Common names for disease: Cotton dust—byssinosis and brown lung. Additionally, an active febrile response to endotoxin has been given many names depending on the source of exposure such as mattress maker’s fever, mill fever, and card room fever.

OCCUPATIONAL SETTING

Agricultural workers and processors of vegetable fibers are most likely to be at risk. Workplaces with potentially high airborne concentrations of endotoxins include cotton and flax mills, grain storage and handling operations, poultry houses and processing plants, saw and paper mills, sewage treatment plants, and swine confinement buildings. Workers may also be exposed during animal handling in various facilities or during composting operations. Contamination of cutting fluids used in machining operations, workplace humidifying systems, or biotechnology processes using Gram-negative bacteria can also result in significant exposure.

EXPOSURE (ROUTE)

Occupational exposures are predominantly by inhalation. Significant exposure can occur wherever aerosols of materials contaminated with Gram-negative bacteria are generated, including office buildings and residential structures. There is no evidence to support a dermal route of exposure for endotoxins.

PATHOBIOLOGY

Endotoxin refers to the lipopolysaccharide (LPS) complex and associated proteins found in the outer layer of the cell wall of Gram-negative bacteria. Although the lipid component (lipid A) is responsible for most of the toxic effects, variability of the biological responses to endotoxin exposures in natural settings may be due in part to influences from associated cell wall components, which differ among Gram-negative bacterial species.

Most clinical attention has focused on the toxicity of endotoxins reaching the bloodstream from endogenous Gram-negative bacterial infections of the host or via contaminated parenteral products.^1,2 However, little endotoxin is detectable in the blood after inhalational exposure. Local uptake in the respiratory system appears to account for the manifestations observed from exposures in the workplace.

ACUTE EXPOSURES

An acute febrile response may follow inhalation of aerosols containing high concentrations of endotoxin.³ This is especially true for individuals with no prior occupational exposure or after a hiatus in exposure for workers with chronic low-level exposures. Historically, process or material-specific descriptors such as mattress maker’s fever, card room fever, mill fever, and grain fever were used to describe acute responses in environments with endotoxin. Monday fever is named for its occurrence on the first workday after a weekend break. In 1986, the generic term organic toxic dust syndrome (OTDS) was proposed to emphasize the commonality of this response to many agents and the nonimmunologic nature of its pathobiology.⁴

The fever usually begins several hours after exposure. Other symptoms include chills, myalgias, malaise, anorexia, headache, cough, and chest tightness. The condition is self-limited and usually lasts only a day or two, but it may be more prolonged if the exposure is massive. Tolerance develops with repeated exposures, and symptoms diminish despite similar doses of endotoxin.

A syndrome commonly described in association with exposure to cotton dust mimics many of the symptoms associated with acute inhalation exposure to endotoxins. Symptoms associated with exposure to cotton dust consist of recurrent chest symptoms that begin several hours into the workshift, often on the first workday after a break. Chest tightness with or without cough or shortness of breath develops gradually and may persist for several hours after the exposure ceases. Symptoms generally remit by the following day and do not recur or are markedly diminished on subsequent days, despite continued exposure. Some, but not all, workers with these symptoms also show a significant decline in ventilatory function over the workshift, with a decrease in the forced expiratory volume in the first second (FEV₁) on spirometry. This value generally returns to pre-exposure levels by the following day. The term “byssinosis” has been used when this syndrome is associated with exposure to cotton dust.

The exact mechanism for the acute symptoms and ventilatory changes has not been established. Animal and human exposure studies have shown that inhaled endotoxins interact with alveolar macrophages, initiating a cascade of events that result in the expression and release of inflammatory mediators such as IL-1 and TNF-alpha.^5,6 Thus, endotoxin inhalation can produce airway inflammation and bronchoconstriction, which could account for the symptoms associated with cotton dust.

