Summary

Inhalation of oxides of nitrogen (NOx) can cause sudden death, pulmonary edema, and/or bronchiolitis obliterans. The oxides causing this silo filler's disease may persist for two weeks in silos newly filled with green chopped plant material. Workers may be exposed when entering a silo, the silo chute, or the adjacent feed room. Most commonly, workers inhale low concentrations of NOx and develop minor transient respiratory symptoms. High concentrations of NOx can kill within minutes. Workers may inhale moderate concentrations for extended time periods without developing symptoms or detecting danger. Reactions to moderate concentrations (pulmonary edema) can be delayed up to 30 hours, with relapse in two to six weeks. Relapse can occur even without reexposure and may be due to the development of bronchiolitis obliterans. Determination of exposure to oxides of nitrogen followed by proper treatment and monitoring of exposed patients are imperative to prevent death or serious complications. Preventing exposure to silo gas could eliminate this occupational illness.

I. OXIDES OF NITROGEN ON THE FARM

Of the several oxides of nitrogen, three may be found in silos on the farm: Nitric oxide (NO), nitrogen dioxide (NO2), and nitrogen tetroxide (N204), collectively referred to here as NOx. Nitrous acid (HNO2) may also be present. Of these compounds, NO2 and N204 are medically significant.

Where are oxides of nitrogen (NOx) found on the farm?

Oxides of nitrogen are formed through fermentation in conventional silos (that is, upright silos that are not airtight) freshly filled with green chopped plant material. (See Fig. 1 ) This chopped corn, alfalfa, oats, or other plant material will form silage which constitutes a crucial portion of the feed ration for beef and dairy cattle; alfalfa silage also may be fed to sheep. Production of NOx commences within four hours after silo filling has begun. Concentrations of NOx reach a maximum in one to two days, after which production continues at a decreasing rate for a week to ten days. Both NOx and carbon dioxide, which are produced simultaneously, lie at or near the silage surface and in depressions in the silage, where they replace oxygen. Because NOx are heavier than air, they will roll down the silo chute like water when a chute door at an appropriate level has been left open, and concentrate in the feed room between the barn and silo, at the base of the silo, or in the chute itself.

Presence and concentration of NOx vary widely among different silo towers, and also depend on qualities of the chopped plant material. The NOx are produced from inorganic nitrates in plants. Higher NOx production is associated with certain crops, certain soil conditions (those heavily fertilized with nitrates), and certain plant growth conditions (prolonged drought with rain just before ensiling, cloudy weather, damage to leaves or roots, harvesting after a frost).

When are farmers exposed to NOx?

Exposure to NOx may occur wherever these heavy gases have concentrated: within a closed silo, higher in the chute (when the worker opens a chute door and a cloud of NOx rolls past), at the base of a silo chute (when a door between the chute and silo has been left open), or in the feed room or barn next to the silo (when gases have flowed through open doors into these structures). (See Fig. 2) Typically, a worker will climb the chute and enter the silo to work at leveling silage or to feed silage out to animals. Either is a potentially fatal task when attempted soon after filling or when the silo has not been ventilated adequately. Because NOx is only mildly irritating to the upper respiratory tract but highly damaging to the alveoli, workers may continue working actively in atmospheres with low to moderate NOx concentrations for hours, inhaling the potent oxides without detecting danger. The high carbon dioxide and low oxygen concentrations induce deep breathing, which speeds penetration of NOx into the alveoli where the gas does its damage. When NOx concentrations are high, a worker may be too weak to retrace his steps. The danger is increased when movement of a worker releases gases trapped in the silage, or when a worker enters or falls into a cavity in the silage.

Gases concentrated in adjacent buildings or at the base of the silo may be inhaled by workers, children at play, or livestock.

The danger from NOx is greatest in summer or early fall, during the two weeks following filling. (Haylage and oatlage may be harvested in summer, corn silage in late summer or early autumn.)

How is NOx detected?

Nitrogen dioxide can be identified by its bleach-like odor and yellow to reddish-brown color. The silage surface, silo wall, base of the chute, or other structures may be stained by the gas. Dead birds or insects may lie at the base of the chute or near the silo. Livestock near the silo may be sick. A physician establishing a diagnosis should question a patient about presence of any of these warning signs.

