A few common gases and liquids have
IDLH values set at 10% of the Lower Explosive Limit (10%
of LEL). IDLH is an acronym for
Immediately Dangerous to Life and Health.
These IDLH concentrations have been established by NIOSH
(National Institute for Occupational Safety and Health)
for several hundred chemicals. For most
chemicals, the IDLH concentrations are based on toxicity
if the chemical is inhaled. For a small
number of chemicals, the IDLH concentration is set at
10% of the LEL even though toxicity information might
allow for a much higher concentration.
An example is propane.
Propane has a lower explosive limit (LEL) of 2.1% and an
upper explosive limit (UEL) of 9.5% at room
temperature. At elevated temperatures these
numbers could be different. The IDLH
concentration is 10% of LEL or 2100 parts per million
(ppm). A 2.1% concentration of propane in
air is equivalent to 21,000 ppm.
Ten percent of 21,000 ppm is 2100 ppm, and this is
the IDLH concentration.
Another example is
methane. Methane is the primary ingredient
of natural gas. Methane has a LEL of 5% and
an UEL of 15% at room
temperature. A concentration of
5% methane in air is equivalent to 50,000
ppm. An IDLH concentration set at 10% of LEL
would be 5,000 ppm.
Butane has an IDLH set at 10% of LEL,
or 1600 ppm. The LEL for butane is
1.6% (concentration of butane in air).
If the chemical is also toxic, the
IDLH will probably be much lower than 10% of LEL.
An example is benzene which has a LEL of 1.2% but the
IDLH value is 500 ppm. Benzene is also a
recognized carcinogen.
Gasoline is a complex mixture of
about 200 different chemicals, many of which are
toxic. The LEL of gasoline is about 1.3% or
1.4% overall. Some of the components of
gasoline have lower LEL values and IDLH values less than
10% of LEL. A major component of gasoline is
heptane which has an LEL of 1.05% (n-heptane) and an
IDLH value of 750 ppm. Benzene is also found
in gasoline. Because gasoline composition
can vary, it is difficult to establish an IDLH number.
Because gasoline components are toxic, if an IDLH number
were established, it would probably be less than 10% of
LEL.
Another problem that arises is the
conversion of 10% of LEL to a value expressed as ppm
(parts per million). The confusion comes from the
fact that LEL is almost always expressed as volume % but
IDLH is typically expressed as ppm (although for
hazardous materials typically found in the form of dusts
or particulates it may be expressed as
mg/m3).
Why the different units when
expressing concentration values? Well primarily
because volume % is used for expressing relatively high
concentrations and ppm is used to express relatively low
concentrations. Volume % can be thought of as
expressing concentrations as parts per hundred vs. ppm
expresses concentrations as parts per million.
So how do we convert from volume % to
ppm? To convert the LEL to an IDLH value based on
its flammability, we first have to figure out what the
10% of LEL value is in the units of volume %. This
is pretty straightforward, if the LEL is 5.5%, we just
multiply the 5.5% by 0.10 or 5.5% x 0.10 = 0.55%.
Now we need to convert the 0.55% (actually 0.55 volume
%) to ppm.
Now we simply move the decimal point
to the right four places, which is equivalent to
multiplying 0.55 by 10,000, so 0.55 volume % becomes
5,500 ppm.
A simple way to keep all this
straight is that 50% (volume %) is the same thing as
500,000 ppm and likewise 1 % (volume %) is 10,000
ppm.
