Science of the Health Effects of Fibers
What This Is All about
Is there a scientific question?
Before we ask why different kinds of fibers yield different responses
when people or laboratory animals are exposed to them, we need to
show that there really is a big difference that needs a scientific
There are now several studies available that show these differences clearly
including these results from chronic inhalation
Why do fibers differ in biological effect?
Now that we have shown that there is a difference among different fibers,
we would like to know what properties of the fibers contribute to these
differences in their biological response.
Recent scientific research has clarified that a fiber must possess all
of the following properties for disease to result:*
It is an axiom of toxicology that a sufficient dose must be present for
disease to occur as a result.
Historical exposures of workers to asbestos fibers were at concentrations
of tens to hundreds of fibers per cubic centimeter (f/cc), whereas
workplace concentrations of modern synthetic vitreous (glass) fibers
are below 1 f/cc and, for most products, below 0.1 f/cc.
Fairly comprehensive information about exposure of residential insulation
workers is given in this publication
and in the references cited there, and information about exposure in manufacturing is
available here, as well as in the
papers cited there.
It is the long, thin fibers that are implicated in disease.
The fibers must be thin to be inhaled into the deep lung - less than
3 µm in diameter for humans.
If the thin fibers are short enough, less than 10 or 20 µm long for
humans, then they are efficiently removed by the lung macrophages in the
same way as ordinary dust particles are removed.
However, if the fibers are long, longer than about 20 µm, then they
are too long to be enveloped and effectively transported by macrophages
even though they are thin enough to be inhaled to the deep lung.
This process is illustrated in these animations.
Fibers, even if they are present as a large dose, even if they are long,
are not associated with disease if they dissolve or break down in the
How rapidly a fiber dissolves is captured in its dissolution rate constant,
which can be measured in animal biopersistence studies as described
this paper or measured in laboratory
experiments using simulated lung fluid as described in
these papers, as well as in papers by
many other scientific groups, most of which are cited in the papers
There is a movie made of time lapse photomicrographs of dissolving fibers
that you may view here.
The effect of fiber dissolution rate on its biological properties may
be made quantitative with a predictive model that allows one to predict
the incidence of disease after exposure to a given dose of long fibers
with a given dissolution rate.
A paper describing the model and verifying it against three different
diseases in a series of inhalation and injection studies is
An animation of this application of the predictive model may be
How is fiber durability engineered and controlled?
The rate at which a fiber dissolves in the lung is determined mainly by its
composition, which is fixed essentially forever when the fiber is
Therefore, the dissolution rate of a fiber is the most important biological
property for a manufacturer of synthetic vitreous fibers.
An extensive series of laboratory investigations
of many fiber compositions combined with the results of animal biopersistence
studies of many different fiber compositions, which are
summarized here, have allowed
the dependence of dissolution rate on fiber composition to be determined.
For a wide range of synthetic vitreous fiber compositions, it is now possible
to estimate the dissolution rate directly from the oxide composition.
A paper that develops this method for borosilicate glass insulation fiber
compositions may be read here.
A further extension of the method to rock, slag, and other fibers is
These methods are implemented in a calculator that runs in these
web pages that you may use by clicking here.
The resolution of these scientific questions allows fibers to be
designed and modified so that they have
dissolution rates high enough to minimize the chance that that disease would occur even
in laboratory animal studies at extremely high exposures.
*Churg, Wright, Bilks, and Dai, J.
"Pathogenesis of fibrosis produced by asbestos and man-made mineral fibers:
What makes a fiber fibrogenic?".
12 (Suppl. 3), 15-26, (2000).
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