Farm workers should understand that even if not smelled or seen, NOx may be present in any recently filled or tightly sealed conventional silo; proper precautions should be taken.

How commonly are farmers exposed to NOx?

Although probably not common, the true scope of exposure to NOx is thought to be underestimated. An unpublished survey in the late 1960's revealed that 4.2% of Wisconsin farm operators had developed symptoms of NOx inhalation when working in or near freshly filled silos. Other statistics on the frequency of agricultural NOx exposure are not available, but a number of deaths have been documented through the years. The severity of the hazard rests partially in the high case fatality rate: 29% of cases cited in medical literature have been fatal.

II. RESPIRATORY EFFECTS OF OXIDES OF NITROGEN

Reactions to NOx depend on the concentration of gas inhaled and the length of exposure.

Relatively mild exposure to NOx produces ocular irritation and a transient upper respiratory tract syndrome manifest as cough, possibly with dyspnea, fatigue, nausea, cyanosis, vomiting, vertigo, or somnolence. Symptoms may be severe enough to induce workers to leave the silo. However, when reactions to NOx are minimal, workers may stay in the silo and increase the probability of a more severe reaction. Although symptoms may persist one to two weeks, chest films, pulmonary function tests, and blood gases are normal, and recovery is complete. These relatively mild responses are more common than the more serious reactions described below.

Very high concentrations of NOx induce immediate distress, resulting in collapse and death within minutes. The mechanism of this reaction is not completely understood: death may be due to airway spasm or laryngospasm, reflex respiratory arrest, or simple asphyxiation due to low ambient oxygen concentrations. 1 Persons who collapse in silos and are rescued immediately, may survive only to experience the respiratory responses described below.

At somewhat lower concentrations, NOx induces pulmonary edema (normally within 30 hours following exposure), (See Fig. 3) bronchiolitis obliterans (within days to weeks), or both. These reactions are commonly called "silo filler's disease." At the time of exposure, patients may have had no or minimal symptoms, or they may have experienced those symptoms associated with mild exposures listed previously. However, a slowly evolving and progressive inflammation of the lungs results in massive pulmonary edema most commonly from 6 to 12 hours later. Death from asphyxiation may occur within hours, but the majority of patients recover completely with appropriate therapy within days or weeks.

In a small percentage of cases, recovery from this first phase of illness may appear to be complete, including clearing of chest films, only to be followed by a relapse characterized by bronchiolitis obliterans, (See Fig. 4) typically two to four weeks following exposure. This fibrocellular obliteration of the bronchioles also may be the initial clinical manifestation. Relapse may lead to death, or to slow recovery over a period of weeks to months. Most patients who survive the pulmonary edema, bronchiolitis obliterans, or both do not develop significant respiratory impairment, although subclinical small airways obstruction may persist. Bronchiolitis obliterans responds to steroids. However, an occasional individual may experience persistent pulmonary dysfunction of variable severity.

The reactions to NOx result from hydrolysis to nitrous and nitric acid when the gases dissolve in mucosal moisture. The delayed and insidious effects of NOx reflect their relatively low solubility, and resultingly slow conversion to acid. These factors also explain the gases' major involvement with lower rather than upper airways. The acid affects peripheral airways, bronchioles, and alveoli, causing extensive tissue damage and formation of methemoglobin. Persons dying from asphyxiation due to pulmonary edema display intra-alveolar edema and exudation and thickening of alveolar walls with lymphocytic cellular infiltrates.

Diagnosis

Patients in the acute stages of silo filler's disease will present with moderate to severe respiratory distress. Systemic hypotension and evidence of severe hemoconcentration may be present, as may methemoglobinemia and severe metabolic acidosis. Leukocytosis is typical. Pulmonary function tests show a reduced vital capacity, increased airways resistance, impaired gas transfer, and hypoxemia.

Because the initial illness may be mild, patients may present to a physician for the first time during a relapse, two to six weeks after exposure to NOx. At this time, cough, tachypnea, dyspnea, fever, tachycardia, cyanosis, or other symptoms of respiratory distress are due to bronchiolitis obliterans. Small, discrete nodules, with or without confluence, will be evident on the chest radiograph.