Although not extensively studied, inhalation of organic dusts or purified endotoxin has been shown to transiently increase nonspecific airway reactivity, as measured by methacholine or histamine bronchoprovocation. Both healthy people and persons with mild atopy or frank asthma have shown bronchial hyperresponsiveness lasting from a few hours to 24 hours after exposure. The clinical significance of these findings is uncertain. Acute airway inflammation may be a contributing factor to these changes.⁷

Some epidemiologic studies of textile workers have found better correlation between chest symptoms and estimated doses of inhaled endotoxin than with crude measures of dust exposure. Controlled exposures of subjects preselected for acute ventilatory responses to cotton dust showed a strong correlation between drop in FEV₁ during exposure and airborne endotoxin concentration, but no association with respirable dust concentration uncorrected for endotoxin content.⁸ These findings suggest that the endotoxin content of cotton dusts and other organic dust aerosols is likely to be an important factor in producing the acute recurrent chest symptoms in exposed persons. This also suggests that measures of airborne endotoxin may be a better predictor of the acute responsiveness to aerosols of cotton dust than gravimetric dust measures. However, other studies have shown biological activity of endotoxin-free extracts of organic dusts. Therefore, the mechanism for responses to natural organic dust exposures may be complex and dependent on interactions of multiple agents.

CHRONIC EXPOSURE

Numerous epidemiologic studies have shown that workers exposed to cotton dust are at increased risk for symptoms of chronic bronchitis and for airway obstruction detectable by spirometry. Retrospective autopsy studies have confirmed the increased prevalence of pathologic changes of chronic bronchitis, even in nonsmoking textile workers. Prospective studies have now confirmed that decrements in ventilatory function in textile workers that exceed the expected rate are correlated with dust exposure. Cigarette smoking is clearly an important factor; however, accelerated loss in FEV₁ also occurs in nonsmokers. Although this condition has been referred to as byssinosis, or brown lung, when associated with cotton dust exposure, workers exposed to other organic dusts are also at risk. A recent review concluded that chronic exposure to organic dust in the textile industry has characteristics similar to both asthma and COPD and that cessation of exposure may lead to improved respiratory function.⁹

The role of endotoxin in the pathogenesis of this chronic condition is not clear.¹⁰ In animals, the inflammatory response seen acutely after endotoxin inhalation appears to diminish with repeated exposures. Also, chronic high-level exposures to organic dusts containing endotoxin do not consistently reproduce the pathologic changes of chronic bronchitis in animal models. Tolerance to the effects of endotoxin given intravenously is well documented in humans as well as animals. In healthy young volunteers, the Th1 type of lymphocyte cytokine response is decreased after intravenous LPS challenge.¹¹ A recent study analyzed the inflammatory response associated with persistent airflow obstruction resulting from chronic exposure to low doses of endotoxin that would be similar to exposure that might occur in occupational environments. After 8 weeks of chronic low-dose exposure, there was an increase in airway hyperresponsiveness and an increase in lung neutrophils was observed that correlated with an increase in proinflammatory cytokines. Evaluation of inflammatory cell subsets showed an increase in proinflammatory dendritic cells (DCs) with a reduced percentage of macrophages. Gene expression profiling showed up-regulation of genes consistent with DC recruitment and lung histology revealed an accumulation of inflammatory aggregates of DCs around the airways. Thus, repeated, low-dose endotoxin resulted in airway hyperresponsiveness, associated with a failure to resolve the proinflammatory response, an inverted macrophage to dendritic cell ratio, and an increase in the inflammatory dendritic cell population. These results demonstrate a possible underlying mechanism of airway obstruction for chronic endotoxin exposure that is consistent with symptoms observed for cotton and other organic dust.¹²

Respiratory effects similar to those seen in textile workers have also been reported in workers exposed to non-cotton organic dust environments with endotoxin. Poultry house workers had drops in FEV₁ over the workshift that correlated with endotoxin exposure.¹³ However, the correlation of lung function decrement with total dust exposure was stronger than that with the endotoxin component. Sewage treatment plant workers had increased nonspecific airway responsiveness measured by methacholine challenge compared to controls, but the difference was small and no difference in lung function was found.¹⁴

DIAGNOSIS

The nonspecific nature of these syndromes makes definitive diagnosis difficult. A careful history is critical to establish a pattern consistent with an association between symptoms and the putative causal exposure. For clinical evaluations, self-measurement and recording of peak flows using portable devices several times per day for 2 weeks may be helpful. Consistent acute worsening of ventilatory function during exposure or a progressive decline during the workweek with improvement on weekends suggests an association with exposure. However, lack of significant findings does not exclude an association between the symptoms and the exposure. Auscultation of the chest will generally not reveal wheezing or rales unless another underlying condition is present, which accounts for the finding. Persons with underlying hyperreactive airways may develop symptomatic bronchospasm with wheezing in response to organic aerosol exposures without detectable sensitization to any components of the aerosol.

