Approximately 194 million people live in rural areas throughout the United States, Canada, and European Union, however there is paucity of exposure assessment data on these individuals.¹ Occupational studies have shown that agricultural workers are regularly exposed to large concentrations of particulate matter (PM) and endotoxin while performing common tasks, such as crop harvesting, grain processing, and livestock production.^2-8 However, the effect of agricultural activities on rural air quality has not been well characterized and population-based exposure information is needed.

PM suspended in the ambient air is a heterogeneous mixture of inorganic and organic substances, the composition of which can vary depending on the source, season, and meteorological conditions.⁹ Health effects from PM are determined by both the pathogenic effect of the substance and the area in which it deposits in the lung.¹⁰ Epidemiological studies have demonstrated a clear association between exposure to PM and a number of adverse health effects including respiratory, cardiac, and all-cause mortality.^11-18 As part of the Clean Air Act, the United States Environmental Protection Agency (EPA) has promulgated air quality standards for two size fractions of particulate, PM₁₀ (aerodynamic diameter ≤ 10 μm) and PM_2.5 (aerodynamic diameter ≤ 2.5 μm).¹⁶ Fine particulate (PM_2.5), produced through combustion processes, is more efficiently inhaled than larger coarse particles (aerodynamic diameter >2.5 μm and ≤ 10 μm) and can potentially deposit deeper in the lungs.¹⁷ Therefore, ambient exposure to PM_2.5 may have a larger impact on human health than PM₁₀.^15,17

The vast majority of air quality studies have focused on urban areas, which compared to rural, may vary considerably in terms of composition of PM.⁹ Agricultural air has a larger fraction of organic dust which is a mixture of plant and animal matter, microorganisms, and bio-aerosols.¹⁹ Exposure to organic dust can cause a variety of acute or chronic conditions that are separate and distinct from health effects associated with urban PM exposure. Occupational workers exposed to large concentrations of organic dust can develop organic toxic dust syndrome, which is characterized by fever, chills, malaise, and dyspnea.^20,21 Long-term exposures can cause decreased lung function as well as chronic bronchitis, asthma-like syndrome, and wheezing.^7,22,23 Adverse health effects have also been linked to populations environmentally exposed. During 1985-1986, a series of asthma epidemics was found to be caused by environmental exposure to soybean dust in Barcelona, Spain.²⁴ Schwartz (1999) concluded that environmental exposure to organic dust among rural populations is one of the most important exposures in the progression of childhood asthma.²⁵

The concentration of endotoxins in the inhaled organic dust fraction appears to be an important factor in the progression and development of respiratory diseases.^26,27 Endotoxins are made up of lipids, proteins, and lipopolysaccharides and are capable of remaining airborne for long periods of time due to their small size.²⁸ However, endotoxins are often attached onto PM, and consequently the majority of endotoxin is found in the coarse fraction as opposed to the fine fraction of particulate samples. ^29-31 Sources of endotoxins in rural environments include animal confinements, grain storage facilities, and row crop harvesting.^{2,7,26,27,32,33}

Indoor and outdoor PM₁₀, PM_2.5, and endotoxin samples were collected from 197 rural households over five continuous days from 2007-2011 in order to characterize exposure to participants in a prospective population-based health study. The goals of this study were to quantify airborne concentrations of PM₁₀, PM_2.5, and endotoxin in an intensely agricultural area and compare findings to reported concentrations from urban areas; identify factors contributing to rural PM and endotoxin concentrations in both ambient and indoor air in homes; and evaluate the effect of seasonal variation on PM and endotoxin levels.

General characteristics of the 197 homes surveyed in the study are shown in Table 1. The majority of were single-family homes (89%), located outside of designated towns (71%), built prior to 1950 (52%), and on gravel roads (55%). Participants typically heated their homes with natural gas or propane (76%), used an electric stove (68%) for cooking, and did not allow smoking inside the home (91%).