Silo filler's disease may be confused with a number of illnesses including hypersensitivity pneumonitis or a toxic organic dust syndrome (TODS, see Unit 2), which result from exposure to moldy hay or grain; exposure to mold typically occurs while uncapping the silo, removing moldy silage from the top silo layers well after the harvest season. The chest radiograph of bronchiolitis obliterans may resemble miliary TB among other diseases; an accurate occupational history and negative sputum smears for acid-fast bacillus will help avoid confusion. A detailed medical and occupational history is crucial to correct diagnosis. In addition to noting exposure to a recently filled silo, most commonly in late summer or early fall, a patient may recall seeing signs of NOx near the silo or experiencing the transient symptoms described previously. Since exposure to silo gas may have occurred from hours to weeks prior to onset of severe respiratory symptoms, the patient may not associate symptoms with exposure to silo gas.

Prompt diagnosis and treatment of patients with acute symptoms are vital to prevent possible death and, in the case of initial illness, to lessen the probability of relapse.

Treatment

Any symptomatic patient who has been exposed to NOx should be monitored closely by a physician for 48 hours because of the possibility of sudden pulmonary edema. Typically, these patients are hospitalized; in certain cases, patients could remain at home but should be warned to report immediately to the physician upon development of respiratory distress. Persons developing pulmonary edema or respiratory embarrassment must be placed on steroids (20 to 120 mg Prednisone/day, tapered over time) for a minimum of eight weeks, to decrease the probability of bronchiolitis obliterans. Persons presenting for the first time with bronchiolitis obliterans also should receive steroid treatment.

Patients may require intensive supportive therapy, including oxygen, bronchodilators, or assisted ventilation. Antibiotics may be required for secondary respiratory infection.

Any symptomatic patient should be followed for six weeks following exposure for development of bronchiolitis obliterans.

III. PREVENTION OF SILO FILLER'S DISEASE

Farmers need to thoroughly understand the hazards associated with newly filled silos. Once filled, no one should enter the silo for at least two weeks. If entrance is imperative during the filling process, the blower should be run for 30 minutes prior to entering the silo and kept running while anyone is inside. All silo doors should be kept closed during and after filling to prevent NOx from flowing down the chute. The door between the silo room and barn should be kept closed. Children and animals should be kept away from the silo and adjacent feed room during filling and for two weeks afterward. A few days before the silo is entered for the first time, the filler opening should be pulled open from the ground (not from the chute) with a rope. The blower should be operated for at least one-half hour prior to entrance, and other means of ventilation should be maximized. Detector tubes (also called colorimetric tubes) that measure the concentration of NO2 are reasonably priced and reliable if properly used. Procedures for measuring concentration of NO2 in silos are described in detail in the patient education brochure entitled "Measurement of Agricultural Dusts and Gases".

Because of the small size of silo doors and deadly nature of NOx, any rescue attempts are extremely difficult and hazardous. Unless testing has been completed to assure the absence of toxic levels of NOx, entrance into the silo within two weeks of filling can only be achieved safely with a self-contained breathing apparatus.

Certain silos (horizontal silos such as stack, bunker, or trench units, or airtight units) decrease the danger of silo gas exposure; however, conventional upright silos are still the most commonly used units. If the slightest throat irritation or coughing occurs while working with fresh silage, a worker should exit the silo immediately. In any case of symptomatic exposure to silo gas, the worker should report to a physician immediately and be monitored for either immediate or delayed reactions. A

תתתתתתתתתתתתתתתתתתתתThe National Dairy Database (1992)תתתתתתתתתתתתתתתתתתתת תתתתתתתתתתתתתתתתתתתתת\NDB\OCCSAFE\TEXT2\OF200800.TXTתתתתתתתתתתתתתתתתתתתתתת

%f TITLE;OXIDES OF NITROGEN
%f COLLECTION;FARM AND OPERATOR SAFETY
%f ORIGIN;Iowa
%f DATE_INCLUDED;June 1992