Silo filler’s disease is a condition caused by inhaling toxic levels of oxides of nitrogen gas. It is primarily an occupational disease associated with working in farm silos where crops are stored. It should not be confused with farmer’s lung, another occupational disease seen in farm workers but arising from the inhalation of molds (fungi) that taint organic substances. Both silo filler’s disease and farmer’s lung are not infections. The toxicity arises from inhaling nitrogen dioxide which is not the same as nitrous oxide (“laughing gas”).
Silo filler’s disease is a relatively uncommon occupational disease among agricultural workers. It is estimated to affect only about 5 out of 100,000 silo workers in the United States. However, it is possible that the incidence is higher and patients probably do not seek medical attention for mild exposure. Furthermore, greater awareness about the condition has possibly contributed to a lower incidence. Silo filler’s disease is potentially life-threatening and suffocation can occur within minutes in the event of exposure to very high concentrations of nitrogen dioxide and carbon dioxide.
Nitrogen dioxide (NO2) found in higher than normal concentrations within farm silos and is the predominant toxin in silo filler’s disease. The gas causes significant irritation of the mucosa lining the eyes, nose and airways. Even acute exposure among non-farm workers can cause mucosal irritation and acute respiratory symptoms. It also worsens pre-existing lung diseases like asthma and has been implicated in sudden infant death syndrome (SIDS / cot death). The effects of nitrogen dioxide is compounded among silo workers because of repeated exposure. Furthermore the high carbon dioxide levels within silos often leads to deeper inhalation and therefore a higher exposure dose of nitrogen dioxide.
Nitrogen dioxide damages the lower airways in particular and the lung tissue. It breaks down into nitrous and nitric oxide within the airways and lungs damaging the ciliated cells that line the airways and the epithelial cells (pneumocytes) that maintain the air sacs (alveoli) of the lungs. Type I pneumocytes are primarily affected in silo filler’s disease. This results in inflammation of the bronchial and bronchioles walls (bronchitis and bronchiolitis) and alveoli (pneumonitis) along with fluid accumulation within the lung (pulmonary edema). It is important to note that neither bronchitis nor pneumonitis in silo filler’s disease occurs due to an infection. However, patients with silo filler’s disease are at a greater risk of developing infectious bronchitis, bronchiolitis and pneumonia.
Apart from the direct effects of nitrogen dioxide as an irritant, it also results in the formation of free radicals which can cause further cellular damage. In addition, nitrogen dioxide impairs immune activity. It therefore increases the risk of infections. By binding to hemoglobin, nitrogen dioxide can affect the oxygen concentration in the blood. This leads to hypoxia meaning that the tissues in the body receives less oxygen than it needs. It occurs by changing normal hemoglobin (oxyhemoglobin) into methemoglobin, a type of hemoglobin that does not carry oxygen. People with silo filler’s disease may therefore have methemoglobinemia, a condition where the methemoglobin in the increases above the normal ranges.
The symptoms present and its intensity depends on several factors such as duration and degree of exposure. Acute symptoms may arise within 24 hours after exposure whereas delayed symptoms can appear days or even weeks later. Suffocation is possible within just a few minutes of exposure to very high concentrations of nitrogen dioxide and carbon dioxide and can be fatal. Most of the symptoms of silo filler’s disease are non-specific meaning that it is not only seen in this disease but many other respiratory conditions.
The acute symptoms of silo filler’s disease are similar to may respiratory infections but without the presence of a fever. These symptoms may include :
- Chest tightness (not pain usually)
- Difficulty breathing
- Wheezing (sometimes)
- Sweating (not fever)
- Lightheadedness and dizziness
- Sore throat (uncommon)
- Red and watery eyes with burning (uncommon)
- Nausea (uncommon)
Delayed symptoms do not occur in every person. Sometimes the acute symptoms resolve and no delayed symptoms may appear. However, when delayed symptoms do appear, it is often persistent for prolonged periods. All of the above early symptoms may appear or persist as well as additional symptoms such as :
- Fever and sometimes chills
- Cyanosis (bluish skin tinge)
- Coughing up blood-streaked sputum (hemoptysis)
- Rapid breathing
- Difficulty sleeping
- Muscle aches
Silo filler’s disease is a result of exposure to nitrogen dioxide. It is a form of chemical injury to the terminal airways and lung tissue. It is one type of chemical pneumonitis that can occur in any person exposed to toxic levels of nitrogen dioxide but is more common among silo workers for several reasons. Freshly cut plant material and harvested crops such as corn and oats may generate nitrogen oxide (NO). In the presence of oxygen this is converted to nitrogen dioxide (NO2).
Nitrogen dioxide is heavier than air and therefore sinks. In 1 to 2 days the nitrogen dioxide levels at the bottom of the silo reaches its peak. Carbon dioxide which is also heavier than air may also accumulate the bottom of the silo. These gases are commonly referred to as silo gas. A worker who will be exposed to this silo air is at risk of inhaling these gases if the silo is not properly ventilated or breathing apparatus is not used. The levels of nitrogen dioxide builds up to toxic levels as it accumulates in a closed silo. The gas may remain there for weeks in well sealed silos.
In most cases the exposure to silo gas is short-lived and the effects are mild. Symptoms tend to resolve spontaneously. However, a person who is exposed to these conditions for prolonged period is at a much greater risk. Although it is an occupational hazard, silo filler’s disease is completely preventable. In severe cases, respiratory distress may occur. Chronic pulmonary conditions like constrictive bronchiolitis (bronchiolitis obliterans) may not be entirely due to the effects of nitrogen dioxide but also with repeated respiratory tract infections that are more likely to arise in patients with silo filler’s disease.
The symptoms of silo filler’s disease are non-specific but the condition should be suspected among farm workers dealing with crops. It is more common during harvest season (September and October in particular). Further diagnostic investigation includes :
- Arterial blood gas
- Quantitative methemoglobin
- Lactate test
- Pulmonary function testing
- Electrocardiography (ECG)
- Pulse oximetry testing
Chest x-rays may reveal some degree of lung inflammation.
Silo filler’s disease is self-limiting when there is mild exposure. In more severe cases, patients need to be hospitalized.
- Oxygen needs to be administered to patients with low blood oxygen levels. Nitric oxide may also be administered in some cases but should be done cautiously.
- Mechanical ventilation is necessary for respiratory failure.
- Methylene blue may be prescribed for methemoglobin.
- Antibiotics are necessary for infections.
- Corticosteroids may help prevent scarring of the bronchioles (bronchiolitis obliterans).