Health & Safety

Extensive research has been conducted to evaluate the potential health effects of many fibrous materials.

• There have been a number of studies on high temperature insulation wools, including animal testing, a long-term human epidemiology study and risk assessments.
• 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”: Dimension, Durability and Dose. The “3Ds” allow us to compare different fibrous materials in terms of their toxicological potency.

Dimension

Fiber dimensions are critical, as only fibers of a certain size can reach the lungs (the target organ). Mineral fibers with a diameter > 3 µm are, in humans, “non respirable” and for the rodents used in animal experiments fibers must be even finer. Even below this respirability threshold only the finest fibers may be deposited into the gas exchange region of the lungs. While respirability is determined by fiber diameter, fiber length is also important. Short fibers behave as if they are compact particles and can be cleared by the normal mechanisms which involve cells called macrophages. However long fibers 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

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). Fibers 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 fibers is typically measured in animal experiments and expressed as their “half life” – that is the time it takes to reduce the number of fibers in the lungs by 50%.

Dose

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 fiber 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 fiber toxicity – summarised in the 3D approach – allows us to compare different fibers 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:

• The development of thicker fibers. If a fiber is thick enough to be non-respirable respiratory health effects are eliminated. This approach has its limitations as the insulation properties of thicker fibers are less favorable and thicker fibers are more likely to cause “itching” (mechanical skin irritation).
• The development of less biopersistent fibers. This approach was the foundation of the development of AES wools which are cleared rapidly from the lung tissue.
• Development of industrial hygiene practices and exposure controls. One way to manage risk in the workplace is by the reduction of the level of exposure to any potentially hazardous material. HTIW Coalition has developed handling advice and guidelines based on thousands of exposure measurements in the PSP 2012 Program. HTIW Coaltion members are therefore well placed to help their customers minimize workplace exposures.