Example – Methyl Mercaptan
This month our example is Methyl
Mercaptan, which has a chemical formula of
(CH3SH). Methyl Mercaptan is listed under the
UN # (United Nations Number) by the US Department of
Transportation: UN 1064. Methyl Mercaptan is listed as
CAS# (Chemical Abstract Service Number) 74-93-1.
Persons exposed only to Methyl
Mercaptan pose little risk of secondary contamination to
personnel outside the Hot Zone.
Methyl Mercaptan is a colorless
flammable gas with unpleasant odor described as rotten
cabbage. It is easily ignited. When heated to
decomposition, it emits highly toxic fumes and flammable
vapors. Vapors from liquified Methyl Mercaptan gas are
heavier than air and may collect in low-lying areas.
Olfactory fatigue may prevent adequate warning of
Methyl Mercaptan is highly irritating
when it contacts moist tissues such as the eyes, skin,
and upper respiratory tract. It can also induce
headache, dizziness, nausea, vomiting, coma, and death.
Ingestion of Methyl Mercaptan is unlikely since it is a
gas at ambient temperatures.
Methyl Mercaptan is produced by the reaction of
hydrogen sulfide with methanol. It is used as a gas
odorant; an intermediate in the production of
pesticides, jet fuels, and plastics; in the synthesis of
methionine; and as a catalyst.
flammable gas at room temperature
properties: Odor does not provide adequate warning
Molecular weight: 48.1
point: (760 mm Hg) = 43 °F (6 °C)
Freezing point: -186
°F (-123 °C)
(liquid): 0.87 at 0 °C
pressure: 1,520 mm Hg at 26 °C
density: 1.66 (air = 1)
solubility: 23.3 g/L at 20 °C
flammable limits 3.9% to 21.8% at room temperature
include methanethiol, mercaptomethane, thiomethanol,
methyl sulfhydrate, and thiomethyl alcohol.
Incompatibilities: Methyl mercaptan is
incompatible with strong oxidizers, bleaches, copper,
aluminum, and nickel-copper alloys.
At room temperature (above 43 °F), Methyl Mercaptan
is a colorless gas with an unpleasant odor described as
rotten cabbage. It is slightly soluble in water. It is
generally shipped as a liquefied compressed gas. When
heated to decomposition, it emits toxic fumes, such as
sulfur dioxide, and flammable vapors. Methyl Mercaptan
should be stored in cool, well-ventilated places. The
main toxic effect of exposure to Methyl Mercaptan is
irritation of the respiratory airway, skin, and eyes.
Standards and Guidelines
(permissible exposure limit) = 10 ppm (20 mg/m3) NIOSH REL
(recommended exposure limit) = 0.5 ppm
IDLH (immediately dangerous to life or health) = 150 ppm
ERPG-2 (maximum airborne concentration below which it is
believed that nearly all persons could be exposed for up
to 1 hour without experiencing or developing
irreversible or other serious health effects or symptoms
that could impair their abilities to take protective
action) = 25 ppm.
Routes of Exposure
Inhalation is the major route of exposure to Methyl
Mercaptan. An odor threshold of 0.002 ppm has been
reported for Methyl Mercaptan, but olfactory fatigue may
occur and thus, it may not provide adequate warning
of hazardous concentrations. Vapors of liquified
Methyl Mercaptan gas are heavier than air and spread
along the ground. Exposure in poorly ventilated,
enclosed, or low lying areas can result in asphyxiation
because of respiratory paralysis.
exposed to the same levels of Methyl Mercaptan as adults
may receive a larger dose because they have a greater
lung surface area:body weight ratios and higher minute
volume:weight ratios. In addition, they may be exposed
to higher levels than adults in the same location
because of their short stature and the higher levels of
Methyl Mercaptan found nearer to the ground.
Contact Direct contact with liquid Methyl Mercaptan
or the gas may cause frostbite injury or irritation of
the eyes and skin.
Ingestion is unlikely to occur because Methyl
Mercaptan is a gas at room temperature.
Methyl Mercaptan gas is irritating to
the eyes, skin, and respiratory tract. Edema of the
airway and lungs can occur. Other possible effects
include headache, dizziness, tremors, and seizures, and
nausea and vomiting, and lack of coordination. The gas
is rapidly absorbed in the lungs. Skin and eye
absorption are minimal; however, upon direct contact
with eyes or skin, liquified Methyl Mercaptan will
likely cause frostbite injury.
Methyl Mercaptan is a central nervous
system depressant that acts on the respiratory center to
produce death by respiratory paralysis.
Individuals with pre-existing
respiratory, cardiac, nervous system, or liver
impairment may be more susceptible to exposure to Methyl
Exposure Methyl Mercaptan inhibits mitochondrial
respiration by interfering with cytochrome c oxidase. It
also inhibits several enzyme systems such as carbonic
anhydrase, beta-tyrosinase, and sodium+,
potassium+ATPase. The enzyme inhibition appears to be
related to a thiol-metal interference.
do not always respond to chemicals in the same way that
adults do. Different protocols for managing their care
may be needed.