No specific diagnostic tests are available for the syndrome of recurrent chest symptoms. No evidence for a direct immunologic mechanism has been found in textile workers with either underlying recurrent chest symptoms (byssinosis) or acute declines in FEV₁ over the workshift. RAST for specific IgE is therefore of no value. Chest radiographs will be normal unless there is other pulmonary disease. Clinical judgment is, of course, necessary to determine if other causes for chest symptoms, such as coronary artery disease, should be pursued. Longer term trends in ventilatory function may show losses in FEV₁ greater than expected from aging alone. Normal variability and the influence of non-work-related exposures, including cigarette smoking, must be considered when interpreting these findings. However, an accelerated decline in function associated with recurrent work-related chest symptoms or acute decrements in function during exposure merits attention and consideration of reduction in exposure.

Demonstration of environmental airborne concentrations of endotoxin of the magnitude that has been associated with these findings in other settings would support a causal relationship. Threshold concentrations of airborne endotoxin for the syndromes described have been postulated for cotton dust environments.³ They vary from 0.5–1 milligram per cubic meter for the febrile reaction to 0.1–0.5 milligram per cubic meter for chest tightness and acute declines in ventilatory function in chronically exposed individuals. Previously nonexposed persons may require higher concentrations (2–3 milligram per cubic meter) to develop acute symptoms of chest tightness. Endotoxin concentrations associated with increased airway reactivity and bronchitis are postulated to be lower, but they are more speculative, given the interaction of the many potential agents to which a worker may be exposed over the course of many years.

Established chronic bronchitis and airway obstruction associated with organic dust exposure has no inherent pathophysiologic features that allow differentiation from that caused by cigarette smoking. A history of prior recurrent chest symptoms or acute ventilatory declines associated with the exposure is helpful, but its absence does not eliminate the possibility that exposure is a causal factor. The magnitude and duration of exposure must always be considered; the greater the cumulative dose, the more likely it is the exposure contributed to the development of the condition. When a history of significant cigarette consumption is also present, the contribution of exposure to the pathogenesis of the condition is difficult to assess.

The threshold endotoxin levels described above are based on the measurement of endotoxin by the Limulus amebocyte lysate (LAL) method. However, the bioavailability of endotoxin in aerosols may vary depending on the nature of the materials. Therefore, concentrations measured in extracts by LAL may not be accurate predictors of the toxic potential in the natural setting. Based on experimental exposures, cell-bound endotoxin has been estimated to be three times as biologically active as isolated endotoxin in equal amounts, as measured by LAL. Also, inter-laboratory variation in measurement of endotoxin is large.

The problem of assessing exposure to airborne endotoxins has been studied for years, but sampling and analysis procedures are still not adequately standardized. The different protocols mean there is broad inter-laboratory variability in the results of endotoxin analyses. Standardized methods for sample collection, storage, extraction, and analysis have been proposed and round robin testing, using standardized protocols, samples, and reagents, has been done. For example, a comparative study showed a sixfold range in reported endotoxin content of the same cotton dust by 11 laboratories experienced with this measurement, despite the use of a common extraction protocol and commercial lots of analytical reagents.¹⁵ A review of the methods for measuring airborne endotoxin and their need for standardization has been published.¹⁶

Until standardization is achieved, endotoxin analyses should be done by a laboratory familiar with measurement of endotoxin in the type of material sampled. Caution should be exercised when comparisons are made between observed concentrations and proposed effect threshold levels.

TREATMENT

Reduction in exposure to the organic aerosol is the treatment of choice. The acute febrile reaction is usually self-limited and can be treated symptomatically if necessary. Recurrent chest symptoms may improve with pre-exposure use of an inhaled bronchodilator, or regular use of inhaled steroid medication, but these methods should not be used in lieu of reduction in exposure.

Established chronic bronchitis and airway obstruction may be treated with inhaled bronchodilators and steroids. Courses of antibiotics may be given when bacterial superinfections occur. Cessation of smoking and avoidance of other respiratory irritants are also indicated. Vaccination against pneumococcal pneumonia and yearly influenza vaccination are recommended for persons with moderate or severe airway obstruction.

