This month our example is
Dichlorosilane which has a chemical formula of
SiH2Cl2. Its chemical structure is
shown in Figure 1. Dichlorosilane is a colorless gas
with an irritating, acidic odor. The immediate health
hazard is that it is a toxic gas. It is flammable, and
may form mixtures with air that are flammable or
explosive. Dichlorosilane is reactive with water and it
fumes in moist air to form hydrogen chloride and
siloxanes. Dichlorosilane is used for deposition of
epitaxial silicon and silicon-based alloys. It is
normally shipped as a liquefied compressed gas.
It has a melting point of -188°F and
a boiling point of 47°F. Its molecular weight is 101.01,
and has a vapor density is 3.52, so it will seek low
areas. It has a vapor pressure of 1,232 mm of Hg at a
standard temperature of 70°F. It is extremely flammable
with a flash point of -62°F. The lower Explosive Limit
(LEL) is 4.7%; the Upper Explosive Limit (UEL) is 96%.
Since it forms hydrogen chloride when in contact with
water or moisture it should be handled with extreme
There is no established IDLH for
Dichlorosilane but Voltaix, Inc., a manufacturer of the
material, recommends a Ceiling of 2.5 ppm, which is half
the Ceiling specified by ACGIH and OSHA for hydrogen
The important thing to remember when
dealing with Dichlorosilane is that it is both a very
flammable substance and its vapors can react with any
moisture to form hydrogen chloride. Therefore if the
material is released from its container, every effort
should be made to eliminate ignition sources and
appropriate PPE must be worn to protect from exposure or
- Keep separated from incompatible substances. Avoid
heat, flames, sparks and other sources of ignition.
- All chemicals should be considered hazardous.
Avoid direct physical contact. Use appropriate, approved
safety equipment. Untrained individuals should not
handle this chemical or its container. Handling should
occur in a chemical fume hood.
- Wear appropriate chemical protective clothing.
- Wear positive pressure self-contained breathing
spills or leaks - Keep sparks, flames, and other
sources of ignition away. Keep material out of water
sources and sewers. Attempt to stop leak if without
undue personnel hazard. Use water spray to knock-down
- May polymerize violently or explosively.
- Reacts with water or moist air to form HCl.
Attacks many metals in presence of water.
Decomposition - The substance may spontaneously
ignite on contact with air. Decomposes on heating or on
burning producing toxic and corrosive fumes including
toxological properties of this substance have not been
- Causes burns.
- Sore throat, cough, burning sensation, shortness
of breath, labored breathing. Corrosive to the
respiratory tract. May cause lung edema. Exposure to
high levels may result in death. Symptoms may be
- Redness, pain, burns, blisters. Exposure to liquid
can cause serious frostbite.
- Pain, redness, severe deep burns, loss of vision.
- Seek medical assistance.
- Move victim to fresh air. Apply artificial
respiration if victim is not breathing. Do not use
mouth-to-mouth method if victim ingested or inhaled the
substance; induce artificial respiration with the aid of
a pocket mask equipped with a one-way valve or other
proper respiratory medical device. Administer oxygen if
breathing is difficult.
- Remove and isolate contaminated clothing and
shoes. Immediately flush with running water for at least
20 minutes. In case of contact with liquefied gas, thaw
frosted parts with lukewarm water.
- Immediately flush with running water for at least
In using the PEAC application we
access information for the chemical by first locating
Dichlorosilane in the database. The following figures
show the screens displayed for chemical properties,
Figure 2 for the PEAC-WMD for Windows application
and Figure 3-5 for the PEAC‑WMD for the Pocket PC
Figure 2 - Using the Lookup By: Name
for Dichlorosilane using the PEAC-WMD for Windows
Review of the information displayed
in the chemical properties screen whether in Figure 2
(above) or Figures 3-5 (below), show chemical properties
values discussed earlier at the top of this discussion.
As you can see, there are no published toxicity values,
e.g., IDLH, ERPGs, or even the TEELs published by
Department of Energy. We will use either the default
Level of Concern or enter a value when we develop the
PAD a little later.
Figure 3 – Selecting
Dichlorosilane using the PEAC-WMD for Pocket PC
Figure 4 – The top portion of
the Chemical Properties Data Display
Figure 5 – The bottom portion
of the Chemical Properties Data Display
A benefit of using the PEAC tool is
assistance in the development of an evacuation zone for
those chemicals that produce a toxic vapor cloud.
Dichlorosilane has a relatively high vapor pressure
(1232 mm Hg) at room temperature, so if the chemical is
released it will vaporize rapidly. As with all of our
examples, AristaTek creates a scenario for a spill or
release of the specific chemical, and then we work
through the development of a PAD (Protective Action
Distance) to demonstrate how the PEAC system works. For
our scenario using Dichlorosilane as the spilled
chemical we’ll set the location to Phoenix and the time
as 12:30 PM on February 12th. A transport truck with 40
cylinders of Dichlorosilane has rolled off the
Interstate 10 close to the center of the city. At least
one of the cylinders has the valve cover knocked off the
cylinder and is leaking vapor very rapidly. The
temperature is about 70°F, the winds are about 5 mph,
and it’s a clear day (no clouds).
As seen at the top of the data
display screens, there is a yellow icon displayed; this
is the PEAC icon for notifying the user that a
Protective Action Distance can be calculated. Clicking
or tapping on the PAD icon will display a screen as
shown in Figure 6. Following through the screens, we
provide information on the Meteorology, Container Size,
and Type of Release (Source). The last screen displays
the PAD based on the provided information. If you decide
to follow along on this example, remember to change the
location to Phoenix and the time to 12:30 PM, February
It’s Phoenix in February and
the temperature about 70°, light wind is set for 5
mph, clear sky so we’ll set cloud cover to 0%, and
the terrain is Urban/Forest since it’s an urban
We have selected from our list
of container sizes the Cylinder selection.
This provides us with a default size that should
get us pretty close to the actual size.
Since the scenario has the
valve cover knocked off we’ve assumed a worst-case
scenario, we’ve selected a Large Rupture as
the Source type of
Figure 6 – Calculating a PAD using
the PEAC System
By pressing the right arrow at the
top of the screen, the PEAC system will display a screen
as shown in Figure 7. This calculates a PAD
(Protective Action Distance) based on the default
Level of Concern, works out to 10% of the LEL or
4700 ppm, since there are no published toxicity levels.
Since we know that Dichlorosilane reacts with moisture
to form Hydrogen Chloride (HCl), we can also use the
IDLH of HCl (50 ppm) as a Level of Concern to calculate
a more appropriate PAD (see Figure 8).
Figure 7 – Default PAD for
Dichlorosilane based on 10% of the LEL as the
Level of Concern.
Figure 8 – PAD for
Dichlorosilane using 50 ppm (the IDLH of Hydrogen
Chloride) as the Level of
Portions of this discussion were
adapted from the WEB site supported by the Hardy
Research Group, Department of Chemistry, The University
of Akron: http://ull.chemistry.uakron.edu/. Additional
information was adapted from Voltaix, Inc., a
manufacturer of Dichlorosilane and available at their