Where's the Leak?
To effectively respond to any chemical odor complaint, one first needs to know the source of the chemical release.
- By Shah Khajeh Najafi, Dyron Hamlin
- Sep 01, 2009
A chemical emitted from an individual
site and the effect of that release — on
not only the emitting site, but also on
neighboring sites and nearby communities
— are of concern to plant personnel,
community leaders, and regulators. The
issue frequently first manifests itself in
terms of someone complaining about an
odor. This may be an employee at the site,
someone at a nearby site, or a resident of a
nearby town. Sometimes the odor may be
easily identified in terms of its substance;
in other cases, not. Often, though, especially
in a situation where a number of
industrial facilities are in close proximity
to one another, it may be difficult to determine
which site is the source of the offending
emission. This results in periods
of uncertainly as to who is responsible.
It may lead to ongoing complaints and
sometimes to good, old-fashioned fingerpointing.
In the afternoon of May 28, 2009, an
unusual number of workers in a petrochemical
plant were rushed to the infirmary
complaining about shortness of
breath, headaches, dizziness, and nausea.
(The plant is located in a rural area and
fl anked by several other industrial sites.)
Most of the complaints came from a unit
that was shut down for routine annual
maintenance. Workers reported smelling
a strong odor, but their description of the
odor characteristics could not establish a
link to a specific chemical.
The plant environmental health and
safety manager was informed. Contact
was made with the managers of different
units within the facility to check for a
possible chemical release during the time
of the reported smell, which was between
4 and 5 p.m. Results of the EH&S manager's
inquiry were negative, and a team
was assembled to identify the source of
the chemical leak.
The plant safety people provided
questionnaires to those who reported
sick. The questionnaire's main queries
were: Where was each individual located
when he or she first detected an unusual
odor? What time did he or she smell the
odor? What chemical odor characteristics
were present (e.g., pungent, rotten egg,
etc.)? What was the intensity of odor on
a number scale of 1 (low), 2 (medium),
and 3 (high)?
From the information gathered, a grid
was superimposed onto a map of the
plant site and each individual's location
at the time the odor was first detected
was marked. (The grid is helpful because
people could specify only the general area
where they started to feel sick, rather than
an exact location.) Five meteorological
towers were then identified: two inside
and three outside the plant's fenceline.
The petrochemical plant also had a
network of sensors for measuring volatile
organic compounds. A copy of archived
hourly average concentration of
measured volatile organic compounds
was obtained. These data could help to
identify chemical(s) with concentrations
beyond he background baseline values.
It was assumed all of the sickness was from
one source and all of the claims were legitimate.
For each location within the grid
using the data gathered via the questionnaires,
the team strove to identify individual
locations and the number of people at
each location. Meteorological data for the
month of May were obtained, from which
the data for the required time window
were extracted. The wind direction for the
period of interest was mainly from east to
The VOC measurement did not identify
any chemical with concentration above
background concentration. The odor description
by individuals was inconclusive,
so the team could not identify the type of
chemical via this method. The focus shifted
into finding the leak source and, most importantly,
whether the source was within
The minimum required information to
run a dispersion scenario is the identity
of chemical, release location, release rate,
and weather data. Meteorological information
was the only data available in this
instance. Armed with only these data, the
team could at best locate the general area
of the source leak but could not quantify
the magnitude of the release. Wind direction
and atmospheric stability were used
to create a reverse corridor for identifying
possible leak source locations. Reverse corridor
is the opposite of a Backtrajectory
method, which is commonly used in air
quality studies to examine the likely path
of a chemical plume released from a point
source. The reverse corridor tries to narrow
the search zone for possible chemical
A reverse corridor is constructed from
the position of each impacted individual.
The corridor is a wedge drawn from the
position of each individual using the opposite
wind direction. So if the wind is blowing
from the south, the reverse corridor is
drawn from the north.
The wind was swung from east to south/
southeast, and the reverse corridors were
drawn, with the hashed area depicting the
possible source locations identified on the
maps. Finally, all of the areas obtained from
different wind directions were combined to
create the overall area for the possible locations
of the odorous chemical release. It
clearly shows the source of the obnoxious
odor was located beyond the petrochemical
plant's property line.
This was both a relief and a concern
for the plant's EH&S personnel. At least
they now knew with a high degree of certainty
that they had not caused the workers'
sickness, and they were relatively sure
who likely had. The next step would be to
contact the facility that was located within
the source location area.
This article originally appeared in the September 2009 issue of Occupational Health & Safety.