Ranch Safety Survey to better understand the magnitude and scope of adult agricultural injuries. The survey was conducted as part of the NIOSH Agricultural Injury and Health Initiative and the NIOSH Childhood Agricultural Injury Prevention Initiative. It collected information on asthma, asthma attacks and attacks that occurred while doing farm work, and asthma-related unscheduled health care visits, including hospitalizations and emergency room visits. In 2011, the Farm and Ranch Safety Survey was repeated and included additional questions on asthma attacks on the farm and associated tasks and exposures.

The National Occupational Research Agenda (NORA) is a partnership program conducted by NIOSH to stimulate innovative research and improved workplace practices. The NORA plan for the US agriculture, forestry, and fishing industries focuses on surveillance; vulnerable populations; and outreach, communication, and partnerships.¹⁹ Of the nine strategic goals, the surveillance-related goal seeks to “describe: the nature, extent, and economic burden of occupational illnesses, injuries, and fatalities; occupational hazards; and worker populations at risk for adverse health outcomes.” To address this NORA goal, we estimate the prevalence of current asthma and the proportion of current asthma that is related to work on the farm among US primary farm operators using data from the 2011 Farm and Ranch Safety Survey.

The US Department of Agriculture National Agricultural Statistics Service (NASS) conducted a random telephone survey of active farm operations for NIOSH in 2011. Of the 25,000 farm operations selected for the survey, 7,497 (30.0%) could not be reached by telephone during the survey period and 5,103 (20.4%) refused to participate in the survey. After excluding 1,190 (4.8%) nonactive farms, the remaining 11,210 (44.8%) active farm operations were surveyed. Data were self-reported either by the primary farm operator or the operator’s spouse; no children were interviewed. Primary farm operator was defined as the individual who runs the farm, making day-to-day management decisions.²⁰ The adjusted survey response rate, excluding noncontacts, was 70.8%. The data are owned and retained by the US Department of Agriculture.

In 2011, of the 2.2 million primary farm operators, 94.7% were over 40 years old (mean age: 59.9 years), 83.7% were males, 83.5% were married or living with a partner, 60.3% were nonsmokers, and 51.7% did not have a second job. Operators were approximately equally divided between managing livestock (51.1%) and crop (48.9%) farms and were more likely to manage smaller farms with less than 101 acres (63.7%) and less than $10,000 in value of sales (55.0%) (Table 1).

An estimated 2.5% of primary farm operators had unknown or missing information on asthma. Compared with those who provided information, operators with missing data on asthma were slightly older (mean age: 66.0 years versus 59.8 years; P < .0001) and more likely to have a second job (72.2% versus 48.6%; P < .0001).

An estimated 5.1% of primary farm operators had current asthma. The mean age at current asthma diagnosis was 33.9 (range: 1–84) years. Current asthma prevalence was significantly higher among female than male operators (7.3% versus 4.6%; P = .0009) and among operators managing farms in the West than in the Midwest (6.6% versus 4.3%; P = .001) (Table 1).

Among primary farm operators with current asthma, an estimated 51.0% (95% confidence interval [CI]: 45.3%–56.7%) had an asthma attack in the prior 12 months. The proportion of operators having an asthma attack was higher among operators who were former smokers than current or nonsmokers, those who had a second job than those who did not have a second job, those who managed farms in the North than in other regions, those who managed farms with <101 acres than >101 acres, and those who managed farms with <$10,000 value of sales than farms with ≥$10,000 value of sales, but these differences were not statistically significant (Table 2).

Of operators with current asthma, 15.4% were ever told by a doctor, nurse, or other health professional that their asthma was related to their work on the farm (Table 2). The proportion of current asthma that may be farm work related was significantly higher among nonsmokers than former smokers, operators managing farms with ≥1,000 acres than farms <101 acres or 101–999 acres, and managing farms with an annual value of sales ≥$100,000 than farms with <$10,000 or $10,000–$99,999 value of sales (Table 2).

Multivariate associations between farm work-related asthma and second job, census region, farm acreage, value of sales, and farm type among primary farm operators with current asthma are shown in Table 2. After controlling for other variables, proportion of current asthma that may be farm work related remained associated with region, farm acreage, and value of sales.

