Permeation vs. Penetration
Safety managers are well-advised to evaluate how chemical protective garments react in these tests.
The selection of any protective garment is complicated
and carries the potential for serious
consequences should the decision prove to be
incorrect or the selection in any way inadequate.
For this reason, standards bodies, vendors, customers,
and workers are engaged -- often in the pages of this very
magazine -- in ongoing conversations to determine the
appropriate levels of protection for a given task.
In the area of liquid chemical protection these conversations
often center around two alternate methods
for testing garments—permeation and penetration.
Often these approaches to evaluating chemical barrier
protection are treated as being equally appropriate. This
approach, however, is overly simplistic and rarely considers
the strengths and deficiencies of each test.
Fundamentally, there is much that is not known regarding
how a wide variety of chemicals react against
skin to accept a compromised testing approach.At their
core, all safety personnel know that what is unknown is,
by definition, unsafe. With that principle in mind, a
thorough investigation of what penetration and permeation
tests are, and what they can achieve, should
prompt safety managers to re-evaluate how they source
chemical protection garments.
In the science of chemical protective clothing, the
terms permeation and penetration represent very different
mechanisms of chemical protection.
Penetration, as it relates to chemical protective garments,
refers to the passage of chemicals through clothing material
without changing its properties. Penetration is
achieved when a chemical finds and passes through
pores and imperfections in the clothing material, closures,
seams, and interfaces. Penetration testing, therefore,
is a defect test—an evaluation of how (or whether),
a substance flows through openings in a material.
On the surface, the concept of a penetration test
sounds entirely appropriate and sensible. In practice,
however, penetration testing is the source ofmuch confusion.
This stems from the way penetration tests are actually
conducted, how the results are reported, and the
way the results are perceived.
There are several factors to consider when evaluating
penetration test results: pressure and duration of the
penetration test, surface energy, viscosity, and chemical
interactions between garment and hazardous liquid.
Pressure: A penetration test infers a
level of pressure is involved in an attempt
to push or force a chemical through a protective
garment—to drive it through the
void. This makes abundant sense; a penetration
test with no pressure involved is
simply a repellency test, the sort of approach
a raincoat manufacturer might take
to evaluate its product.
As much sense as this makes, however,
some manufacturers report penetration test
results without specifying the pressure and
duration of testing.Others fail to explain the
implications of a given pressure. Small variations
in pressure can mean the difference
between a Pass or Fail in penetration testing
(especially when chemical properties and
viscosity are also taken into account). Penetration
tests use a variety of pressures, and
only by examining the method and with full
reporting of the test conditions can you
begin to relate the penetration results to the
Chemical of the hazardous liquid: When
conducting penetration tests, the physical
and chemical properties of the hazardous
liquid are also of note.While it is best to use
the actual chemical of concern in penetration
testing, this is sometimes not practicable
or is considered “too” expensive.As a result,
manufacturers often use surrogate
liquids and assume that a surrogate of
equal surface tension and viscosity will be
Much like pressure, small variations in
viscosity and surface energy can have a large
impact on the ability of liquid to penetrate a
garment. Small amounts of soaps and other
surfactants can alter surface tension (the
ability to “wet-out”) significantly and, in
turn, reduce pressure required to penetrate.
Some liquids/fabric interactions do not follow
simple penetration models based on
pore size, viscosity, and surface tension.
Chemical interactions between the liquid
and fabric lead to behaviors different than
predicted by surface energy, pore size, and
viscosity. This makes selecting the right surrogate
liquid especially important, if one
must be used.
Is the penetration performance of the
surrogate representative of the hazard? Failure
to evaluate the physical and chemical
equivalence of the surrogate is as equally
troubling as a failure to explore the pressure
and duration of the test. One successful penetration test, regardless of the pressure and
duration, is not sufficient, in and of itself, to
ensure protection against a different liquid
chemical with different chemistry, viscosity,
and surface energy.
As opposed to penetration testing, permeation
testing evaluates how chemicals move
through protective garments without passing
through voids or imperfections. Permeation
testing looks at how a given test chemical
is absorbed into the contact surface of a
garment, the diffusion of the chemical
through the material, and desorption of the
chemical from the opposite side.Permeation
can occur in the absence of penetration.
Like penetration testing, there are some
variations involved with permeation testing.
The military, for instance, gauges permeation
of field clothing by “spotting” a
material with drops of a given chemical.
The theory behind this is that armed forces
in warfighting situations are far less likely to
encounter pools of chemicals than dispersed
Some tests are also constructed around
intermittent splashes of a given chemical.
The chemical is splashed against a fabric at
a given interval -- once every 10 minutes, for
instance. The time sequence for intermittent
splash permeation testing is as much a judgment
call as the pressure/time sequence used
in penetration testing.As will all other forms
of permeation testing, the material is monitored
for the presence of the chemical on its
By far the most thorough permeation
test -- and the most thorough liquid chemical
barrier test in general -- is to ensure the
entire surface of the material is covered by
the chemical in question. In these tests, the
material is used to create a division between
two chambers. On one side is a large pool of
liquid chemical; the other is empty. Over
time, the unexposed side is tested for the
presence of the chemical.
Testing for Safety
For some, the differences between penetration
and permeation testing are no cause for
concern. Garments that pass penetration
tests are said to be “safe” for certain activities
where minimal exposure is expected; the
more stringent permeation tests are said to
be for garments that will be used in extreme
conditions. But who and what determine
“minimal” and “extreme”?
This minimal-exposure argument is the
justification some manufacturers have used
to promote garments with higher levels of
moisture transport for the purpose of reducing
heat stress on workers. These garments
are reported to pass some penetration
tests and are, therefore, said to be safe for activities
involving the potential for liquid
This is where the element of the unknown
comes into play.Are the penetration
tests representative of the hazard? The
chemical industry does not have a large
body of reliable information on how chemicals
interact with skin. It simply does not
have good skin toxicity data and does not
know the skin exposure limits for a large
number of chemicals—either liquid or
vapor. Furthermore, the industry doesn’t
know how these chemicals will interact with
the skin under different conditions: dry skin;
sweat-saturated skin; in the presence of
sweat-soaked clothing; when exposed to
damaged skin, abrasions, or cuts. There are
many unknown factors.
Furthermore, the benefits of limited-use materials—
namely the ability to reduce heat stress—have not been
well established. True, at low-levels of work activity,
and in air with low moisture content, there is some evidence
to support some lower heat stress burden. Beyond
that, however, there is no data that demonstrate
heat stress is eliminated at normal working conditions.
Yet advertising still promotes that barrier should be
sacrificed to comfort.
This total lack of reliable data -- relative to both heat
stress and the interaction of chemicals with skin --
means safety managers are not equipped with the information
necessary to be able to select a lower level of
protection for their workers with any peace-of-mind.
The solution, of course, is to stick to the principle mentioned
above: unknown is unsafe. Especially considering
that the solutions being proposed do not eliminate
the heat stress issue—it must still be managed.
With this principle in mind, the clear choice of manufacturers
and safety managers is to err on the side of
caution. In this case, caution means focusing on more
sensitive, more stringent, testing. While penetration
tests look for holes and imperfections, permeation tests,
by their nature, test both permeation and penetration—
after all, any voids or imperfections will be just as
apparent in this test as they are in a standard penetration
test.As a result, safety managers are well advised to
evaluate how chemical protective garments react in
This article originally appeared in the July 2008 issue of Occupational Health & Safety.