MEDICAL SURVEILLANCE

Periodic medical questionnaires focusing on respiratory symptoms are helpful. The association of symptoms with exposure should be explored carefully with questions referring to the intensity of symptoms at home compared to at work, improvement during days off or vacations, and changes in severity related to specific job activities or use of personal protective equipment.

Periodic measurement of pre- and post-shift spirometry may show significant reductions in ventilatory capacity after exposure. Normal variability in spirometry must be considered when interpreting these results. The acute response to inhalation of organic dust may vary significantly on different occasions. Therefore, repeated observations are most helpful to determine if a meaningful pattern is present.

Traditionally, most surveillance examinations have been done on a yearly basis. However, modification of the interval based on the intensity of exposure and the presence or absence of symptoms is justifiable. If work-related symptoms are absent and exposure is relatively low, ventilatory function testing every 2 years is probably adequate to detect important long-term trends. Changes in intensity of exposure or the onset of symptoms are indications for consideration of more frequent monitoring.

PREVENTION

Currently, there are no enforceable standards that limit exposure to endotoxins. However, there are legal limits for exposure to selected types of organic dust. In Denmark, the exposure limit for “organic dust” is 3 mg/m³ of “total” dust; in Norway and Sweden, it is 5 milligram per cubic meter. In the United States, the permissible exposure limit for grain dust is 10 mg/m³ for total grain dust (established by the Occupational Safety and Health Administration (OSHA) in 1989) and the OSHA cotton dust standard limits exposure to 0.2 mg/m³ PEL 8 hours time weighted average. Suggestions for other threshold limit values for different types of organic dust have been made; however, these were outside a formal standard making process.

Prior to 2010, the only recommendation for a health-based occupational exposure limit for endotoxin was 50 EU/m³ as an 8 hours TWA by the Health Council of the Netherlands. ¹⁷ (Note: There are approximately 10 endotoxin units (EUs) per nanogram of endotoxin.) This recommendation was modified in 2010 to 90 EU/m³ (8 hours TWA) and was based on the lung function results of a series of experimental cross-sectional studies exposing healthy individuals to cotton-derived endotoxins⁸ and of an epidemiological cohort study among grain processing and animal-feed industry workers.^18,19 In addition to the exposure limit, the Health Council also recommended a specific endotoxin analysis method.²⁰

However, until a reliable and reproducible method for measuring endotoxin is developed, the most effective primary preventive measure is to reduce exposure by decreasing workplace concentrations of endotoxin-containing materials. Traditional methods of reducing airborne contaminants include enclosure of aerosol-generating processes and increasing general ventilation. Decreasing the endotoxin content of organic dusts by selective removal has not yet reached commercial feasibility. Water washing of cotton prior to processing has been shown to reduce both the endotoxin content of the dust generated during processing and the acute dilatory functional decline associated with exposure. Other methods of detoxification, such as heat treatment, have not been successful due to the relative stability of endotoxin and the toxicity of cell wall components of nonviable bacteria.

The use of personal protective equipment to reduce the dose of inhaled aerosol can be effective. However, because the higher concentration of endotoxin appears to be associated with the smallest particulates in an organic dust-contaminated environment and workers dislike using respirators, due to their discomfort and interference with job activities, reliance on this technique is questionable. Nevertheless, a respirator program with adequate training and monitoring of use can be a valuable adjunct to other measures and may be necessary for adequate protection of especially sensitive workers. There is both in vitro and in vivo evidence that atopic persons have an increased risk for adverse effects. Atopic persons showed an increase in nonspecific airway reactivity when exposed experimentally to cotton dust.²¹ Atopic individuals also had increased nasal airway eosinophils when challenged with LPS compared to after saline challenge.²² Lymphocytes from persons allergic to birch pollen had increased expression of CD154, a component of the signaling pathway for allergic inflammatory responses, after incubation with pollen and LPS compared to incubation with pollen alone.²³ However, despite these findings, at present there is not sufficient justification for excluding atopic persons from jobs with potential exposure for endotoxin. If work-related symptoms and acute decrements in ventilatory function are found, reduction in exposure is clearly indicated.

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