The mean age of farm operators with farm work-related asthma was 59.7 (range: 34–84) years and their mean age at asthma diagnosis was 31.6 years. Most operators with farm work-related asthma were males (83.9%), married or living with a partner (87.2%), nonsmokers (77.5%), had a second job (51.2%), and operated livestock farms (52.5%). Significantly more operators with farm work-related asthma managed farms located in the South or West than in the North, farms with <101 or 101–999 acres than with ≥1,000 acres, and farms with <$10,000 or with $10,000–$99,999 in value of sales than with ≥$100,000 value of sales (Table 3).

Among operators with farm work-related asthma, 54.8% (95% CI: 41.4%–68.2%) had an asthma attack in the prior 12 months and 33.3% (95% CI: 21.2%–45.4%) had an asthma attack that occurred while doing farm work. Of operators who had an asthma attack that occurred while doing farm work, 65.0% reported that plant/tree materials triggered their asthma attacks. Farm work reported during these asthma attacks included baling hay or straw, handling grain or animal feed, bush/brush hogging, corn planting, or cutting grass/weeds. Other exposures were reported by a small number of operators (estimated proportions of attacks associated with these exposures were not reliable) and include miscellaneous chemicals, physical factors, animal material, and mold.

This cross-sectional study indicates that 5.1% of primary farm operators had current asthma. Similar asthma prevalence among farmers and rural populations has been reported from other studies.^24–27 Within these studies, prevalence of current asthma among farmers ranged from 2.7% to 7.7%. This finding is similar to that previously reported using the 2006 Farm Safety Survey data (4.9%; 95% CI: 4.4%–5.4%) and is lower than the estimated national prevalence of current asthma among adults employed at any time in the prior 12 months (7.2%).^18,28 Several factors have been proposed that may explain the lower prevalence of asthma in farmers and include the healthy worker effect among farmers (i.e., susceptible persons may leave dusty work areas early in the course of the job, leaving behind a relatively less susceptible group of workers who are potentially more able to tolerate the work environment)^17,29 and the “hygiene” hypothesis (i.e., early in life exposure to bacterial and viral agents through contact with other children or farm animals result in a reduced incidence of atopy and asthma).^30–36

In this study, over half of the primary farm operators with current asthma had an asthma attack requiring the use of an inhaler or other medical treatment in the last 12 months. These results are consistent with other studies reporting that 49.1% of adults with current asthma had an asthma attack in 2010.²

Of primary farm operators with current asthma, 15.4% had asthma that may be related to work on the farm. This proportion is greater than the proportion of current asthma that is work-related asthma among all ever-employed adults that was estimated using data from the Behavioral Risk Factor Surveillance System (BRFSS) Asthma Call-back Survey conducted in 38 states and the District of Columbia (9.7%) and lower than that previously reported using the 2006 Farm and Ranch Safety Survey data (24.8%).^7,18 Although some of the differences between the populations may be due to differences in survey methodologies, both BRFSS and Farm and Ranch Safety Survey used similar question on health care professional diagnosis of work-related asthma (BRFSS Asthma Call-back Survey questionnaires are available at http://www.cdc.gov/brfss/acbs/index.htm). Thus, it is unlikely that differences in methodologies can explain the substantially greater proportion of asthma that may be work related in farm operators. The differences can be explained, in part, by the presence of exposures (e.g., dusts, agricultural chemicals, bacteria, fungi, toxic gases) at substantially higher concentrations in agriculture than in other industries.^14,30 The reason for the lower proportion of current asthma that may be farm work related found in this study is unclear. Longitudinal follow-up studies of children of farmers or new farmers could confirm this change.

We found that female primary farm operators have a significant higher prevalence of current asthma than male farm operators. These findings are consistent with our previous report that an estimated 62.8% of ever-employed adults with asthma in 38 states and District of Columbia are females.¹¹ In addition, although we found a higher proportion of asthma that is farm related for male operators than for female operators (16.9% versus 10.4%), this difference was not statistically significant. This was consistent with a report by Torén and Blanc where proportion of asthma that could be attributed to occupational exposures was 9.1% for males and 11.5% for females.¹² Moreover, we found that among operators with farm work-related asthma 83.9% were males. This finding is slightly higher than previously reported results from California, Massachusetts, Michigan, and New Jersey work-related surveillance systems where 68.6% of individuals with work-related asthma in the agriculture, forestry, and fishing industry were males.³⁷ Other studies from work-related asthma surveillance systems reported that more of the work-related asthma patients are females.^38,39 However, in these reports no separate results for men and women by industry and occupation were presented.