Summary results for indoor and outdoor PM and endotoxin data are shown in Table 2. The range of indoor concentrations of PM spanned two orders of magnitude (PM₁₀: 4.1 to 173.3 μg m⁻³; PM_2.5: 1.4 to 187.7 μg m⁻³), while outdoor PM levels were less varied and spanned only a single order of magnitude (PM₁₀: 6.2 to 56.2 μg m⁻³; PM_2.5: 1.5 to 24.1 μg m⁻³). Geometric mean (GM) indoor PM₁₀ (21.2 μg m⁻³) and PM_2.5 (12.2 μg m⁻³) concentrations tended to be larger than outdoor PM₁₀ (19.6 μg m⁻³) and PM_2.5 (8.2 μg m⁻³) concentrations (PM₁₀ p= 0.086; PM_2.5 p <0.001). Conversely, GM outdoor endotoxin concentrations (1.93 EU m⁻³) were significantly larger than indoor (0.32 EU m⁻³) (p<0.001).

A subset of homes (n=15) were re-sampled for indoor and outdoor PM₁₀ and PM_2.5 (Table 3). Due to flow rate and sampling time restrictions, only indoor PM₁₀ measurements contained all fifteen matched samples. Since the number of re-sampled homes was small, Wilcoxon signed-rank tests were used to evaluate pairwise differences in re-sampled homes. Results showed no significant difference in PM concentrations in re-sampled homes between sample periods; however, a lack of power may be responsible for the null finding.

Bivariate analysis was conducted to determine significant differences in ambient concentrations of PM and endotoxin by season (Table 4). No seasonal trend was observed in ambient PM₁₀ concentrations. A seasonal trend was found in the outdoor endotoxin measurements, with autumn (2.63 EU m⁻³) having approximately seven-times larger endotoxin concentrations compared to winter (0.39 EU m⁻³). A seasonal trend was also detected in ambient PM_2.5 levels. Compared to other seasons, winter (10.6 μg m⁻³) had significantly larger concentrations of PM_2.5, while autumn had the smallest (6.8 μg m⁻³).

In mixed regression analysis (Table 5) the majority of agricultural and property variables were not found to be significantly associated with outdoor PM and endotoxin levels. One variable that was found to be associated with outdoor PM₁₀ levels was home location (town vs. agricultural). After adjusting for significant covariates, residents living in agricultural areas had significantly larger PM₁₀ concentrations (20.8 μg m⁻³) than residents living in designated towns (17.6 μg m⁻³). Interestingly, when controlling for home location, no significant increase in PM₁₀ concentrations was found between homes situated on a paved roads compared to a gravel roads (p= 0.297). Additionally, no significant association was observed between ambient endotoxin concentrations and presence of livestock, swine confinements, and/or grain bins on the property. However, unmeasured variables such as distance and direction were not taken into consideration in the model and the magnitude of the association may have been attenuated.

Multiple linear regression analysis of the indoor sample results is presented in Table 6. Smoking, outdoor PM concentrations, and indoor relative humidity were all significantly (p<0.05) associated with indoor PM concentrations. When controlling for seasonality, indoor fine particulate concentrations were significantly larger in homes using a gas furnace and without central air conditioning. However, these factors did not affect indoor PM₁₀ or endotoxins levels. One of the major predictors of indoor PM₁₀ and endotoxin levels inside the home was cleanliness. Compared to a residence that scored high on the scale, a home that rated low had a mean increase of 7.8 μg m⁻³ of PM₁₀ and 0.12 EU m⁻³ of endotoxin. A positive association (p=0.006) was also observed between indoor endotoxin levels and having a grain storage bin on the property. Adjusting for significant co-variates, homes with a grain bin on the property had a mean increase of 0.08 EU m⁻³. Smoking was negatively associated with indoor endotoxin concentrations; however only 7% of homes sampled for endotoxin reported smoking in the home.

