This scientific research has used "in vitro" testing (using cell cultures or simulated lung fluids), animal testing and epidemiological human studies. As a consequence it is now known that many types of dusts and fibres, reaching the gas exchange region of the lungs can cause chronic inflammation.
This may develop further into fibrosis (scarring of the lung tissue) which can cause, or predispose to, lung cancer.
Fibres passing into the lining of the chest cavity may also cause inflammation, fibrosis or tumours (mesotheliomas) at this site. Injecting fibres directly, into the abdominal cavity of rats can also cause mesotheliomas.
Consideration of all the available results indicates that three factors are important in the ability of a fibrous dust to cause disease. These are often called the "3Ds":
Fibre dimensions are critical, as only fibres of a certain size can reach the deep lung (the target organ). Mineral fibres with a diameter > 3 µm are, in humans, considered essentially "non respirable", and for the rodents used in animal experiments fibres must be thinner still to reach the deep lung. While respirability is predominantly determined by fibre diameter, fibre length is also important. Short fibres behave as if they are compact particles and can be cleared by the normal mechanisms which involve cells in the lung called macrophages. However, long fibres frustrate this mechanism and are also, for some still unknown reason, more biologically active. This is also true both in tissue culture experiments and after injection – circumstances in which clearance mechanisms are not involved.
Durability in this context describes the ability of a material to persist in the lung and is more accurately called "biopersistence". There are several different mechanisms involved in the removal (clearance) of foreign particles from the lungs. Fibres can dissolve and/or they may break into shorter pieces which can then be removed by macrophages or be transported through the lymphatic system. The rate of removal of different fibres, typically measured in animal experiments, is expressed as the "half life" – that is the time it takes to reduce the number of fibres in the lungs by 50%.
The last "D" – dose – is related to the other two parameters and is often referred to as "lung burden". With chronic exposures the lung burden is the result of ongoing deposition (determined by workplace exposure levels and fibre dimensions) and clearance (determined by fibre biopersistence). If the exposure is high enough and clearance slow then a sufficiently large dose may accumulate and adverse health effects occur.
The scientific knowledge about fibre toxicity – summarised in the 3D approach – allows us to compare different fibres in terms of their toxicological potency, and has driven several initiatives to reduce potential risks in the workplace.
This risk reduction can be achieved by:
Survey of the Biological Effects of Refractory Ceramic Fibres