Occupational and environmental lung disease

The airways of the lung derive from the trachea (wind pipe) downwards by progressive division into two (or more) branches. Those airways beyond the trachea that contain cartilage are called bronchi. The airways lacking in cartilage beyond the bronchi are the bronchioles. These lead into hollow spaces called alveoli which have a diameter of about 0.1 mm each. There are approximately 300 million alveoli and their total surface area is about 140 m². The conducting airways are lined by cells with cilia (small motile surface projections). Interspersed between these cells are mucus secreting cells. Secreted mucus spreads over the cilia which direct it upwards to the larger airways by rhythmic undulating movements, thus helping to clear deposited dusts.

The respiratory units, i.e. the alveoli and the smallest bronchioles called respiratory bronchioles are responsible for the exchange of gases. They are lined mainly by flat, extremely thin cells which permit easy diffusion of oxygen through them from the air in the alveolar spaces to the blood in the capillaries and easier diffusion of carbon dioxide in the opposite direction. Alveolar macrophages are very abundant, mobile and phagocytic cells mainly responsible amongst other functions for the ingestion of foreign matter. The lining of the outside of the lung and the inside of the chest wall is called the pleura.

Deposition and host defence of inhaled dusts and mists

Aerosol is an all-embracing term including all airborne particles small enough to float in the air. Dusts are solid particles dispersed in air. Mists are liquid droplets formed by the condensation of vapours, usually around appropriate nuclei or the 'atomisation' of liquids. The aerodynamic diameter of a particle is the diameter of a sphere of unit density that would settle at the same rate.

When airborne particles come in contact with the wall of the conducting airway or a respiratory unit they do not become airborne again. This constitutes deposition and can be achieved in one of four ways:

• Sedimentation is settlement by gravity and tends to occur in larger airways.
• Inertial impaction occurs when an airstream changes direction especially in the nose but also in other large airways.
• Interception applies mainly to irregular particles such as asbestos or other fibrous dusts which by virtue of their shape can avoid sedimentation and inertial impaction. However they are intercepted by collision with walls of bronchioles especially at bifurcations or if the fibres are curved.
• Diffusion is the behaviour of very small aerosol particles which are randomly bombarded by the molecules of air. It significantly influences deposition beyond the terminal bronchioles.

Most compact particles larger than 20 microns aerodynamic diameter and about half of those of 5 micron aerodynamic diameter are filtered within the nose during breathing at rest. However there is a wide variation in the efficiency of this among apparently normal subjects. Moreover conditions which favour mouth breathing, (e.g. high ventilation rates and obstructive disease of the nasal airways) will cause large particles to bypass this filter. Alveolar deposition is appreciable at particle diameters of between 1 and 7 microns (respirable particles) and probably maximal at aerodynamic diameter of between 2 and 4 microns. During regular breathing at rest only about 10% of compact particles of 0.5 to 1 micron diameter are deposited in the lung (alveoli), the bulk being again exhaled.

During exertion, increase in tidal volume (i.e. the volume of air inspired with each breath) and particularly in respiratory minute volume (i.e. the product of tidal volume and the number of breaths per minute) is the single most important determinant of the total load of particles in the alveoli and hence the total volume of particles deposited for a given aerosol. Several other factors may influence particle deposition. Insoluble particles deposited in the conducting airways are propelled towards the larger airways by the cilia and then rapidly coughed or swallowed. This may be delayed by factors such as tobacco smoking. In the respiratory units, ingestion by macrophages is necessary before the particles are carried to the larger airways. Particles may also penetrate the deeper lung tissue where they may stay for years or be transported by macrophages to the lymph nodes.

Vapours and gases

Vapours are substances in the gaseous phase at a temperature below their boiling point. Gases produce their harmful effects in the following ways (as described below):

1. They can cause asphyxiation (deprivation of oxygen to the tissues)
2. They can cause irritation of the airways and the lungs
3. After entering the body through the lungs they can cause damage to other tissues of the body

Nuisance dusts are relatively inert and, by definition, cause no serious health effects although they may be irritant to the upper airways. Examples include chalk, limestone and titanium dioxide (provided they are free of toxic impurities). They may cause radiographic changes without disease. Dusts should be considered as nuisance dusts only when there is good evidence that they are inert and free from significant health effects not when evidence for an effect is lacking. Moreover there is now good evidence that ultrafine particles of dusts previously considered inert, such as titanium dioxide can be toxic.

Books on occupational lung diseases include those by:

• Morgan and Seaton
• Parkes
• Banks
• Agius, R. Respiratory Toxicology. in Duffus JH & Worth HGJ (Eds). Fundamental Toxicology for Chemists. 1996, The Royal Society of Chemistry, pp 129-135, ISBN 0-85404-529-5

• Occupational and environmental skin disease
• Martin, Lawrence: "Pitfalls in diagnosis of occupational lung disease for purposes of compensation", Journal of Law and Health, Vol. 13, 1998