Few published studies have characterized rural populations’ air pollution exposure. Therefore, we were interested in comparing indoor and outdoor PM and endotoxin concentrations from an intensely agricultural area to measurements taken in urban centers. Mean concentrations of ambient PM₁₀ and PM_2.5 observed in Keokuk County were approximately 35% smaller than levels recorded across 15 metropolitan sites in the US from 2005-2007.¹⁶ Indoor PM₁₀ and PM_2.5 levels also tended to be smaller than levels found in previous North American urban studies.^40-43 Smoking prevalence among agricultural populations is generally smaller than urban and this may partially account for the decreased levels of indoor PM observed in this study.⁴⁴

In contrast to PM measurements, ambient endotoxin levels measured in the study area were larger than studies conducted in non-rural settings using a similar size selective sampler. Geometric mean endotoxin concentrations in Keokuk County (1.19 EU m⁻³) were approximately three-times larger than ambient levels found in Southern California (0.44 EU m⁻³), while endotoxin levels were an order of magnitude larger than measurements recorded in the urban areas of Germany and Sweden (0.05 EU m⁻³).^29,31,45 Although outdoor endotoxin concentrations were larger in the study area compared to urban areas, indoor levels (0.21 EU m⁻³) were on the same order of magnitude as concentrations found in Baltimore (0.13 EU m⁻³; PM₁₀ sample)⁴⁶, Paris (0.512 EU m⁻³ and 0.553 EU m⁻³; total dust sample)⁴⁷, and Boston (0.77 EU m⁻³; total dust sample)⁴⁸. Geometric mean endotoxin concentrations in Keokuk County were similar to levels found in rural-Canada, which sampled 146 homes over five days during the winter (indoor=0.14 EU m⁻³ vs. outdoor=0.12 EU m⁻³) and summer (indoor=0.47 EU m⁻³ vs. outdoor=1.57 EU m⁻³) of 2007 using a coarse PM sampler.⁴⁹ However, maximum five-day concentrations observed in this study were larger than levels found in Canada, with outdoor levels in Keokuk County reaching 13.00 EU m⁻³compared to 6.41 EU m⁻³.⁴⁹

We expected airborne PM₁₀ levels to be significantly larger during the autumn, when row-crop harvesting generates large amounts of airborne dust. Results show that PM₁₀ concentrations outside the home were not significantly increased and five-day mean concentrations were comparable to other seasons. Although mean levels were not significantly affected, autumn had the largest range (6.2-56.2 μg m⁻³) and GSD (1.84) of any season. This large variation was also reflected when PM₁₀ measurements were stratified by quartiles. Only 26% of the PM₁₀ measurements were recorded in the autumn, yet 38% of measurements were in the upper quartile, while 40% of measurements were in the lower quartile. This finding indicates that during certain times in autumn, ambient levels of PM₁₀ can be elevated, but quickly return to background levels, usually within a week. Future rural air quality studies may benefit from a shorter sampling period and identification of local agricultural activities in order to achieve better temporal resolution to determine peak exposures during harvest season.

Unlike PM concentrations, ambient endotoxin concentrations were significantly larger during autumn, a finding that is unique to this study. Two previous urban air studies found no significant increase in endotoxin concentrations during this season.^45,50 A study conducted outside Munich, Germany observed a strong positive correlation between ambient temperature and increased endotoxin levels, with peak concentrations occurring during June and July, while mean concentrations in the autumn were comparable to levels found in the winter time.⁵⁰ Additionally, a 2004 study conducted in Southern California found no seasonal pattern in endotoxin concentrations. ⁴⁵ Although more data are needed to assign causality, harvesting appears to be responsible for this seasonal trend, since urban studies did not find elevated concentrations of endotoxin during the autumn.

A major goal of this study was to determine if agricultural variables were predictive of indoor and outdoor PM₁₀, PM_2.5, and endotoxin concentrations. One of the most significant factors influencing airborne PM₁₀ and endotoxin levels inside homes was the qualitative assessment of home cleanliness. This is consistent with previous studies which have found home cleanliness, assessed either through questionnaire data or interviewer rated, was associated with decreased levels of endotoxin in settled dust and airborne samples.^46,47,51,52 Adjusting for significant covariates, homes that scored in the lowest of the three categories for home cleanliness had an average increase of 7.8 μg m⁻³ of PM₁₀ and 0.12 EU m⁻³ of endotoxin compared to homes rated cleanest. This finding has potential implication for children’s health. An epidemiological study of asthmatic children in inner-city Baltimore, found a significant increase in the incidence of cough, wheezing, and chest tightness for every 10 μg m⁻³ of PM_2.5-10.⁵³ While a study using total dust samplers, conducted in Prince Edward, Canada, found an increase of 0.49 EU m⁻³ was significantly associated with larger incidences of respiratory illnesses in children below the age of two years.⁵⁴ Compared to total dust samplers, PM₁₀ samplers may underestimate endotoxin concentrations and consequently health effects may be detected at lower concentrations. Although home cleanliness was found to be a significant predictor of PM₁₀ and endotoxin it only explained 4% and 10% of the variability in indoor measurements, respectively. Consequently, visual inspection alone would not serve as a surrogate for quantitative exposure measurements.

