This scientific research has used “in vitro” testing (using cell cultures or simulated lung fluids), animal testing and epidemiological human studies. The findings of all these has resulted in the a view that many types of dust, including any type of fibre, 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, but not other dusts, 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 “3D”s:
Fibre dimensions are critical, as only fibres of a certain size can reach the lungs (the target organ). Mineral fibres with a diameter > 3 µm are, in humans, “non respirable” and for the rodents used in animal experiments fibres must be even finer. Even below this respirability threshold only the finest fibres may be deposited into the gas exchange region of the lungs. While respirability is 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 called macrophages. However long fibres frustrate this mechanism and also, for some still unknown reason, are more biologically active. This is also true in tissue cultures 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 body and so is more accurately called “biopersistence”. There are several different “clearance” mechanisms causing foreign particles to be removed from tissue (in this case the lungs). Fibres can dissolve or they may break into shorter pieces which can then be removed to the airways or through the lymphatic system. The rate of removal of different fibres is typically measured in animal experiments and expressed as their “half life” – that is the time it takes to reduce the number of fibres in the lungs by 50%.
The last “D” is the result of the other two parameters and is often referred to as “lung burden”. With chronic exposures the lung burden is the result of ongoing deposition (driven by workplace exposure levels and fibre dimensions) and clearance (driven by biopersistence). If the exposure is high enough and clearance slow then a sufficiently large dose will accumulate for adverse health effects to result.
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 also 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