Acute inhalation exposure can irritate the mucous
membranes of the respiratory tract. This may cause
cough, dyspnea, a sensation of tightness of the chest,
and subsequent cyanosis. Respiratory depression, apnea,
and pulmonary edema were observed in animals.
may be more vulnerable to gas exposure because of
relatively higher minute ventilation per kg and failure
to evacuate an area promptly when exposed.
Severe hemolytic anemia may occur in people with
glucose-6-phosphate dehydrogenase deficiency.
Restlessness, headache, staggering, and dizziness
may develop; severe exposure may lead to convulsions and
Frostbite injury can occur from contact with the
liquified gas. Because of their relatively larger
surface area:weight ratio, children are more vulnerable
to toxicants that may affect the skin.
concentrations of Methyl Mercaptan can cause eye
Although ingestion is unlikely, irritation of the
mouth, throat, and esophagus are possible. Nausea and
vomiting may occur even with inhalation exposure to the
Potential Sequelae Methyl
Mercaptan exposure may result in altered heme synthesis.
Chronic Exposure Dermatitis
can occur with chronic exposure to Methyl Mercaptan.
Mercaptan has not been classified for carcinogenic
Reproductive and Developmental
Effects No information is available regarding
reproductive or developmental effects of Methyl
Mercaptan in experimental animals or humans. Methyl
Mercaptan is not included in Reproductive and
Developmental Toxicants, a 1991 report published by
the U.S. General Accounting Office (GAO) that lists 30
chemicals of concern because of widely acknowledged
reproductive and developmental consequences.
Using the PEAC tool
In using the PEAC application we
access information for the chemical by first locating
Methyl Mercaptan in the database. The following figures
show the screens displayed for chemical properties,
Figure 1 for the PEAC-WMD for Windows application
and Figure 2-4 for the PEAC‑WMD for the Pocket PC
Figure 1 - Using the Lookup By: Name
for Methyl Mercaptan using the PEAC-WMD for
Review of the information displayed
in the chemical properties screen whether in Figure 1
(above) or Figures 2-4 (below), show chemical properties
values discussed earlier at the top of this discussion.
As you can see below, the published toxicity values,
e.g., IDLH, ERPGs, and the TEELs (Temporary Emergency
Exposure Limits) published by Department of Energy are
provided. We will use the IDLH as the Level of Concern
when we develop the Protective Action Distance (PAD) a
Figure 2 – Selecting Methyl
Mercaptan using the PEAC-WMD for Pocket PC
Figure 3 – The top portion of the
Chemical Properties Data Display Screen
Figure 4– The bottom portion of
the Chemical Properties Data Display Screen
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. As
with most 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
For our hypothetical scenario using
Methyl Mercaptan as the involved chemical we’ll set the
location to be natural gas processing facility located
outside Houston, TX. The date is May 13, 2004, at
midnight with a temperature of 65°F, wind speed of 2 mph and a clear
sky. The hypothetical release involves a portable tank
that contains Methyl Mercaptan and a ½” transfer line
from the tank has been severed and can’t be isolated.
Vapor is flowing from the tank through the ½” line. The
PEAC tool can provide guidance with regards to toxic
vapor cloud that is released.
If you decide to follow along as we
proceed through these examples, remember to set the
location to Houston and set the date and time to the
proper values, otherwise you’ll compute different
values. We’ll use a terrain type of urban/forest since
this is a manufacturing facility and has buildings and
processing equipment in the immediate area.
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 5. Following through the screens, we
provide information on the Meteorology, Container Size,
and Type of Release (Source). The following figures
demonstrate how we would work through our scenario to
see what our Protective Action Distance should be.
It’s midnight Houston in May
and the temperature about 65°, wind is set for 2
mph, clear skies and the terrain is
Urban/Forest since it’s a processing
We have selected from our list
of container sizes the Portable Tank
selection. This gets us a quick estimate of
how much material might be involved.
We have selected a Hole or
Pipe Release, and set the hole size to 0.5”.
Figure 5 – Calculating a PAD using the
PEAC‑WMD System for May 13th
By pressing the right arrow at the
top of the screen, the PEAC system will display a screen
as shown in Figure 6. This calculates a PAD
(Protective Action Distance) based on the default
Level of Concern the IDLH of 150 ppm. This
evacuation or standoff distance is based on the toxicity
of Methyl Mercaptan, not the flammability.
Figure 6 – Default PAD for Methyl
Mercaptan using the IDLH of 150 ppm
If we want to calculate a PAD based
on a toxicity level other than the IDLH, we can enter a
value in the field for Level of Concern or we can
select a value from our list of toxicity values shown in
Figure 7. In this figure we select the ERPG-2 value or
Figure 7 – Selecting another Level of
The calculated PAD will be displayed, see Figure
Figure 8 – Calculated PAD using the
ERPG-2 Level of Concern
In addition to the toxicity of the
released material, the user should also remember the
flammability issue with Methyl Mercaptan and eliminate
all ignition sources. At either the IDLH concentration
of 150 ppm, or the ERPG-2 concentration of 25 ppm, there
will be many complaints about the odor.
Substantial portions of this
discussion were adapted from the Agency for Toxic
Substances and Disease Registry (ATSDR) Web site for
Medical Management Guidelines at: http://www.atsdr.cdc.gov/.