The prevalence of current asthma among primary farm operators was not associated with the farm size or annual value of sales. However, the proportion of farm operators with current asthma that may be farm work related increased with increasing farm size and value of sales. In addition, more operators with asthma that may be farm work related operated farms located in the North. No information on possible factors (e.g., farm operator’s recognition of the asthma work-relatedness, opportunities to discuss asthma work-relatedness with health care providers, access and barriers to medical care, health insurance, income, presence of occupational health medical surveillance on the farm, number of hours spent doing farm activities) that might explain these differences was available. More research is needed to better understand the current findings that the proportion of current asthma that may be farm work related is greater among operators managing larger farms, farms with greater value of sales, and farms located in the North.

In this study, smoking prevalence among farm operators was 9.2% and was lower than the 2011 estimate for all US working adults (19%) and for all working adults in the agriculture, forestry, fishing, and hunting industry (18.5%).⁴⁰ Lower smoking prevalence among farmers and other agricultural industry workers has been previously reported.²⁶ It is likely that some operators use smokeless tobacco. In 2005, an estimated 8.8% of working adults in the agriculture, forestry, fishing, and hunting industry used smokeless tobacco.⁴¹ No data were available to assess smokeless tobacco use among farm operators in this study.

Early identification of persons with work-related asthma is essential for preventing worsening of symptoms and outcomes. Patients with sensitizer-induced work-related asthma may need to be removed from exposure in addition to other asthma management. For those with irritant-induced asthma management should aim at optimizing treatment and reducing the exposure to workplace triggers. Moreover, the diagnosis of work-related asthma should be considered an occupational sentinel health event and serve as a warning signal that other workers in the same workplace may also be exposed and that implementation of primary preventive measures may be warranted (eg, elimination, substitution, process modification, respirator use, and engineering control).⁵ If elimination of exposures is not feasible, other preventive measures play an important role in reducing exposures. Studies showed that providing education on occupational health and safety (e.g., safety training, familiarity with material safety data sheets, knowledge of legislation) and asthma causes and prevention (e.g., recognition of respiratory hazards, work/environmental hygiene practices, use of respirators) is feasible and effective, and has a positive effect on farmer health and farm practices.^42–46 In addition, in workplaces with high prevalence and incidence of work-related asthma, implementation of medical surveillance (i.e., secondary prevention) may assist in measuring the impact of primary prevention and, in work environments with potential exposure to sensitizers, in early detection of sensitized workers.^5,47

This cross-sectional study has some limitations. The Farm and Ranch Safety Survey was designed to collect data only on primary farm operators and no information on other farm workers was available. Diagnosis of asthma or asthma in relation to farm work was not validated; thus, estimates may be subject to misclassification. Also, information on race/ethnicity or family income was not available to assess their associations with asthma and asthma attacks. Moreover, the survey was designed to allow for both self- and proxy report. No information was available to assess the number of proxy responses and their potential bias. In addition, although the survey asked for information on asthma attack in the prior 12 months, some respondents may report events outside this time frame, thus overestimating our results.⁴⁸ Finally, because of small sample sizes resulting in unreliable estimates, we could not include marital status and smoking status in the logistic regression models, estimate the prevalence of current asthma by specific farm type, and examine associations between asthma attack and specific exposures on the farm.

This study documents the prevalence of current asthma and farm work-related asthma among US primary farm operators in 2011. The results suggest that the proportion of farm operators with current asthma that may have farm work-related asthma is higher than the proportion of US ever-employed adults with current asthma who have work-related asthma. In addition, the proportion of farm work-related asthma varies by farm size, value of sales, and region, indicating that certain groups of farm operators might experience greater benefits than others from work-related asthma prevention interventions. Future studies should examine operators’ asthma history and work-related factors to fully characterize the burden of work-related asthma among primary farm operators.