Module 4: Organic Dusts - Health Effects Of Grain Dust

HEALTH EFFECTS OF GRAIN DUST:

Effects of grain dusts on the lung:

The effects of grain dust on the lungs are difficult to define precisely because:

1. The complex nature of the dust.
2. The spectrum of clinical syndromes it causes.
3. The difference in the characteristics of exposure to grain dust in various occupations.
4. The multitude of pathogenetic mechanisms.
5. The changing pattern of illness with progressive reduction in exposure.

Levels of ambient particles:

The levels of ambient particles in grain elevators depend on:

1. The type of grain handled.
2. The degree of activity.
3. The efficiency of engineering controls (exhaust ventilation) and housekeeping practices.

Composition of grain dust:

1. Various grains (wheat, oats, barley, rye etc.)
2. Microflora (Cladosporium, Alternaria, Ustilago, Aspergillus, Mucor, Gram negative bacteria).
3. Mites (Glycyphagus, Tyrophagus, Acarus, Goheiria).
   Note: House dust mites are NOT found in grain dust.
4. Inorganic matter (soil, silicon dioxide, trace elements).
5. Herbicides and pesticides (Aluminium phosphide).
6. Animal matter (weevils, insects, rodents and birds excreta).

Physical properties:

Particle size of wheat/barley dusts: 10 µm (diam); wheat starch grains 10-40 µm (diam).

Biochemical properties:

1. Histamine release.
2. Activation of complement (d.t grain-derived tannin).
3. Neutrophil chemotaxis.
4. Lymphocyte stimulation (prod. interlukin-1).

Clinical syndromes:

1. Acute effects:
   • Asthma.
   • Asthma-like syndrome.
   • Grain fever.
   • Skin and mucous membrane irritation.
2. Chronic effects:
   • Chronic obstructive lung disease.
   • Interstitial lung disease (?).

ASTHMA:

• Prevalence 2-40%.
• Immediate and dual responses.

Allergens identified:

• Asthma not due to a single allergen, but different allergens may be responsible in different grain handlers viz:
  1. Storage mite +++ (Lepidoglypus destructor, Acarus siro, Glycyphagus destructor)
  2. Durum wheat extract(++), durum wheat airborne dust.
  3. Grain weevil (Sitophilus granarius).
  4. Aspergillus, Cladosporium, Ustilago.
  Note: In baker's asthma wheat antigen and alpha-amylase positive.

Antigen identification for a specific grain e.g. wheat:

• 40 different wheat antigens.
• Antigenicity dependent on: different components of wheat (protein fraction)
  • altered grain while processing wheat;
  • cross-reactivity of different grain antigens (wheat & rye);
  • route of exposure;
• Produce Type I immediate response (Ig E);
• Positive RAST to wheat antigen accompanied with elevated total IgE highly predictive of baker's asthma and strongly positive RAST values correlate well with history and bronchial provocation challenge.

ASTHMA-LIKE SYNDROME:

• Prevalence: 3.9-11%.
• Acute changes in lung function (fall in FEV1>10% over shift with or without respiratory symptoms).
• Not due to asthma nor the result of a Type 1 allergic reaction.
• Across shift changes in lung function have been shown to be related to:
  • Level of dust exposure.
  • Increase in peripheral blood leucocyte count.
• Not related to: age; duration of employment; smoking or atopic status.
• Across shift changes have been shown to be a predictor of longitudinal decline in lung function.

GRAIN FEVER:

• Prevalence: 6-33%.
• General constitutional symptoms shortly after exposure to high levels of grain dust.
• Leukocytosis and a left shift in the differential leukocyte count.
• Diffuse airway obstruction on lung function tests.
• No CXR abnormalities and no precipitating antibodies.
• May be due to endotoxin or mycotoxin.

CHRONIC OBSTRUCTIVE LUNG DISEASE:

• Lung function decline irrespective of smoking habits.
• Related to high dust concentrations.
• Tolerance to grain dust exposure.
• Initial changes in lung function among young workers may be partially or total reversible.
• Eventually chronic irreversible airway obstruction.

OTHER HEALTH CONDITIONS ASSOCIATED WITH GRAIN DUST:

Silo fillers lung:

• Common among farmers.
• Due to inhalation of oxides of nitrogen in a recently filled silo.
• Results in a toxic pneumonitis and sometimes pulmonary oedema after few hours or days.
• Accompanied by residual airway hyper-reactivity and asthma.
• No accompanying fever or constitutional symptoms.
• Immediate medical treatment (steroids) required.

Organic dust toxic syndrome:

• Common among farmers.
• Due to removal of mouldy silage.
• Febrile illness associated with constitutional symptoms 4-12 hours after exposure.
• Rarely requires medical treatment.

PREDISPOSING HOST FACTORS:

Importance of environmental factors and host factors depend on the type of clinical syndrome. For grain fever and asthma-like syndrome (across-shift changes in lung function), the degree of exposure is more important. For grain dust asthma, host susceptibility factors are more important. These include:

1. Atopy (response to more than one common aeroallergen).
2. Smoking (additive/synergistic effect).
3. Alpha-1-Antitrypsin deficiency.
4. Non specific bronchial hyperreactivity.

ENVIRONMENTAL FACTORS:

• ACGIH: TLV = 4 mg/m³ (but still associated with respiratory hazards).
• Dutch expert committee (1997) recommends an 8-hr TWA of 1 mg/m³. More recent studies among bakers suggest that work-related sensitisation risk will be negligible if average exposure concentrations of inhalable dust is reduced to 0.5 mg/m³ and of wheat below 0.2 µg/m³.
• In South Africa: no differentiation between different types of dust exposure irrespective of potential to cause sensitisation. Grain dust defined as "dust arising from harvesting, drying, handling, storage or processing of barley, wheat, oats, maize, rye, including contaminants".
• exposure regulated by the Regulations for Hazardous chemical substances (sections 36;41-43, Annexure 7 ). It has been designated a Control limit of 10 mg/m³ TWA (total inhalable dust) and is denoted as a sensitiser (exposure should be prevented especially activities giving rise to short-term peak concentrations).
• Since particle size of wheat and barley dusts are 10 µm in diameter, the TWA of 10 mg/m³ is too high and the high sensitisation potential of the dust makes this standard unacceptable in protecting the health of workers (we need better standards).

REFERENCES:

1. Chan-Yeung M, Enarson D, Kennedy S. Sate of the Art - The impact of grain dust on respiratory health. Am Rev Resp Dis 1992; 145:476-487.
2. O'Hollaren MT. Bakers' asthma and reactions secondary to soybean and grain dust. In: Bardanna EJ, Montanaro A, O'Hollaren AT, eds. Occupational asthma. Hanley & Belfus, Inc, 1992;107-116.
3. Chan-Yeung, Kennedy S, Schwartz D. Grain dust induced lung disease. Asthma in the Workplace. Eds Bernstein IL, Chan-Yeung M, Malo JL and Bernstein DI. Marcel Decker Inc. (2nd ed) 1999; 32:617-633.
4. Jeebhay MF, Stark J, Fourie A, Robins T, Ehrlich R. Grain dust allergy and asthma among grain mill workers in Cape Town. Current Allergy and Clinical Immunology, 13(3), 23-25, 2000.