Another factor which was shown to significantly increase indoor endotoxin levels was the presence of grain storage bins on the property. Since outdoor levels were unaffected, grain bins may be a source of take-home exposure. Multiple agricultural studies have shown increased levels of pesticides inside rural households from take-home sources.^55-57 Recently, a study from the United Kingdom found larger levels of flour dust, an allergic sensitizer associated with occupational asthma, inside bakers’ homes compared to non-bakers.⁵⁸ In the present study it is not clear whether larger endotoxin levels are associated with grain bins themselves or whether the bins are a proxy for unmeasured agricultural variables. Although more work is needed to determine the source, greater education among farmers about improved hygiene practices may decrease indoor endotoxin levels.

Gravel roads are often a source of nuisance dust in rural areas and can negatively impact EPA PM₁₀ attainment status.⁵⁹ In multivariate modeling no significant increase in ambient PM₁₀ was observed in samples collected outside homes located on unpaved roads. The lack of a significant increase was likely due to low vehicle traffic in the county (less than 100 vehicles per day)⁶⁰ and five-day averaging time. Findings from this study suggest that paving rural roads in low-vehicle traffic areas would do little to reduce ambient PM₁₀ exposure near homes and would not be beneficial given the increased costs of maintenance and construction.

This study had several limitations including non-specific survey questions to categorize exposure variables, potential underestimation of outdoor endotoxin levels, possible lack of generalizability due to the recruitment strategy of households, and small sample size for certain household characteristics. First, the lack of specificity in the environmental questionnaire may have caused possible misclassification of residential and agricultural variables. For example, regarding smoking status inside the home, participants were asked if household members or guests ever smoke in the residence. However, it was not known whether individuals smoked during the time of the sample collection. Consequently, estimation of the effect of predictors on concentrations of particulate and endotoxin may have been attenuated due to misclassification. Second, only the coarse fraction of the outdoor particulate sample was analyzed for endotoxin. As a result, this may have underestimated rural populations’ exposure to airborne endotoxin. Third, households were recruited into the study through non-random sampling. This may limit the generalizabilty of this study if fundamental differences exist between homes selected for assessment and the underlying eligible population. Also, we could not account for changes in ambient PM and endotoxin concentrations by different years, since homes were not sampled in all season every year. Finally, smoking and the use of biomass for residential heating has been associated with increased indoor endotoxin levels in previous studies.^61-63 However, due to the small number of participants who smoked or burned biomass, we were unable to achieve enough power or large enough sample size to generalize results found in this study to the larger rural-population.

Results from this study show ambient endotoxin concentrations in an agricultural county in the Midwest US were elevated compared to those previously reported in urban areas; however, indoor and outdoor PM₁₀ and PM_2.5 concentrations were smaller. Contrary to our initial hypothesis, there was no significant increase in five-day averaged outdoor PM₁₀ during the harvest season. Conversely, concentrations of ambient endotoxin were significantly increased, a finding that seems unique to rural areas. In general, agricultural and housing variables were found to be poorly associated with indoor and outdoor PM₁₀, PM_2.5, and endotoxin concentrations. One variable that was found to be highly associated with indoor PM₁₀ and endotoxin was our qualitative assessment of home cleanliness. Compared to a residence that scored high on the scale, a home that rated low had a mean increase of 7.8 μg m⁻³ of PM₁₀ and 0.12 EU m⁻³ of endotoxin. This study demonstrated that a complete evaluation of exposures to PM_2.5 and endotoxin among residents of agricultural communities of the Midwest United States should incorporate both indoor and outdoor measurements.